I

EFFECTS OF CATTLE MANURE AND MINERAL NITROGEN

FERTILIZER APPLICATIONS ON NITROUS OXIDE EMISSION

AND NITRATE LEACHING IN A WETLAND CROPPING SYSTEM

IN ZIMBABWE

BY JOHNSON MASAKA

A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE

REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY

DEPARTMENT OF SOIL SCIENCE AND AGRICULTURAL

ENGINEERING

FACULTY OF AGRICULTURE

UNIVERSITY OF ZIMBABWE

NOVEMBER 2013

II

TITLES................................................................................................................... I

LIST OF CONTENTS.............................................................................................. II

LIST OF TABLES……………………………………………………………………….. VI

LIST OF FIGURES……………………………………………………………………… VII

LIST OF SYMBOLS AND ACRONYMS……………………………………………… IX

DEDICATION………………………………………………………………………….. X

ACKNOWLEDGEMENTS…………………………………………………………….. XI

ABSTRACT…………………………………………………………………………….. XII

1.0 INTRODUCTION.....................................................................................................

1

1.1 The nature and use of wetlands in Zimbabwe…………………………………... 1

1.2 Justification of the study …………………………………………………........... 3

1.3 Objectives of the study………………………………………………………………. 5

1.3.1 Main objective………………………………………………………………….. 5

1.3.2 Hypotheses……………………………………………………………………... 5

1.3.3 Specific objectives……………………………………………………………... 6

1.4 Thesis structure…………………………………………………………………… 6

2.0 LITERATURE REVIEW ………………………………………………………… 7

2.1 Wetland vegetable production practices in Zimbabwe…………………………. 7

2.2 Availability and quality of cattle manure in Zimbabwe………………………… 8

2.3 Biogeochemistry of wetlands soils……………………………………………… 10

2.4 Biochemical transformation pathways of N in soils……………………………. 11

2.4.1 Mineralization and immobilization………………………………………………... 11

2.4.2 Nitrification…………………………………………………………………….. 13

2.4.3 Loss of N by ammonia volatilization…………………………………………. 14

2.4.4 Loss of N by denitrification…………………………………………………… 14

2.4.5 Loss of N by leaching.................................................................................... 15

2.5 Effect of mineral N fertilizer and cattle manure applications on N2O emission and

nitrate leaching………………………………………………………………………..

17

2.5.1 Effect of manure and mineral N fertilizer application on N2O emission............ 17

2.5.2 Effect manure and mineral N fertilizer application on nitrate leaching.......... 21

2.5.3 Cattle manure quality, N2O emission and nitrate leaching…………………… 23

III

2.6 Gas sampling techniques for measurement of N2O fluxes in soil……………… 26

2.7 Lysimeter installations for NO3 leaching measurements……………………….. 27

2.7.1 Uses of lysimeters……………………………………………………………… 27

2.7.2 Lysimeter soil filling technique for measuring nitrate fluxes in leachate…….. 22

2.7.3 Lysimeter size and material used………………………………………………. 28

2.7.4 Placement of lysimeters in the field…………………………………………......... 29

3.0 GENERAL MATERIALS AND METHODS………………………………………. 30

3.1 Site description……………………………………………………………........... 30

3.2 Experimental soil description............................................................................ 31

3.3 Dufuya wetland cropping and hydrology……………....................................... 32

3.4 Land preparation and plot establishment........................................................... 35

3.5 Cattle manure used in the study........................................................................ 35

3.6 Test crops and crop management..................................................................... 36

3.7 Nitrous oxide flux measurement...................................................................... 36

3.8 Lysimeter experiments....................................................................................... 39

3.9 Mineral N measurements................................................................................... 41

3.10 Dry matter yield and N uptake………………………………………................. 41

3.11 Weather conditions……………………………………………………………... 42

4.0 EFFECTS OF CATTLE MANURE QUALITY ON NITROUS OXIDE EMISSION

AND NITRATE LEACHING...................................................................................

44

Abstract.................................................................................................................... 44

4.1 Introduction………………………………………………………………………. 45

4.2 Hypotheses and objectives……………………………………………................ 46

4.3 Materials and methods…………………………………………………………… 46

4.3.1 Experimental manure …………………………................................................. 46

4.3.2 Experimental design and treatments............................................................... 46

4.3.3 Nitrous oxide, mineral N and dry matter measurements................................ 47

4.3.4 Lysimenter study........................................................................................... 47

4.3.3 Statistical analysis.......................................................................................... 47

4.4 Results.............................................................................................................. 47

4.4.1 Ammonium N concentration in soil................................................................ 47

4.4.2 Nitrate N concentration in soil…………………………………………………. 49

4.4.3 Nitrate N concentration in leachate.......................................................................... 51

IV

4.4.4 Nitrous oxide fluxes on soil…………………………………………………… 53

4.4.5 Estimated total N lost in leachate following application of high and low

N manure………………………………………………………………………………

54

4.4.6 Estimated total N lost in N2O emission following application of high and low N

manure…………………………………………………………………………………

57

4.4.7 N uptake following application of high and low N manure...................................... 59

4.4.8 Correlations between measured variables................................................................ 60

4.5 Discussion…………………………………………………………………………. 65

4.5.1 Cattle manure quality, nitrate N leaching and nitrous oxide emissions.................... 65

4.5.2 Regression analysis between measured variables..................................................... 68

4.5.3 Effect of manure quality and application rates on soil N uptake.................... 69

4.6 Conclusions....................................................................................................... 70

5.0 EFFECTS OF APPLICATION RATES OF MINERAL N FERTILIZER AND

CATTALE MANURE ON N2O EMISSION AND NITRATE LEACHING........... ........

72

Abstract……………………………………………………………………………….. 72

5.1 Introduction………………………………………………………………………. 73

5.2 Hypothesis and objective.................................................................................. 74

5.3 Materials and methods...................................................................................... 74

5.3.1 Experimental design and treatments................................................................. 74

5.3.2 Experimental manure..................................................................................... 75

5.3.3 Nitrous oxide and nitrate leaching measurements.......................................... 75

5.3.4 Dry matter sampling and analysis……………………………………………… 75

5.3.5 Statistical analysis………………………………………………………………. 75

5.4 Results.............................................................................................................. 76

5.4.1 Ammonium N concentrations in soil after N fertilizer and manure application 76

5.4.2 Ammonium N concentrations in soil after manure application…………….... 77

5.4.3 Nitrate N concentrations in soil after N fertilizer and manure application….. 79

5.4.4 Nitrate N concentrations in soil after manure application……………………. 80

5.4.5 Nitrous oxide fluxes in soil following application of mineral N fertilizer and

manure…………………………………………………………………………..

82

5.4.6 Nitrous oxide fluxes in soil following application of manure……………….. 83

5.4.7 Nitrate N concentration in leachate and leachate volume after application of N

fertilizer and manure……………………………………………………………

85

V

5.4.8 Nitrate N concentration in leachate after manure application………………… 88

5.4.9 Correlations between selected variables……………………………………….. 89

5.4.10 N uptake following application of mineral N fertilizer and manure……….. 91

5.4.11 N uptake following application of high N manure.................................................. 92

5.4.12 Estimated total N lost in leachate following application of N fertilizer and

manure…………………………………………………………………………………..

92

5.4.13 Estimated total N lost in leachate following application of cattle manure…. 94

5.4.14 Total N lost as N2O following N fertilizer and manure application........................ 95

5.4.15 Total N lost as nitrous oxide after application of cattle manure……………. 97

5.4.16 Total N in N2O emission and nitrate leaching per unit dry matter………….. 97

5.5 Discussion………………………………………………………………………… 98

5.5.1 Mineralized N concentrations in fertilized soil……………………………….. 98

5.5.2 Nitrate leachating in fertilized cropping.................................................................... 99

5.5.3 Nitrous oxide fluxes from soil................................................................................... 102

5.5.4 Total N lost as nitrous oxide and nitrate leaching…………………………….. 103

5.5.5 Regression analysis…………………………………………………………….. 104

5.5.6 N uptake by the crops............................................................................................... 105

5.5.7 Loss of N in N2O emission and nitrate leaching per unit dry matter........................ 105

5.6 Conclussion.................................................................................................................. 106

6.0 THE EFFECTS SEASONAL SPLIT APPLICATION OF CATTLE MANURE ON

NITROUS OXIDE EMISSION……………………………………………………….

107

Abstract............................................................................................................. 107

6.1 Introduction………………………………………………………………………. 108

6.2 Objective and hypothesis.................................................................................. 108

6.3 Materials and methods...................................................................................... 108

6.3.1 Experimental manure..................................................................................... 108

6.3.2 Experimental design and treatments…………………………………………… 109

6.3.3 Gas sampling and analysis……………………………………………………... 109

6.3.4 Statistical analysis………………………………………………………………. 109

6.4 Results…………………………………………………………………………….. 110

6.4.1 NH4-N concentrations in soil following single and split application manure 110

6.4.2 NO3-N concentration in soil following single and split application of and manure 112

6.4.3 Nitrous oxide fluxes on soil following single and split application of high manure 113

VI

6.4.4 Soil factors - N2O emission relationships........................................................ 115

6.4.5 Total N lost as nitrous oxide…………………………………………………… 116

6.4.6 N uptake and aboveground dry matter yield............................................................. 118

6.5 Discussion.................................................................................................................... 120

6.5.1 Effect of seasonal split manure application on mineral N and N2O emission 120

6.5.2 Regression analysis........................................................................................ 122

6.5.3 Effect of single and seasonal split application of manure on soil N uptake 122

6.6 Conclusions………………………………………………………………………. 124

7.0 OVERALL DISCUSSIONS, CONCLUSIONS AND RECOMMENDATIONS 125

7.1 Nitrous oxide emissions from wetland soil amended with cattle manure and n

fertilizer...................................................................................................................

126

7.1.1 Gaseous losses of N following N fertilizer and cattle manure application...... 125

7.1.2 Effects of cattle manure quality on N2O emissions…………………………… 127

7.1.3 Effects of seasonal split application of cattle manure on N2O emission……. 127

7.2 Nitrate leaching in wetland soil amended with cattle manure and N fertilizer… 129

7.2.1 Nitrate N leaching following application of N fertilizer and cattle manure…. 129

7.2.2 Nitrate N leaching from wetland soil following application of high and low N

manure…………………………………………………………………………………

130

7.3 Implications for farmers…………………………………………………………. 132

7.4 Suggested further research……………………………………………………….. 133

References.............................................................................................................. 134

Appendices............................................................................................................. 152

Appendix 1: Summary of the results on N2O emissions and nitrate leaching.................... 152

Appendix 2: Journal publications....................................................................................... 154

LIST OF TABLES

Table 3.1 Chemical and physical properties of the experimental soil…………….. 32

Table 3.2 Selected chemical properties of cattle manure from communal and

commercial farming areas.......................................................................................

36

Table 4.1: Estimated total N lost in leachate following application of low N manure 55

Table 4.2: Estimated total N lost in leachate following application of high N manure 56

Table 4.3: Estimated total N lost through N2O emission following application of low N

manure………………………………………………………………………………..

57

Table 4.4: Estimated total N lost through N2O emission following application of high N

VII

manure……………………………………………………………………………….. 58

Table 4.5: Dry matter yield and N uptake by aboveground plant biomass following

application of high N cattle manure………………………………………………….

59

Table 4.6: Dry matter yield and N uptake by aboveground plant biomass following

application of low N manure…………………………………………………………

60

Table 5.1: Dry matter yield and N uptake by aboveground plant biomass following

application of N fertilizer and manure………………………………………………

92

Table 5.2: Estimated total N lost through leaching following application of N fertilizer

and manure……………………………………………………………………………

94

Table 5.3: Estimated total N as N2O after application of N fertilizer and cattle manure 96

Table 5.4: Loss of N in N2O emission and nitrate leaching per unit matter yield 98

Table 6.1: Estimated total N lost through nitrous oxide emission following seasonal

split application of manure………………………………………………………………

117

Table 6.2: N uptake and aboveground dry matter yield after split application of manure 119

LIST OF FIGURES

Fig 2.1: Transformation of mineral N in the soil..................................................... 19

Fig 3.1 Study site location in Dufuya wetland and Zimbabwe................................ 30

Fig 3.2 A typical shallow well less than 2 m deep at Dufuya........................................... 34

Fig 3.3 Experimental plots at Dufuya wetlands................................................................ 35

Fig 3.4 Static chamber for nitrous oxide gas sampling in a plot with a tomato crop 37

Fig 3.5 Aerial view of the lysimeter station for the experiments.............................. 40

Fig 3.6 Daily rainfall, air temperature at the study site...................................................... 42

Fig 4.1: NH4-N concentration in soil following application of high and low N manure 48

Fig 4.2: NO3-N concentration in soil following application of high and low N manure 50

Fig 4.3: NO3-N concentration in leachate following application of high and low N

manure....................................................................................................................

52

Fig 4.4: Nitrous oxide fluxes on soil following application of high and low N manure.

53

Fig 4.5 Regression analyses showing the effects of soil factors on selected variables

following application of low N manure...................................................................

61

Fig 4.6 Regression analyses showing the effects of soil factors on NO3-N in leachate

VIII

following application of high N manure............................................................................ 62

Fig 4.7 Regression analyses showing relationships between mineral N, N2O and soil

moisture after application of high N manure...........................................................

64

Fig 4.8 Regression analyses showing relationships between mineral N, N2O and soil

moisture after application of low N manure...........................................................

65

Fig 5.1 NH4-N concentration in soil following application of N fertilizer and manure 77

Fig 5.2: NH4-N concentration in soil following cattle manure application........................ 78

Fig 5.3: NO3-N concentration in soil following application of N fertilizer and manure 80

Fig 5.4: NO3-N concentration in soil following cattle manure application........................ 81

Fig 5.5: Nitrous oxide fluxes on soil following application of N fertilizer and manure 83

Fig 5.6: Nitrous oxide fluxes on soil following manure application.................................. 84

Fig 5.7: NO3-N leaching in soil following application of N fertilizer and manure........... 86

Fig 5.8: Cumulative precipitation and leachate volumes following various application

rates of N fertilizer and cattle manure to rape and tomato crops at Dufuya wetland........

87

Fig 5.9: NO3-N concentration in leachate following manure application......................... 88

Fig 5.10: Regression analyses showing the relationships between soil measured

variables and NO3-N in leachate after applications of N fertilizer and manure........

91

Fig 5.11: Regression analyses showing relationships between N2O flux on surface soil

and mineral N concentration in soil........................................................................

91

Fig 6.1: NH4-N concentration in soil following single and split application of high N

manure....................................................................................................................

111

Fig 6.2 NO3-N concentration in soil following single and split application of high N

manure....................................................................................................................

112

Fig 6.3: Nitrous oxide fluxes from wetland soil following single and split application of

high N manure...................................................................................................................

114

Fig 6.4 Regression analyses showing relationships between mineral N, N2O and soil

moisture after split application of manure..............................................................

116

IX

LIST OF SYMBOLS AND ACRONYMS

GHG: Greenhouse gas

C: N: Carbon to N ratio

USA: United States of America

FAO: Food and Agriculture Organization

USDA: United States Department of Agriculture

NJ: New Jersey

CA: California

TX: Texas

USEPA: United States Environmental Protection

Agency

IPCC: International Panel on Climate Change

LSD: Least significant difference

X

DEDICATION

To my wife Sibongile, daughters Ruramayi and Hazel, my son Tawonashe, and my

elder brother M.D. Masaka. Thank you for taking me this far.

XI

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my first main supervisor Professor

Justice Nyamangara who was always willing to give academic support and advice at

every stage of this study. Despite his tight and busy schedules, Professor J.

Nyamangara found time and space on the many occasions he passed through Gweru

town for short discussions on the study. The contributions by the second academic

supervisor Professor Francis Themba Mugabe are also recognized sincerely. The

second main academic supervisor Dr M. Wuta’s diligence in guiding the write up of

the thesis and encouragement to continue with the work contributed immensely to the

success of this study. The greater part of the field experiments at Dufuya was financed

by the Midlands State University through Research Board funding during the period

of hyperinflation. The University imported 1200 glass vials for N2O gas sampling

from USA and allocated scarce fuel resources for field trips to Dufuya at a time when

it was facing unbearable financial challenges. The contributions of the Environmental

Management Agency (Gweru and Head Office, Harare) and the Heifer Project

International in initiating the study at Dufuya wetlands were vital. The Department of

Soil Science and Agricultural Engineering of the University of Zimbabwe gave the

researcher a rare opportunity to carry out this study at this level. All members of the

department were instrumental in one way or another. Thank you.

XII

ABSTRACT

The uncertainty in the contributions of N2O emissions from African regions to the

global anthropogenic N2O emissions is largely due to the scarcity of data and low

frequency of sampling in African tropical studies. The overall objective of this study

was to determine the effect of cattle manure and mineral N fertilizer applications on

N2O emissions and nitrate leaching in a wetland cropping system in Zimbabwe.

Experimental treatments varying in rates of application, manure quality and timing of

application were used as follows: Control; 100 kg N +15 Mg high N manure

(1.36%N) ha-1; 200 kg N + 30 Mg high N manure ha-1; control; 15 Mg low (0.51%N)

and high N manure ha-1; 30 Mg high and low N manure ha-1; control; 15 Mg high N

manure; 30 Mg high N manure split into four applications of 3.75 and 7.5 Mg ha-1 per

cropping event. A completely randomized block design was used in the experiments.

Fluxes of N2O on soil were measured using static chamber techniques and gas

chromatography. Nitrate leaching was measured using zero tension lysimeters for

leachate collection.

Study results have confirmed increased N2O emissions and nitrate leaching with

increasing N fertilizer and manure applications to tomato and rape crops. Increasing

the application rates of N fertilizer and cattle manure from 100 kg N + 15 Mg high N

manure to 200 kg N + 30 Mg high N manure ha-1significantly (p<0.05) increased the

loss of N by N2O emissions by 40% and 45% for the tomato and rape crops

respectively. The same practice increased significantly the loss of N in nitrate

leaching by 64% and 56% for the tomato and rape crops, respectively. The

substitution of 15 and 30 Mg low N manure ha-1 with the same rates of high N manure

increased N2O fluxes from wetland soil surface by 41% and 50%, respectively. When

15 and 30 Mg low N ha-1 manure were substituted with the same rates of high N

manure total N lost through N2O emission increased by 59% and 31%, respectively.

The proportion of applied N lost as N2O emission and nitrate leaching was higher in

the rape crop than in the tomato crop. When 30 and 15 Mg of high N manure ha-1

were applied once in four cropping events N2O emissions were significantly higher

than those recorded on plots that received split applications of 3.75 and 7.5 Mg ha-1

manure at least up to the second test crop. Thereafter N2O emissions on plots

subjected to split applications of manure were higher or equal to those recorded in

plots that received single basal applications of 15 and 30 Mg manure ha-1.

When the applications rates of mineral N fertilize and manure were doubled, the loss

of N in N2O emissions per unit dry matter yield decreased significantly (p<0.05) by

0.02 – 0.03 kg N2O-N/Mg of harvested dry matter. The loss of N from applied

fertilizer per unit harvested dry matter yield decreases significantly with increasing

harvested dry matter yield. The rates of nitrate N leaching losses of applied N in the

current study were comparably similar with global average nitrate N leaching losses

of 19% of applied. The percentage of N applied lost as N2O-N when N fertilizer and

manure were applied was generally lower than the global default value of 1.25% of N

applied. The most recent IPCC methodology for estimating direct N2O emission from

applied fertilizer is based on emission data in temperate regions. These data are scant

especially in sub-tropical regions of Africa. The current study is of important

significance in providing a more complete data set from sub-tropical Africa to IPCC

for incorporation into the predictive models for global estimations of N2O emissions.

1

CHAPTER ONE

1.0 INTRODUCTION

1.1 THE NATURE AND USE OF WETLANDS IN ZIMBABWE

The southern, western and central parts of Zimbabwe are covered predominantly by

semi-arid areas under Agro-ecological Regions III, IV and V, where rainfall is

generally low and erratic (300 - 800 mm per year) for reliable rain-fed cropping

(Vincent and Thomas, 1960; Mugandani et al., 2012). About 74% of the smallholder

areas in Zimbabwe are located in these fragile agro-ecological environments (Ministry

of Environment and Natural Resources Management, 2010) where availability of

water is the dominant factor controlling crop production.

The assured availability of water in wetlands has made wetland utilization a wide

spread farming practice in the smallholder areas (Owen et al., 1995). Wetlands are

drainage depressions that occur in most parts of the Zimbabwe. Areas of granitic

rocks along the watershed may contain wetlands on more than one third of the area

(Whitlow, 1988, Owen et al., 1995). Wetlands vary strongly in their morphologic,

hydrologic and pedologic characteristics, so that each one can be considered unique.

Surface runoff and seepage of groundwater from catchment areas over an

impermeable substratum towards lower lying areas, together with incident

precipitation contribute largely to the water budget of wetlands (Ramsar Managing

Wetlands, 2010). Attempts to classify wetlands in the 1960s in Zimbabwe according

to texture, effective top soil depth and duration of wetness had limited success.

Texture of wetland soils in Zimbabwe range from sands to clays, while profiles range

from undeveloped to strongly developed. Factors such as morphogenesis, location,

hydrologic regimes, lithologic origins and climatic conditions account for the wide

soil variation between wetlands (Brinkman and Blockhuis, 1986).

Estimated world area coverage of wetlands range between 700 million ha (Mitsch and

Gosselink, 1993) and 1024 million ha (Ramsar, 1999), which is about 5 to 7% of the

Earth’s terrestrial surface area. There are 200 million ha of wetland in tropical sub-

2

Saharan Africa that exists in the form of coastal wetlands, lakes, river flood plains and

small inland valleys (Neue et al., 1997). The importance of wetlands to global

biochemistry, water balance, wildlife and human food production is much greater than

their proportional surface area. Wetlands contain the most productive agricultural and

natural ecosystems on Earth (Ramsar, 2010). Wetlands in Zimbabwe cover

approximately 1.28 million hectares, of which 25% are found in the smallholder areas

(Ministry of Environment and Natural Resources Management, 2010).

At its Sixth Conference of the Contracting Parties in 1996, the Convention on

Wetlands (Ramsar, 1993) adopted Resolution VI. 23 entitled ‘Ramsar and Water’.

This resolution recognized the important hydrological ground water recharge, water

quality improvement, flood alleviation and the inextricable link between water

resources and wetlands (Ramsar, 2010). In Zimbabwe, the Environmental

Management Act (2002) (Chapter 20:27) governs wetland utilization. Under this Act,

wetlands can only be legally cultivated by obtaining a special consent with respect to

the legislation. The Act forbids wetland cultivation in order to preserve downstream

dry season river flow. The Act also bans the cultivation of any land within 30 m of a

stream bank in order to reduce erosion and river siltation.

Research in Zimbabwe and Malawi reported that the essential cause of accelerated

erosion in the smallholder wetlands is deforestation and cultivation of the sandy

interfluves (Whitlow, 1988). Although there are a number of theories relating

accelerated erosion to land use changes due to legislation, it has not been conclusively

established that there is a direct cause and effect linkage (Owen et al., 1995). Present

research in Zimbabwe has not generated a clear hydrological model for wetlands,

although considerable progress has been made in recent years. This is due in part to

the complexity of wetland hydrology and the very high degree of heterogeneity

among wetlands (Mitsch and Gosselink, 1993). It is generally agreed that in terms of

water resources the catchment area and the wetland operate as a unit (Brinkman and

Blockhuis, 1986). The principal aquifer storage during the dry season lies beneath the

upper dry land areas of the catchment. Therefore management of the upper catchment

is seen as an integral part of the wetland management (Owen et al., 1995, Neue et al.,

1997).

3

In many southern African countries, wetlands have been traditionally cultivated in

small individually owned gardens using a bed-and-furrow system, with rice grown in

the furrows and other crops grown on raised beds (Whitlow, 1988). These practices

have resulted in suitable aeration of the beds, organic matter accumulation in the

furrows, avoidance of stagnant water and interception of runoff, and have minimized

erosion (Ministry of Environment and Natural Resources Management, 2010). The

concept that wetlands are areas of high soil fertility needs to be redefined. They are

productive only so far as they have available water where other areas do not. Some

wetland soils are strongly leached and poor in nutrients and others have high organic

matter content which improves their physical structure, fertility and water holding

capacity. Results from research highlight the potential productivity of wetlands under

proper management. With the addition of fertilizer, especially manure, good yields of

maize can be obtained (Whitlow, 1988, Owen et al., 1995).

1.2 JUSTIFICATION OF THE STUDY

Water is one of the most critical factors that limit smallholder crop production in the

semi-arid areas of Zimbabwe (Nyamangara et al., 2000; Qadir et al., 2006). The

integrated resource management concept, which has become a dominant paradigm in

sustainable rural development in Zimbabwe, has encouraged the tapping of local

water resources by smallholder farmers in order to improve food security systems

(Nzuma and Murwira, 2000). Wetlands are important in crop production in the

smallholder semi-arid areas of Zimbabwe, because they have enough water for a

longer period to allow crops to be grown throughout the year. The assured availability

of water in wetlands, which can be extracted without large capital-intensive measures

(Fig 1.1), has enticed smallholder farmers to intensively utilize wetlands under

cropping (Kuntashula and Mafongoya, 2009).

Cattle manure commonly used by smallholder farmers in Zimbabwe has relatively

low nutrient content in comparison with commercial fertilizers. In an effort to

increase wetland vegetable crop yields farmers apply manures in combination with

mineral fertilizers at rates far in excess of those employed in commercial agriculture

(De Lannoy, 2001, Zotarelli et al., 2009; Lin et al., 2011). Vegetable production

systems in the tropics and elsewhere are mostly intensive because vegetables are high-

value crops. The consequences of the larger inputs of N applied to wetland vegetables

4

are worsened by the fact that vegetables have a low N recovery rate (Vegetable

Production Management, 2011). Lowrance and Smittle (1988) measured 290 kg N

ha-1 available for leaching out of 388 kg N ha-1 applied to a vegetable-cereal

production system. As a result, the unused N applied to crops with fertilizer and N

mineralised from soil and applied manures, under anaerobic conditions may be lost to

the atmosphere in gaseous forms through denitrification (Van der Meer, 2008;

Mapanda et al., 2011). Furthermore, under water saturated wetland soil conditions N

is also lost by leaching into underground water systems (Silva et al., 2005; Salo and

Turtola, 2006).

With the need to quantify all possible sources of N2O, there is a renewed interest in N

balance studies (Wrage et al., 2001). Research during the past several decades has

improved our understanding of how N2O is produced, the factors that control its

production, source/sink relationships, and gas movement processes. However, despite

extensive knowledge of the processes involved scientists are only beginning to be able

to predict the fate of a unit of N that is applied or deposited on a specific agricultural

field (Mosier et al., 2003). While studies on the loss of N by denitrification processes

are extensive and thorough in Europe, North America and south-east Asia, there has

been a lack of research on the same subject in Africa. Exploring greenhouse gas

emissions that occur at the soil-atmosphere interface is an essential part of the effort

to integrate land management strategies with climate change mitigation and

adaptation in southern Africa (Mapanda et al., 2011).

Both conservationists and agriculturalists have recognised the potential vulnerability

of wetland environments to the effects of increasing wetland soil fertility, but there is

little scientific information about the effects of elevated fertilizer applications and

manure derived N on losses of N through gaseous emissions and nitrate leaching in

Zimbabwe. A study on N cycling and loss of N through emissions of N2O by

Chikowo et al. (2004) in Zimbabwe was carried out on dry land cropping systems at

Domboshava Training Centre. Nitrate N leaching studies by Kamukondiwa and

Bergstrom (1994a) at Grasslands Research Station; Hagmann (1994) and Vogel et al.

(1994) at Domboshava Training Centre in Zimbabwe were both carried out on dry

land sandy loams and not on wetland soil.

5

Consequently, there was a need to conduct studies to determine the extent of gaseous

and leaching losses of nitrate N under wetland cropping systems, and to establish

fertilizer management practices that can be used to reduce these N losses. This study

is of special significance in view of the potential damage that nitrate leaching could

cause to the ecology of public water bodies through eutrophication and its detrimental

effects to human health (Stewart et al., 1982). Nitrous oxide, one of the products of

denitrification, depletes the atmospheric ozone and is a potent greenhouse gas (GHG)

(Van der Meer, 2008; Mapanda et al., 2011). The influence of fertilizer and manure

N applications to wetland vegetable crops on denitrification rate could also be of

environmental significance. The concentration of N2O in the atmosphere has

increased considerably over the last century and is set to rise further at a rate of 0.4%

per year principally from microbial denitrification and nitrification in soils (Mosier

and Kroetze, 1999; Hoogendoom et al., 2008). Besides posing an environmental

pollution problem, N fertilizer constitutes a major input cost in smallholder crop

production in Zimbabwe (Kamukondiwa et al., 1994a; Bin-Le Line et al., 2001;

Nyamangara et al., 2003). This study was carried out to establish the effect of

wetland cropping amended with ammonium nitrate and aerobically composted cattle

manure on nitrous oxide emissions and nitrate leaching.

1.3 OBJECTIVES OF THE STUDY

1.3.1 Main objective

The overall objective of the study was to investigate the effects of cattle manure and

mineral N fertilizer applications on nitrous oxide emission and nitrate leaching in a

wetland cropping system in Zimbabwe.

1.3.2 Hypotheses

i. The application rate of smallholder cattle manure and mineral N fertilizer to

tomato (Lycopersicon esculentum, Mill) and rape (Brasica napus, L) crops

has no effect on the emission of N2O and nitrate leaching in wetland soil.

ii. The quality of cattle manure applied to tomato and rape crops has no effect on

the emission of N2O and nitrate leaching in wetland soil.

iii. The seasonal split application of cattle manure to tomato and rape crops have

no effect on the emission of N2O from wetland soil.

6

1.3.3 Specific objectives

The specific objectives of the study were:

i. To determine the effect of smallholder cattle manure and mineral N fertilizer

application rate to tomato and rape crops on N2O emission and nitrate

leaching in wetland soil.

ii. To determine the effect of cattle manure quality applied to tomato and rape

crops on N2O emission and nitrate leaching in wetland soil.

iii. To determine the effect of seasonal split application of cattle manure to

tomato and rape crops on N2O emission from wetland soil.

1.4 THESIS STRUCTURE

The thesis is organized into 7 chapters. The first chapter consists of an introduction,

which describes the wetland as a land unit and the use of wetland in the smallholder

farming sector in Zimbabwe. The rationale, hypotheses and objectives of the study

and an outline of the thesis structure are also given in this chapter. The second

chapter consists of the literature review of the major N transformations in soils and

literature for the results based chapters. This chapter also covers the general

considerations in gas sampling on plots, zero-tension lysimeter installations and

general wetland hydrology. The third chapter covers site description and general

methodology. The fourth chapter is based on experiments designed to measure

gaseous emission of N2O and nitrate leaching in wetland soil after application of two

cattle manure quality types. The fifth chapter covers the field experiments designed

to measure emission of N2O and nitrate leaching in wetland soils treated with mineral

N fertilizer and cattle manure. Chapter six is based on the field experiment set up to

determine the effect of split application of cattle manure on N2O fluxes in wetland.

The seventh chapter covers a synthesis of chapters four to six where the overall

conclusions, recommendations, practical implications of the entire study are drawn.

Further studies on the topic are also suggested here.

7

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 WETLAND VEGETABLE PRODUCTION PRACTICES IN ZIMBABWE

A survey carried out by Jackson (1988) in Mashonaland West and Mashonaland East

provinces revealed that leafy vegetables (rape and cabbages), tomatoes, onion, green

beans, sweet potatoes and pumpkins are the most important vegetable crops grown by

smallholder farmers in small-scale informal irrigation schemes. The smallholder

farming sector in Zimbabwe had in excess of 20 000 ha under vegetables on small-

scale irrigation schemes and wetland gardens (Department of Irrigation, 2008). The

smallholder farmers usually rely on use of cattle manures and mineral fertilizers

applied in combination or separately to increase productivity in the vegetable

cropping systems. Recommended manure application rates range from 25 – 50 Mg

ha1. Compound S (5%N, 7.9% P, 16.6% K, and 8% S) is used as basal fertilizer at a

rate of 1000 kg ha-1 when available on the market. A 100% irrigated crop rotation

system that includes tomato (September to February) and rape (March to August) on

raised plots is most common farming practice (Owen et al., 1995; De Lannoy, 2001).

Raised plots allow free movement of water and air and excess water is carried away in

furrows that separate the plots (Andreini et al., 1995).

Rape or canola has been an important arable crop for a couple of decades, grown

primarily for its oil in northern Europe and North America and its addible leaves in

Africa and Asia (Zhang et al., 2010; Keogh et al., 2012). In Zimbabwe and other

southern African countries, rape is commonly grown under irrigation. Smallholder

farmers often apply about 30 Mg ha-1 of cattle manure for the production of rape

under small-scale informal irrigation to achieve yields levels of 25-30Mg of leaf mass

ha-1 (De Lannoy, 2001).

High rates of N fertilizers (200 – 400 kg N ha-1) are usually applied to rape in order to

obtain maximum yield (De Lannoy, 2001; Lin et al., 2011). Rape is a highly N

demanding crop, with economically optimal fertilization levels often exceeding 200

kg N ha-1 for the more common winter varieties sown in autumn and harvested in

8

early or mid summer (De Lannoy, 2001). The main effects of increasing N status in

rape are enhanced leaf expansion and leaf chlorophyll content (Henke et al., 2009;

Keogh et al., 2012). Usually, only about 50% of applied fertilizer N is recovered in

the harvested biomass. The relatively small recovery rate of applied N by the rape

crop increase risks for significant losses of N to the environment (Krouk et al., 2010,

Liu et al., 2010; Mishima et al., 2011).

Tomato is one of the world’s most important vegetable crops, with a worldwide fresh

weight production of 80 million Mg and a total cropped area of about 3 million ha

(FAO, 1995). The crop is grown typically on shallow water table sites (wetlands or

flatwoods) using high rates of N fertilizers (Agele et al., 2011). Growth characteristics

of tomato are governed by genetic traits and management practices. For field

production, the use of transplants is common. Transplants are typically 5 week old,

having 3 to 5 leaf-bearing nodes, an initial leaf area of 15 to 40 cm2, and a dry weight

of 0.2 to 0.30 g plant-1 (Owen et al., 1995).

Commercial tomato growers apply N fertilizers in amounts of 300 - 400 kg N ha-1. In

many cases, N is the element that limits crop growth, especially on coarse-textured,

low organic matter soils. Under wetland cropping conditions, N is readily lost by

leaching and denitrification, with N concentrations in shallow ground water reaching

hazardous levels (Mishima et al., 2011). For sub-irrigated mulched tomato crop, all N

fertilizer is normally applied at planting. For drip-irrigated tomato crop on coarse-

textured soil, fertilizer N is typically applied both before planting (20 - 40% of total)

and during vegetative period as split applications (60 - 80%) (Zotarelli et al., 2009).

2.2 AVAILABILITY AND QUALITY OF MANURE IN ZIMBABWE

Fertilizer use in many subsistence agricultural systems remains insufficient to meet

the N demand of crops. Cattle manure is the major source of nutrients for plant

growth in the smallholder-farming sector. However, the low efficiency of smallholder

cattle manures as a source of N has prompted farmers to supplement the manures with

inorganic N fertilizer (Nyamangara et al., 1999; Mafongoya and Hove, 2008; Wuta

and Nyamugafata, 2012).

9

According to Commercial Farmers Union of Zimbabwe (2012), the number of cattle

in Zimbabwe was estimated at 5 156 753. About 58% of the cattle were in communal

areas, giving a potential annual manure production of 8.2 million tonnes (Nyamangara

et al., 1999). The amount of cattle manure available per household has declined since

1990 due to persistent droughts, which have reduced the number of cattle.

Smallholder wetland farmers without cattle resort to use of mineral fertilizers (Owen

et al., 1995). The availability and access to cattle manure in the smallholder areas of

Zimbabwe is largely related to cattle ownership (Mugwira and Murwira, 1997;

Materechera, 2010). A survey conducted by Shumba (1985) in Mangwende

Communal Area in 1982 concluded that cattle ownership within households was 20-

75%. The study also revealed that 52% of cattle owners applied manure in the first

year of a maize-based rotation system compared to only 12% of non-cattle owners. Of

the 57% of smallholder farmers that owned cattle, 47% had less than 8 head of cattle.

Given the low cattle ownership and assuming that the average manure production per

livestock unit is equivalent to 1.5 Mg year-1 (Zingore et al., 2008; Wuta and

Nyamugafata, 2012), it is apparent that many smallholder farmers cannot afford to

apply cattle manure at the recommended rate of 37 Mg ha-1 to maize in a four year

crop rotation (Materechera, 2010).

A high proportion of manure in smallholder areas is not directly usable as the dung is

deposited in the grazing areas (Mafongoya and Hove, 2008). Fortunately, some

nutrients are recycled through grazing, and a small proportion through direct

collection of dung from grazing areas. A survey carried out in 1994 by Campbell et al.

(1998) in Mutoko, Uzumba and Mangwende Communal Areas revealed that about 34

kg of manure per household per year (or 0.3 kg N household-1 yr-1) was directly

collected from grazing area by farmers and applied to the field. During the dry season

some of the dung and urine are directly deposited in the fields as the cattle feed on

crop remains. The quantity of usable manure deposited in cattle pens largely depends

on design and location of the pen, stabling period, amount of organic residues added

as bedding or to absorb nutrients, and efficiency of collection of the manure

(Nyamangara and Nyagumbo, 2010; Wuta and Nyamugafata, 2012).

Cattle manure from smallholder areas is generally of poor quality due to inadequate

and low quality grazing (Zingore et al., 2008), and inappropriate handling of the

10

manure, which results in both excessive NH3 volatilisation and mixing with soil in the

kraals (Khombe et al., 1992; Murwira, 1995). Mugwira (1984) reported that soil

constituted 57.4% of cattle manures collected from Chihota and Svosve smallholder

farming areas. However, Mugwira and Murwira, (1997) recorded as much as 87% soil

content of manures from smallholder areas due to soil either ingested by the grazing

animals or mixed in as a result of trampling of manure in the kraal.

Tanner and Mugwira (1984) reported depressed plant growth during the first weeks

due to N immobilisation in low quality manure (less than 1% N) applied at an

equivalent of 80 Mg ha-1 in a greenhouse experiment. Communal area cattle manures

in Zimbabwe are aerobically composted. The prolonged aerobic composting of

manure can increase C stabilization resulting in a low rate of N mineralization

(Mapfumo et al., 2007; Wuta and Nyamugafata, 2012). N concentration is generally

higher in manures from commercial farming areas compared to that from communal

areas, although the general composition from both sectors is variable (Nyamangara

and Nyagumbo, 2010). According to Mugwira and Mukurumbira (1984), cattle

manures from communal areas and commercial feedlots can be classified in terms of

N concentration as low nutrient (0.62-0.93% N), medium nutrient (1.00-1.22% N),

and high nutrient manures (1.38-2.25%).

2.3 BIOGEOCHEMISTRY OF WETLANDS SOILS

Wetland biogeochemistry is controlled by flooding and the resulting status and pattern

of oxidation and reduction reactions. The terrestrial ecosystems within a watershed

affect flooding pattern, groundwater level, water quality, sedimentation and erosion

for wetlands. Irrigation, water harvesting, drainage, and cultivation practices influence

wetland soil biogeochemistry. Flooding a soil drastically changes its hydrosphere,

atmosphere, biosphere and biogeochemistry (Brinkman and Blokhuis, 19986).

In a review on the chemistry of adversely flooded soils, Neue and Zhong (1990) and

Neue et al. (1997) reported that flooding decreases atmospheric oxygen diffusion to

the soil by a factor of 105 and sets in motion a series of unique physical, chemical and

biological processes not found in dry land soils. In a study on the effect of N fertilizer

management and water-logging on nitrous oxide emission from subtropical cropped

land Allen et al. (2010) concluded that the nature, pattern and extent of the reduction

11

processes depend on the physical and chemical properties of the soil, flooding

duration, floodwater quality, soil and floodwater biosphere, management practices,

plant growth, and climatic conditions. The chemistry and wetland soil fertility issues

were extensively described by Neue, (1991). After O2 is depleted by aerobic

respiration, facultative and obligate anaerobic organisms progressively use oxidized

soil substrates as electron acceptors for their respiration (Czobel et al., 2010).

Denitrifying bacteria use nitrate an alternative electron acceptor for the oxidation of

organic matter below a redox potential of +420 mV pH 7 (Allen et al., 2010).

The magnitude and intensity of denitrification is controlled by the amount of easily

degradable organic matter, their rate of decomposition, amount and kinds of reducible

nitrates, and organic substrates. For a range of wetland soils, Gaunt (1993) showed

that the C/N ratio of decomposable organic matter influenced the soil reduction rate

upon wetting whereas the active iron content defined the reduction capacity of the

soils (Venterea and Rolston, 2000; Allen et al., 2010, Wang et al., 2012; Mapanda et

al., 2012b; Nyamadzawo et al., 2012).

2.4 BIOCHEMICAL TRANSFORMATION PATHWAYS OF N IN SOILS

2.4.1 Mineralization and immobilization

Soil N is continuously changing from one form to another because of the activities of

plants and microorganisms. The decomposition of organic matter converts some soil

organic N into mineral N in a process termed mineralization in which ammonium

(NH4+), nitrite (NO2

-) and nitrate (NO3-) are generated (van der Meer, 2008, Mapanda

et al., 2011; Mapanda et al., 2012a). In the process of N mineralization, heterogeneous

soil microorganisms simplify and hydrolyse the organic N compounds, ultimately

producing the NH4+ and NO3

+ ions (Equation 1).

The N content and C to N ratio of applied organic residues is important for N

mineralization dynamics after incorporation (van der Meer, 2008; Nyamangara and

Makumire, 2010). The C to N ratio is widely used in predicting N

mineralization/immobilization from organic materials added to a soil (Markewich et

al., 2010; Mutsamba et al., 2012). Generally, when the C to N ratio is less than 25, net

mineralization is expected, and when the ratio is greater than 25, net N

12

immobilization is expected (Van Der Ploeg et al., 2007; Abro et al., 2011; Wuta and

Nyamugafata, 2012).

Mineralization

+H2O, H+ +O2 +O

RNH2 ROH + NH4+ NO2

- + 4H+ NO3- (1)

-H2O -O2 -O

Immobilization

(Brunn et al, 2006, van der Meer, 2008, Mutsamba et al., 2012)

Immobilization is the conversion of inorganic N ions (NH4+, NO3

-) into organic

forms. As carbonaceous organic residues are decomposed in the soil, the growth of

microbial colonies may require more N for cell substance synthesis than is contained

in the residues themselves. The microorganisms then have to incorporate mineral N

ions to synthesize cellular components, such as proteins. When the microorganisms

die, some of the organic N in their cells may be converted into NH4+ and NO3

- ions

(Markewich et al., 2010). In this respect, mineralization and immobilization occur

simultaneously, whether the net effect is an increase or a decrease in the mineral N

available depends primarily on the ratio of carbon to N (C: N ratio) in the organic

residues undergoing decomposition (van der Meer, 2008, Mapanda et al., 2011,

Mutsamba et al., 2012).

The application of mineral N fertilizer as a supplementary source of nutrient N to

manures is common practice in smallholder vegetable production (Materechera,

2010). Normally, organic N mineralization rate does not match demand for available

N by the vegetable crop (Mapfumo et al., 2007). Mineral N fertilizer is applied in

combination with animal manure in order to increase the availability of soil N to

plants. The application of mineral N fertilizer as a supplement has the effect of

narrowing the C: N ratio to within ranges of net mineralization of organic N leading

to increased pool of solubilised N exposed to leaching and loss as gaseous N

emissions (Ajdary et al., 2007).

13

2.4.2 Nitrification

Nitrification is a microbial process by which reduced N compounds (primarily

ammonia) are sequentially oxidized to nitrite and nitrate (Equation 2). The

nitrification process is primarily accomplished by two groups of autotrophic nitrifying

bacteria that can build organic molecules using energy obtained from inorganic

sources; in this case ammonia or nitrite (Gonzalez-Chavez et al., 2009). In the first

step of nitrification, ammonia-oxidizing bacteria oxidize ammonia to nitrite according

to equation (2).

NH3 + O2 → NO2- + 3H+ + 2e- (2)

Nitrosomonas is the most frequently identified genus associated with this step,

although other genera, including Nitrosococcus, and Nitrosospira. Some subgenera,

Nitrosolobus and Nitrosovibrio, can also autotrophically oxidize ammonia

(Klemedtsson et al., 1999, Woznica, 2013). In the second step of the process, nitrite-

oxidizing bacteria oxidize nitrite to nitrate according to equation (3).

NO2- + H2O → NO3- + 2H+ +2e- (3)

Nitrobacter is the most frequently identified genus associated with this second step,

although other genera, including Nitrospina, Nitrococcus, and Nitrospira can also

autotrophically oxidize nitrite. Various groups of heterotrophic bacteria and fungi can

also carry out nitrification, although at a slower rate than autotrophic organisms

(Gonzalez-Chavez et al., 2009).

Besides being a N2O emitting process, nitrification is essentially a prior step to the

accelerated N2O emission during denitrification of nitrate N in anaerobic soil

conditions in wetlands. In addition, the transformation of organic N in applied cattle

into mineralized NO3-N form exposes nutrient N to leaching losses especially under

conditions in wetlands (Berdad-Haughn et al., 2006; Van Der Ploeg et al., 2007).

14

2.4.3 Loss of N by ammonia volatilization

Ammonia gas (NH3) can be produced under alkaline conditions in the soil-plant

system. The source of ammonia gas may be animal manure and urine (hence the

familiar ammonia smell around cattle kraals), fertilizers, decomposing plant residues

or even vegetative mass of living plants (Fan et al., 2011; Nasima et al., 2012).

Gaseous ammonia loss from N fertilizer and animal manure applied to the surface of

wetland soil under cropping can be substantial even under slightly acidic soil

conditions (Woznica, 2013). The applied fertilizer material stimulates algal growth on

wetland soil surface. As the algae photosynthesize, they extract CO2 from water and

reduce the amount of carbonic acid formed thereby increasing soil alkalinity

especially during daylight (Nasima et al., 2012). This has the effect of accentuating

volatilization of ammonia from the fertilized wetland soil (van der Meer, 2008).

Ammonia volatilization is also dependent on climatic conditions such as considerable

air movement above the soil surface, method and rates of fertilizer application,

chemical and physical properties of the soil (Hotta and Funamizu, 2007).

2.4.4 Loss of N by denitrification

N may be lost from soil when nitrate ions are converted to gaseous forms by a series

of biochemical reduction reactions termed denitrification (Gonzále-Chavez et al.,

2009; Abro et al., 2011). In denitrification, NO3-N replaces O2 as the electron

acceptor in the soil microbial respiration, and in the process, NO3- is reduced to N2O,

NO2 and N2 gases that are lost to the atmosphere (Venterea and Rolston, 2000; Allen

et al., 2010, Wang et al., 2012). Denitrification may take place on wetland soil when

the diffusion rate of O2 in the soil system does not meet the microbial respiration

demands (Chirinda et al., 2010; Lesschen et al., 2011; Wang et al., 2012).

Flooded soils have aerobic and anaerobic zones, allowing both nitrification and

denitrification to take place simultaneously. Since the first process produces the

substrate for the second, N losses can be very high when the two processes are

associated. As much as 60 to 70% of applied N may be lost as oxides of N or

elemental N (Markewich et al., 2010; Kamaa et al., 2011). The exact magnitude of the

15

denitrification losses is difficult to predict and largely depends on management

practices and soil conditions (Gonzále-Chavez et al., 2009; Mapanda et al., 2012a).

The bacteria that carry out denitrification are facultative anaerobes, which are

predominantly heterotrophic. At least 23 genera of denitrifying bacteria have been

identified such as Pseudomonas, Bacillus, Micrococcus, and Achromobacter

(Gonzále-Chavez et al., 2009). Significant denitrification can take place when NO3-N

is available, readily decomposable organic compounds are present and the soil air

contains less than 10% O2 or less than 0.2 mg L-1 of O2 dissolved in the solution.

Prevalence of optimum temperatures ranging from 25 to 35ºC increases loss of N by

denitrification (Ha et al., 2008; Geeta et al., 2011).

Low-lying, organic-rich wetland areas may lose N 10 times as fast as the average rate

for a typical field. Although as much as 10 kg ha-1 of N may be lost in a single day

from the sudden wetting of well-drained, humid region soil, such soils rarely lose

more than 5 to 15 kg N ha-1 annually by denitrification (Snyder et al., 2009 ). But

where drainage is restricted and where large amounts of N fertilizer are applied,

substantial losses may occur (Dale, 1993). Losses of 30 to 60 kg N ha-1 year-1 N have

been recorded in agricultural wetland systems (Lohila et al., 2010).

Nitrous oxide is a greenhouse and ozone-depleting gas (Mosier and Kroetze, 1999;

IPCC, 2001; Vasileiadou et al., 2011; Mapanda et al., 2012a) whose atmospheric

concentration is currently > 310 nLL-1 and increasing at a rate of approximately 0.4%

per annum (Mosier and Kroetze, 1999). Nitrous oxide is estimated to account for

some 6% of the greenhouse warming (Ma et al., 2007). Nitrous oxide has a global

warming potential of 270 – 320 times compared to carbon dioxide (Snyder et al.,

2009; Smith, 2012). Nitrous oxide gas can last 150 years in the atmosphere (Munoz et

al., 2010; Saggar et al., 2010). The major sink for N2O is the stratospheric reaction

with atomic oxygen to NO, which induces the destruction of stratospheric ozone

(Levy et al., 2011).

2.4.5 Loss of N by leaching

The concentration of nitrate in ground water, rivers, and lakes has been increasing

steadily over the past thirty years in large parts of the world (Addiscot and Benjamin,

16

2000, Venterea and Rolston, 2000). The loss of N through leaching is of serious and

ubiquitous concern for three basic reasons. Such loss represents an impoverishment of

the ecosystem whether or not cultivated crops are grown. Leaching of nitrate causes

several serious environmental problems. In addition to these, nitrate-N contamination

is a potential health hazard to humans and animals (Min et al., 2010).

Once it arrives in surface waters, nitrate, along with other forms of N and phosphorus

from agricultural sources, contribute to the problem of eutrophication. Eutrophication

may stimulate the growth of large masses of algae, certain species of which produce

flavors and toxins that make the water unfit to drink. The depletion of dissolved

oxygen that ensues when algae die and decompose severely degrades the aquatic

ecosystem, resulting in the death of fish and other aquatic organisms (Fan et al., 2011;

Rasiah et al., 2010; Mapanda et al., 2012a).

High input nitrate has the potential to cause toxic effects on human and animal life

(Addiscott et al., 1991). Methemoglobinemia, also called blue body syndrome, occurs

when certain bacteria, found in the guts of ruminant animals and human infants,

convert ingested nitrate into nitrite (Addiscott and Benjamin, 2000, Liu et al., 2010).

Wetland smallholder farmers often sink shallow wells for the supply of drinking water

at household level (Grant, 1995). The nitrite then interferes with the ability of the

blood to carry oxygen to the body cells. Regulatory agencies in most countries limit

the amount of nitrate permissible in drinking water to less than half the concentration

known to cause toxicity. In the USA, the limit on nitrate is 45 mg L-1 nitrate (or 10 mg

L-1 N in nitrate form). In Europe, the standard is 50 mg L-1 nitrate (Addiscott and

Benjamin, 2000).

Key drivers of nitrate leaching from cropping systems were described extensive in

related studies by Nicholson et al. (1997), Ren and Wang (2010) and Mishima et al.

(2011). When soil becomes excessively wet, the soil will reach a point where it cannot

hold any more water. This happens because the air spaces between soil particles

become filled with water. As these air spaces fill, gravity will cause water to move

down through the soil profile (Ren and Wang (2010). An important factor that can

affect the degree of leaching is how much water a soil can hold. For example, by their

17

nature sandy soils cannot hold as much water as clay soils. This means that leaching

of nitrates will take place much more easily in a sandy soil compared to a clay soil

(Fan et al., 2011; Rasiah et al., 2010). Other factors that can affect nitrate leaching

include amount of rainfall, amount of water use by plants and how much nitrate is

present in the soil system. The magnitude of N loss is proportional to the

concentration of nitrates in soil solution and the volume of leaching water (Mishima

et al. (2011).

Irrigated vegetable production systems represent one of the most intensively fertilized

and cultivated production systems. The conditions of high N inputs (in forms of

fertilizer and manure), frequent cultivation, relatively short periods of plant growth

and low nutrient use efficiency by many vegetable crops make the vegetable

production system highly vulnerable to nitrate leaching (Rasiah et al., 2010). There

are studies on the effect of manure and mineral N fertilizer applications to crops on

nitrate leaching under dry land conditions (Nyamangara et al., 2003). In a study on

fertilizer use efficiency and nitrate leaching in a tropical sandy soil in Zimbabwe,

Nyamangara et al. (2003) reported 47% of applied mineral N fertilizer lost in leachate

under dryland maize. However, there has been limited research on the effect of

manure and mineral N fertilizer applications on nitrate leaching under wetland

vegetable cropping systems in sub-tropical Africa.

2.5 EFFECT OF MINERAL N FERTILIZER AND CATTLE MANURE

APPLICATIONS ON N2O EMISSION AND NITRATE LEACHING

2.5.1 Effect manure and mineral N fertilizer application on N2O emission

The traditional understanding of the N cycle is that N from compounds in applied

cattle manures and inorganic N fertilizer are lost to the atmosphere either as NOx

during denitrification or ammonia volatilization (Van Der Salm et al., 2006; Allen et

al., 2010; Mapanda et al., 2012a). However, over the past few years, it has become

accepted that this is an over-simplification and that other N gases (N2O, N2) may

result from denitrification.

Several workers have reported that NOx is produced following the breakdown of N

compounds in fertilizer (Wrage et al., 2003; Wang et al., 2012) and manures (Van Der

18

Ploeg et al., 2007; Smith, 2012) that have been spread on the soil. Agricultural soils

are a primary source of anthropogenic trace gas emissions (Munoz et al., 2010). The

concentration of N2O in the atmosphere is increasing, as a result of biotic and

anthropogenic activities, at a rate of about 4 Tg N2O-N year-1 above the rate of

decomposition in the atmosphere (IPCC, 1996). Kroetze et al. (2003) estimated that

total global N2O emissions in 1994 were approximately 17.6 Tg N, with about 55%

(9.6 Tg) of the total arising from relatively natural terrestrial and aquatic systems and

about 8 Tg derived from anthropogenic sources. Of human initiated N2O emissions,

about 70% is thought to result from emissions from agriculture, both crop and

livestock production (Lesschen et al., 2011). With the need to quantify all possible

sources of N2O, there is a renewed interest in N balance studies (Smith et al., 1997).

Data drawn from temperate agricultural crop production systems demonstrates that

N2O is emitted in response to N fertilization (; Khalil and Renault, 2003; Mosier et al.,

2003; Ha et al., 2008). A related study conducted at Domboshava Training Centre

under dryland conditions in Zimbabwe by Chikowo et al. (2004) concluded that it is

neither moisture nor tillage per se that leads to increased N2O fluxes from cropping

systems, but rather accelerated soil N cycling. However, Dobbie et al. (1999)

measured emissions of nitrous oxide from extensively managed agricultural fields

over three years and found exponential increases in nitrous and nitric oxides flux with

increasing soil water-filled pore space, temperature and soil mineral N. In a study of

effects of temperature, water content and N fertilization on emissions of nitrous oxide

by soils, Smith et al. (1997) confirmed exponential relationships between N2O flux

and both water-filled pore space and temperature are only observed when soil mineral

N is not limiting.

The flux of N2O from cultivated land ranges from 0.003 to 2.95 mg m-2 h-1 (Takaya et

al., 2003; Levy et al., 2011). Factors such as soil water content, precipitation, soil

temperature (Ma et al., 2007; Allen et al., 2010), soil organic C (Soren et al., 2006;

Chirinda et al., 2010; Lesschen et al., 2011) and soil pH (Lohila et al., 2010) have

been proposed as regulators of N2O emissions. Soil surface N2O fluxes are also

influenced by form and quantity of added N (Schils et al., 2008; Jassal et al., 2011).

Mineral fertilizer-derived N2O emissions under field conditions range from 0.001 to

6.84% of N applied. Organic amendments supply additional quantities of C and N and

19

increase N2O fluxes from the soil especially under wetland conditions (Frimpong and

Baggs, 2010).

Coupled nitrification-denitrification is an important source of N2O emission from

wetland soil amended with manure and mineral N fertilizer. The term is used to stress

that denitrifiers can utilize NO2- or NO3

- produced during nitrification. This coupling

between nitrification and denitrification can take place in soils where favourable

conditions for both nitrification and denitrification are present in neighbouring

microhabitats (Allen et al., 2010). N2O is mainly produced at the aerobic-anaerobic

interface from where it could diffuse to the soil surface where it is lost to the

atmosphere (Venterea and Rolston, 2000; Ma et al., 2007). This suggests that the

production and emission of N2O is highest at conditions that are sub-optimal for both

nitrifiers and denitrifiers. A wetland soil with fluctuating water table best represents

such conditions. Here, nitrification can take place in aerobic surface layers or cracks.

Denitrification is mostly confined to anaerobic deeper layers, waterlogged areas or the

interior of soil aggregates. The interface between these areas in wetland soil amended

with manure and mineral N fertilizer are the soil profile micro-sites where the

production of N2O is highest (Mosier et al., 2003; Ma et al., 2007; Lohila et al., 2010).

NO2- NO N2O N2

Denitrification

NO3-

Nitrifier Denitrification pathway

Nitrification

N2O

NH3 NH2OH NO2- NO N2O N2

Coupled

nitrification-

denitrification

Fig 2.1: Transformation of mineral N in the soil (redrawn from Wrage et al., 2001)

20

Nitrifier denitrification is another pathway that can contribute to the production and

emission of N2O. In nitrifier denitrification, the oxidation of NH3 to NO2- is followed

by the reduction of NO2- to N2O and N2 (Allen et al., 2010). This sequence of

reactions is carried out by only one group of microorganisms, namely autotrophic

NH3-oxidizers. Thus, nitrifier denitrification contrasts with coupled nitrification-

denitrification in that different groups of coexisting microorganisms can together

transform NH3 to finally N2. The first part of nitrifier denitrification (oxidation of NH3

to NO2-) has been attributed to nitrification (NH3 oxidation), whereas the reduction of

NO2- is regarded as denitrification (Mosier et al., 2003; Czobel et al., 2010; Lohila et

al., 2010).

A good deal of research has been done to estimate emissions of N oxides (NOx) from

soils. Although numerous measurements have been made, emissions from soils show

variability based on a number of factors. Differences in soil type, moisture,

temperature, season, crop type, fertilization, and other agricultural practices

apparently all play a part in emissions from soils. The quantity of NOx emitted from

agricultural land is dependent on fertilizer application and the subsequent microbial

denitrification of the soil. Nitrous oxide emissions from fertilizer use can be estimated

using the following equation (IPCC, 2001):

N2O Emissions = FC * EC * 44/28 (1)

Where, FC is fertilizer consumption (Mg N-applied); EC is emission coefficient =

0.0117 Mg N2O-N/Mg N applied; and 44/28 is the molecular weight ratio of N2O as

N (N2O/N2O-N). The emission coefficient of 0.0117 Mg N/Mg N-applied represents

the percent of N applied as fertilizer that is released into the atmosphere as nitrous

oxide.

Research during the past several decades has improved our understanding of how N2O

is produced, the factors that control its production, source/sink relationships, and gas

movement processes. However, despite extensive knowledge of the processes

involved, researchers are only beginning to be able to predict the fate of a unit of N

that is applied or deposited on a specific agricultural field (Mosier et al., 2003).

21

Existing data on emissions of NOx is extracted from research generated in western

Europe, north America and south-east Asia (Kroeze et al., 2003) despite the fact that

the tropics and subtropics contribute greatly to the emissions (Grenon et al., 2004;

Billy et al., 2010), particularly since 51% of world soils are in these climate zones

(Mosier et al., 2003). The incorporation of data on N2O emissions from African

tropical and sub-tropical regions in the near future will lead to realistic and more

appropriate emission factors being used by the IPCC (Kroeze et al., 2003; Van Der

Salm et al., 2006). Apart from environmental problems posed by N2O emissions to the

atmosphere, such losses of N lead to higher expenses on fertilizers (Kroeze et al.,

2003; Van Der Salm et al., 2006). An understanding of the contribution of manure

and mineral N fertilizer applications to global atmospheric N2O loading and NO3-N

leaching is needed to evaluate agriculture’s contribution to the global warming

process and ground water pollution (Mapanda et al., 2012a). The objective of this

study was to quantify the effects of combined application rates of aerobically

decomposed cattle manure and mineral N fertilizer on N2O fluxes from a wetland

field during the growing seasons of rape and tomato crops under sub-tropical

conditions in Zimbabwe.

2.5.2 Effect of manure and mineral N fertilizer application on nitrate leaching

Sustainable fertilizer application should provide sufficient nutrients for growth of

crops while simultaneously avoiding the risk of environmental pollution due to

nutrient surpluses (Gonzalez et al., 2011; Surya and Rothsten, 2011). Soluble

nutrients leached beyond the root zone are a potential threat to the quality of

groundwater. Increased use of N fertilizer and animal manures has accentuated

nitrate-N contamination, because nitrate leaching in the ground water is related to N

fertilization rate (Stadler et al., 2008; Claret et al., 2011). This occurs because of

excessive nitrate-N accumulation in the soil profile (Wang et al., 2010) due to N

fertilization rates that exceed crop requirements, accompanied by poor soil and crop

management practices (Fan et al., 2011).

In sub-tropical regions of Africa, manures play an important role in soil fertility

management through their short-term effects on nutrient supply and long-term

contribution to the soil organic matter (Lekasi et al. 2002; Ondersteijn et al. 2002;

22

Groot et al. 2006; Mutsamba et al. 2012). Although organic resources used alone offer

insufficient nutrients to sustain crop yields and soil fertility build-up (Groot et al.,

2006; Mapfumo et al., 2007; Materechera, 2010;), they continue to be a critical

nutrient resource as most smallholder farmers in the sub-tropics are unable to access

adequate quantities of mineral fertilizers (Zingore et al., 2008; Nyamangara and

Nyagumbo, 2010).

The low efficiency of manures from most smallholder areas as sources of N has

prompted farmers to supplement manures with inorganic N fertilizer (Nyamangara et

al. 2003; Wuta and Nyamugafata, 2012). The application of mineral N fertilizer

upgrades the mobilization of organic N in applied manure into available forms of

mineralized N which have a greater susceptibility to leaching that threaten

environmental quality (Krouk et al., 2010; Liu et al., 2010).

Wetland agriculture receiving excessive mineral N fertilizer and animal manures are

highly susceptible to NO3-N leaching (Owen et al., 1995). Before manure N is

available to plants, nitrogenous organic compounds must undergo decomposition and

N mineralization. The microbial degradation of nitrogenous organic substance (in

manure crude protein) in soil may yield net mineralized N when N is turned into

available/soluble forms (NH4-N and NO3-N) or immobilized N (assimilated into

microbial cell substance, and therefore temporarily sequestrated from plant uptake

and nitrate leaching losses) (Van Der Ploeg et al., 2007; Mafongoya and Hove, 2008;

Wuta and Nyamugafata, 2012). However, whether organic N in applied manure is

immobilized or mineralized depends on the concentration of available N in soil

against the content of C in applied manure (Silva et al. 2005).

Barraclough and Jarvis (1989) introduced the concept of a break point for nitrate

leaching, a level of fertilizer N application above which, nitrate leaching increased

markedly. Based largely on direct measurements of leaching and denitrification, they

suggest break points of 150 - 200 kg N ha-1 for grazed swards and between 250 and

350 kg N ha-1 for cut grass. Nevertheless, Kirkham and Wilkins (1993) reported that

the risk of substantial nitrate leaching begins between 50 and 100 kg N ha-1 yr-1,

increasing markedly at rates above 100 kg N ha-1. In a related study on a modeling

approach to global nitrate leaching caused by anthropogenic fertilization, Bin-Le Lin

23

et al. (2001) reported 19% of applied N fertilizer lost as NO3-N leaching from soil at

global level.

Recent decades have seen an increase in the attention paid to pollution by fertilizer

applications in intensive agricultural practices mostly in Europe, North America and

south-east Asia (Venterea and Rolston, 2000). Giller et al. (1997) reported that the

average intensity of fertilizer use in Africa was only 8 kilograms per hectare of

cultivated land, much lower than in other developing regions. Even when countries

and crops in similar agro-ecological zones are compared, the rate of fertilizer use is

much lower in Africa than in other developing regions, and crop yields are

correspondingly lower. However, De Lannoy (2001) and Zotarelli et al. (2009) in

related studies on wetland vegetable production in sub-tropical regions reported

higher N fertilizer and manure applications by smallholder farmers in order to avoid

yield reduction of the high value crops. Under wetland conditions, excess mineralized

N may be exposed to nitrate leaching losses to the terrestrial aquatic environment.

Both conservationists and agriculturalists have recognized the potential vulnerability

of wetland environments to the effects of increasing wetland soil fertility for increased

food crop production. There is little scientific information about the effects of

supplementing manure applications with mineral N fertilizer on nitrate leaching in

wetland soil under smallholder vegetable crops in the African sub-tropical regions.

A better understanding of the regulation of nitrate leaching following manure and

mineral N fertilizer applications may result in improved crop management practices

that increase crop uptake of mineral N and reduce its loss through nitrate leaching.

The main objective of the current study was to determine the effects of cattle manure

application in combination with mineral N fertilizer on NO3-N leaching.

2.5.3 Cattle manure quality, N2O emission and nitrate leaching

In Zimbabwe, cattle manures are largely prepared by subjecting cattle excreta together

with maize stover to prolonged aerobic composting. This increases C stabilization

resulting in a low rate of N mineralization (Zingore et al., 2008; Materechera, 2010;

Wuta and Nyamugafata, 2012). The quality of cattle manure reflects the average

concentrations of nutrients in the livestock diet. Livestock diet from communal

grazing areas in Zimbabwe are, in most cases, of poor forage value (Nyamangara and

24

Nyagumbo, 2010). Cattle manures from smallholder areas in Zimbabwe are generally

regarded to be of lower quality than manures from commercial farming areas (Zingore

et al., 2008). As a generalization, cattle manure has relatively low nutrient content in

comparison with commercial fertilizers (Nyamangara and Nyagumbo, 2010; Wuta

and Nyamugafata, 2012).

Organic matter decomposition dynamics largely determines the rate of release of

mineralized N, which may be subjected to loss by N2O emission and leaching.

Although there is a general trend relating net mineralization/immobilization to the C:

N ratio, there is no critical precise value, which marks the reversal from

immobilization to mineralization. Other aspects such as substrate quality, which

include lignin and polyphenol contents, have a major impact on rate and direction of

decomposition (Vitten and Smith, 1993; Mtambanengwe et al. 1998). Taylor et al.

(1989) and Silva et al. (2005) concluded that the C: N ratio is a better predictor of

decay rate for substrates low in lignin when comparing substrates with a wide range

of lignin contents. Organic materials rich in N-free lignins and low-N residual

substances of soils are poor energy sources for most microorganisms (Yates et al.

2006). Polyphenols affect the release of N from decomposing organic material by

forming stable complexes with proteins thereby stabilizing the organic material

(Mafongoya et al. 1998a; Yates et al. 2006). Thus, application of poor quality cattle

manure (low N, high C) might be a way to reduce the risk of NO-3-N leaching from

soil due to immobilization of N by heterotrophic microorganisms. Jarvis et al. (1989)

and Nicholson et al. (1997) reported that incorporation of carbon-rich organic

materials to a soil significantly decreased NO3-N leaching.

Addition of cattle manures to wetland soils under cropping increases the amount of

readily decomposable organic matter. This enhances the potential for denitrification

and increased emissions of nitrous oxide gas through a general stimulation of

microbial respiration, causing rapid oxygen consumption and consequently an

increase of anaerobic conditions (Jassal et al., 2011). The rates of denitrification are

highly correlated with readily oxidizable C or water-soluble organic C (Mapanda et

al., 2012b). In a study on N leaching and nitrous oxide emissions from croplands at

the University of Zimbabwe Research farm in Domboshava, Mapanda et al. (2012b)

reported increased annual N2O emissions with increasing N fertilizer rates. In a

25

related study of NO and N2O emissions from arable organic and conventional

cropping systems, Chirinda et al. (2010) observed that N2O emissions averaged 6% of

total N emissions.

The forms of oxidized and reduced N range from highly reactive gaseous nitric acid

(HNO3), ammonia (NH3) through nitric oxide (NO), N dioxide (NO2) and nitrous acid

(HONO), all of which have relatively short (< 1 week) atmospheric lifetimes, to very

long lived nitrous oxide (N2O), which has an atmospheric lifetime of 100 years or

more (IPCC, 2001). Nitrous oxide emissions from soils vary (Freney, 1997).

Depending upon fertilizer type and manure quality, 0.07-2.7% may evolve as N2O

(Munoz et al., 2010). On average, 0.3 - 2.25% of applied N to agricultural soils may

be emitted as N2O (Mosier et al., 2003; Saggar, 2010).

Nitrous oxide emissions from rivers, estuaries and continental shelves increase with

increasing N loading from 0.3 - 3% or even 6% of denitification rates; thus

approximately 1% of total N input into these systems may be emitted as N2O

(Johnson et al., 2005). Evidently, the contamination of the subsurface environment

with nitrate has the potential for increasing the contribution to atmospheric N2O

(Nyamadzawo et al., 2012). In fact, direct N2O emissions (2.1 Tg N) may equal

indirect emissions (2.1 Tg N) resulting from agricultural N input into the atmosphere

and aquatic systems (Mosier et al., 1994). Thus, a NO3 water quality problem (Ajdary

et al., 2007) may be traded for an atmospheric problem (Van Der Salm, 2006).

A better understanding of the regulation of episodic denitrification following manure

application may result in improved management practices that increase crop uptake of

manure N and reduce its loss through N2O emissions. Quantification of N losses by

denitrification following cattle manure application is also necessary to assess the fate

of excess NO3-, that is, whether it is denitrified. In addition, a better understanding of

the contribution of manures to global atmospheric N2O loading is needed to evaluate

agriculture’s contribution to global warming. Consequently, a two-season study was

carried out at a wetland site in Zimbabwe in order to determine the effect of cattle

manure application rate, quality and timing of application on N2O emission from soil

surface.

26

2.6 GAS SAMPLING TECHNIQUES FOR MEASUREMENT OF N2O

FLUXES ON SOIL

It is well recognized that the amounts of N2O evolved during denitrification vary

widely within a single soil, and even a single site, depending upon the prevailing

physical, chemical and environmental conditions (Kroon et al., 2008; Levy et al.,

2011). The global N2O emission estimates are based on limited data sets, which

cannot account for the large spatial and temporal variability of N2O emission from

soils. During the last decade, emissions at the field scale have been principally

measured by chamber methods. Such devices are simple and especially adapted to

study the relationships between the fluxes and soil chemical or microbiological

factors. However, they are less effective for quantifying fluxes at the landscape level.

Indeed, many studies have demonstrated a large spatial and temporal variability of

N2O sources at the field scale (Kroon et al., 2008; Akiyama et al., 2009). Because the

area enclosed in a chamber is typically smaller than 1 m2, it is necessary to use a large

number of chambers to get a representative estimate of the fluxes at field scale

(Smemo et al., 2011). In addition, chamber techniques have disadvantage in that they

tend to modify the environment conditions during gas sampling (Galle et al., 2003).

Their use is labour-intensive because, in the case of N2O, measurements are difficult

to automate. The most suitable static chamber type is the non-steady-state and non-

flow-through chamber (Livingston and Hutchinson, 1993).

An alternative is to continuously monitor the N2O flux at the field scale by using

micrometeorological methods. These methods require fast gas analysis with

sensitivity better than 1 nLL-1. They are also costly and difficult to maintain, requiring

constant and continuous monitoring (Denmead et al., 2000). Micrometeorological

methods integrate fluxes over larger space (i.e., field scale). The technique has

capacity for the experimental measurement of controlling factors. The resulting data

can therefore be used to verify the performance of process-based models for

estimating the flux at the field scale (Arnold and Walker, 2010). The chamber

technique of sampling N2O gas from soil was chosen in this study for the reason that

it is simple, cheap and adapted to study the relationships between fluxes and soil

chemical factors.

27

2.7 LYSIMETER INSTALLATIONS FOR NO3 LEACHING

MEASUREMENTS

2.7.1 Uses of lysimeters

Measurements of soil nitrate movements in soil are difficult to perform owing to the

physical and chemical complexity of most natural soils. Lysimeters, which are

essentially blocks of soil enclosed in suitable containers, have been used to determine

water percolation over the past 300 years (Goss and Ehlers, 2009). During the past

few decades, many types of lysimeter have been developed and used for a great

variety of purposes. The differences usually concern size, soil filling techniques and

methods for measuring nitrate fluxes. The choice of a particular lysimeter type is

generally governed by the type of experiment planned and required measurement

precision.

Lysimeters can also be grouped as free draining (zero tension) or suction-controlled

(tension lysimeter). In free draining lysimeters, water percolates through the soil

column by gravity. Tension lysimeters consist of a suction device that is installed at

the bottom of the lysimetrs which allows water to be sucked at a given tension

through a porous plate (McMahon and Thomas, 1974). Aboukhaled et al. (1982)

distinguished between two basic types of lysimeters: weighing and non-weighing

types. Losses or gains of measured variable in weighing lysimeters are determined by

weighing the whole lysimeter. In non-weighing lysimeters, fluxes of leachate are

either measured by volume after water has drained or been extracted out of the soil

(Bergstrom, 1987).

2.7.2 Lysimeter soil filling technique for measuring nitrate fluxes in leachate

The physical conditions of an isolated soil under study in a lysimeter have a major

influence on the outcome of measurements. The filling of lysimeters with soil has to

be given considerable attention in order to create conditions representative of field

conditions. There are basically two soil filling methods: either by enclosing an

undisturbed soil monolith in a container, or filling the container with disturbed soil

(Bergstrom, 1987; Goss and Ehlers, 2009). McMahon and Thomas (1974) found that

the solutes displaced in undisturbed lysimeters had a more asymmetric breakthrough

curve compared with those displaced in repacked lysimeters of the same soil, even

28

though the two types of columns had similar pore volumes. These differences are

more pronounced for well-aggregated and stratified soils. Sandy soils with a single

grain structure do not appear to suffer as much from these disadvantages when

installed in lysimeter.

When considering repacked lysimeters, the soil profile under study is removed from

the field and filled back into the lysimeter container, layer by layer. The most crucial

step during reconstruction of the soil profile is to keep the bulk density as similar as

possible to that of the undisturbed soil. This more or less limits use of the technique to

light-textured soils. Accordingly, the majority of studies with repacked lysimeters

have been performed in unstructured soils (Bergstrom, 1987); although lysimeters

containing disturbed clay soil profile have also been used (Marin et al., 2010). After

filling the lysimeter with soil, it is commonly subjected to wetting and drying cycles

to promote settling of soil particles (Hang et al., 2010).

2.7.3 Lysimeter size and material used

One problem related to lysimeter size is the potential water flow that may occur along

the lysimeter wall, often referred to as a sidewall flow. Sidewall flows are often

limited to soils susceptible to shrinkage such as 2:1 clays and soils with high organic

matter content. Sidewall flow becomes naturally more accentuated with decreasing

lysimeter size (Goss and Ehlers, 2009). One way to prevent sidewall flow between the

soil and lysimeter wall is to provide the wall with a rough surface by covering it with

asphalt-based waterproofing paint.The size of lysimeters is determined by the rooting

characteristics of the test crop. The depth of the lysimeters is chosen after considering

the rooting depth of the test crops (Aboukhaled et al., 1982).

A great variety of materials are used to manufacture lysimeter containers. The choice

of material involves considerations such as lightness, ease of handling, cost, thermal

conductivity and inertness. Steel and stainless steel containers have been used in

studies with both disturbed and undisturbed soil, which is also the case for fiberglass

and various types of plastic containers. To prevent soil-to-wall contact and to avoid

rusting, steel containers are usually painted with some inert waterproofing paint

before they are filled with soil (Marin et al., 2010).

29

All lysimeters used in the measurement of NO3-N fluxes need to have arrangements

for draining off excessive water that has percolated through the soil profile. There are

two main ways to arrange this: either by letting water drain freely through the soil, i.e.

free drainage (zero tension lysimeter), or by installing some kind of suction device at

the bottom of the lysimeter, which allows water to be sucked through porous

collectors at different tensions (Goss And Ehlers, 2009). A free drainage system is

easy and cheap to install and requires very little maintenance. Water that has

percolated through the soil is either collected in a layer of course material at the

bottom of the lysimeter (Bergstrom, 1987) or in a collection vessel outside the

lysimeter. Although zero tension lysimeters are relatively easy and cheap to install

and maintain the soil water conditions throughout the soil profile can be modified.

This is due to the fact that a water-saturated zone occurs at the bottom of the soil

profile before drainage, due to the resistance formed by the surface tension at the soil-

air boundary. This can be rectified by the adding gravel at the bottom of the lysimeter.

Therefore, zero tension lysimeters are more appropriate for mimicking solute

movement in soil with a shallow groundwater table such as that in wetland. However,

with zero tension lysimeters it is feasible to sample water moving preferentially

through large pores when the surrounding soil matrix is unsaturated (Bergstrom,

1990).

2.7.4 Placement of lysimeters in the field

Installation of lysimeters in the field can have a major influence on their water

balance and thus also on NO3-N leaching measurements. The main goal of lysimeter

studies is to simulate actual field conditions as closely as possible. The majority of

lysimeters are sunk into the ground to soil level, ensuring realistic temperatures inside

it. Standard cultural practices such as ploughing test crop, weeding, and fertilization

should be mimicked during the experiment. The lysimeters should be sited to ensure

that rainfall, temperature and wind conditions compare to the general field situation.

Adequate guard rows should be established around the lysimeters to reduce edge

effects. The depth of the soil column depends on the rooting depth of the test crop and

the drainage system (Bergstrom, 1992).

30

CHAPTER THREE

3.0 GENERAL MATERIALS AND METHODS

3.1 SITE DESCRIPTION

The study was conducted between 2007 and 2009 in a typical wetland garden at

Dufuya (1917 S; 2921 E, 1260 m above sea level) wetlands in Chief Sogwala area

of Lower Gweru Communal Lands, about 42 km west of the city of Gweru,

Zimbabwe (Fig 3.1).

Fig 3.1 Study site location in Dufuya wetland and Zimbabwe

31

The field experimental site is in Agro-ecological Region III, which receives total

rainfall ranging from 650 to 800 mm per annum (average 725 mm) and mean annual

temperature is 21C with insignificant frost occurrence in the months of June and July

(Vincent and Thomas, 1960; Mugandani et al., 2012). Rainfall occurs during a single

rainy season extending from November to April. The experimental soil is a deeply

weathered course textured loamy sand topsoil over sandy loam subsoil derived from

granite and classified as Udic Kandiustalf (USDA) and Gleyic Luvisol (FAO) (FAO,

1988, Nyamapfene, 1991, Soil Survey Staff, 1992). The soil is perennially moist in

part of the profile and smallholder farmers have established vegetable gardens along

the wetland. Vegetable production is all year round. The site had been under alternate

rape, tomato, and maize crops for several years. Rape is cultivated as a leaf vegetable

in Zimbabwe (De Lannoy, 2001).

3.2 EXPERIMENTAL SOIL DESCRIPTION

Initial soil characterization was done by collecting twenty soil samples from randomly

selected points of the experimental site at depths of 0 – 20; 20 – 60 and 60 - 100 cm

using a soil auger. The soil samples were mixed thoroughly in a clean plastic bucket

to obtain a composite sample. The composite sample was air-dried, sieved (<2mm)

and characterized (Table 3.1). Soil organic carbon was determined by the Walkely

and Black method (Nelson and Sommers, 1996). Soil texture was determined by the

Bouyocous hydrometer method (Bouyocous, 1965). Soil pH was determined by

weighing a 15 g soil sample in a 200 ml honey jar to which 75 ml 0.1M CaCl2 were

added. The mixture was shaken mechanically for 30 minutes and pH was determined

using a digital pH meter (Model: Orion 701, Orion Manufacturing, MI, USA). Total

N was measured by the Kjeldahl method using concentrated H2SO4, K2SO4 and HgO

to digest the sample (Bremner, 1996).

Soil bulk density was determined by the core method (Black and Hartge, 1986).

Twenty soil cores for bulk density determination were randomly collected from the

experimental site. Bulk density (Db) was calculated using the following equation:

𝐷𝑏 = 𝑀𝑠 𝑉𝑡⁄ (1)

32

Where Ms is mass of oven dry solids and Vt is total soil volume. The soil cores were

oven-dried at 105C (to constant weight) for determination of mean gravimetric water

content.

Taking particle density (Pd) of soil to be 2.65 g cm-3 total porosity was calculated as:

1– Db Pd⁄ (2)

Total N in soil was measured by the Kjeldahl method using concentrated H2SO4,

K2SO4 and HgO to digest the sample (Bremner 1996).

Table 3.1 Chemical and physical properties of the experimental soil

Soil

depth,

(cm)

Soil pH

(H2O)

Org-C

(%)

1N

mgkg-1

Sand

(%)

Clay

(%)

Silt

(%)

Total

porosity

(cm3cm-3)

Bulk

density

(gcm-3)

Saturation

gravimetric

water

(gg -1)

0-20 5.5 0.4 24 85 10 5 0.46 1.28 0.51

20-60 5.8 0.2 20 80 15 5 0.43 1.34 0.67

60-100 5.7 0.2 20 78 17 5 0.41 1.39 0.69

3.3 DUFUYA WETLAND CROPPING AND HYDROLOGY

The Dufuya watershed (724 ha) is largely covered by loamy sand gently sloping

southwards at an almost homogeneous slope of 4%. Long narrow fields (300 x 40 m)

under rain-fed maize cropping cover the greater part of the watershed (Fig 3.1). The

fields are bordered with trees or small shrubs with contour ridges separating the

narrow fields. There are no significant signs of surface runoff problems. The area has

several wetlands on streams running down slope. The wetlands, which are seasonal,

are covered almost entirely by a dense growth of bulrushes. One of these wetlands is

the Dufuya. Below the wetland, an intermittent stream meanders downstream into a

system of dotted small gardens located on a perennially damp and marshy strip, which

bisects the system. The wetland gardens are usually placed immediately adjacent to

one another often sharing common fences (Owen et al., 1995).

Underground water draining beneath Kalahari sands first emerges from a huge sponge

just above the Sogwala road, which was constructed at the break in slope below the

southern flank of a long Kalahari sand ridge. The Dufuya wetland lies close to the

33

headwaters of the fluvial network, which is a common characteristic of most

wetlands. Consequently, estimates of water available within the wetland are not

complicated by stream flows originating in extensive and complex systems upstream.

The formulation of an expression for the water balances and relative water supplies of

the system provide the background to the wetland water management issues, which

are of fundamental importance in the choice of sustainable land use options in

wetlands (Levine, 1982).

The smallholder farmers at Dufuya wetlands practice intensive vegetable production

in small gardens under informal irrigation. In wetland systems, water is not conveyed

by large engineered channels nor pumped from deep wells. It arrives as groundwater

and is usually lifted by hand buckets from less than 2 m deep (Fig 3.2).

Tomato and rape are high-value vegetable crops grown under informal irrigation by

smallholder farmers at Dufuya. The crops are grown typically using high rates of N

fertilizers and cattle manure in order to avoid yield depression. Because of lack of

availability and higher cost, smallholder farmers have resorted to use of cattle manure

which are readily available without chemical fertilizers. However, some wetland

farmers with financial resources from vegetable sales apply cattle manure in

combination with mineral fertilizers in order to increase nutrient availability to the

vegetable crops. Usually, 15 Mg ha-1 of cattle manure is applied by wetland farmers

with limited number of cattle (less than 6). On average, 30 Mg cattle manure ha-1 is

applied by wetland farmers with larger cattle herds (more than 6). Dufuya wetland

farmers with limited financial resources apply cattle manure without mineral fertilizer

applications. In some cases, smaller doses of cattle manure (4 – 8 Mg ha-1) in every

cropping event are used. Mineral N fertilizer applications in combination with cattle

manure are applied in rates of 100 to 200 kg N ha-1 depending on the financial

resources of the farmer. The fertilizer rates were used as treatments in the experiments

in order to capture the common farmer practice. Under wetland cropping conditions,

N is readily lost by leaching and denitrification, with nitrate concentrations in shallow

ground water reaching hazardous levels.

Small-scale wetland irrigators at Dufuya have developed sustainable water

management strategies that have evolved in response to the socio-technical constraints

34

they are facing. In wetland systems, water is conveyed by simple water engineering

techniques. It arrives as groundwater and is usually lifted by hand buckets from less

than 2 m deep.

The bulk of the water demand by vegetable crops in wetland gardens is met directly

by uptake from the groundwater. Irrigation rates range from 5 mm in summer to 600

mm ha-1 during the dry season (De Lannoy, 2001; Lin et al., 2011). In wetlands, the

majority of irrigation is done in winter when the wetlands are drier. In summer,

irrigation is employed simply to establish the crops and as a supplement between

storms (Kuntashula and Mafongoya, 2009). Current informal irrigation in wetlands

amounts to nearly 20 000 ha and has potential to be considerably increased. In

contrast, state supported irrigation for smallholder farmers is in the region of 7 000

hectares (Department of Irrigation, 2008).

Fig 3.2 A typical shallow well less than 2 m deep at Dufuya

wetlands

35

3.4 LAND PREPARATION AND PLOT ESTABLISHMENT

The field experiments commenced in September 2007. The site had been under

alternate rape, tomato, and maize crops for several years. The land was cultivated

using a hand hoe to a depth of about 30 cm and then leveled using a rake. Raised

plots, which measured 5 by 1.5 m, were then carefully marked out. Small 20 cm high

ridges were established around each plot to avoid cross-contamination by surface run-

off (Fig 3.3).

3.5 CATTLE MANURE USED IN THE STUDY

Two types of cattle manure were used in the field plot and lysimeter experiments

namely: cattle manure collected from a homestead in the surrounding communal area

(smallholder cattle manure, high N (HN) manure) and manure collected from a farm

in the adjacent Vungu commercial farming area (commercial farming area manure,

low N (LN) manure). Ten randomly selected samples were collected from a pile of

each type of manure and thoroughly mixed in a plastic bucket. Three replicate

composite samples were taken for laboratory analysis. The samples were air-dried,

passed through a 2 mm sieve, and analyzed for organic C (Nelson and Sommers

1982), total N using the Kjeidahl procedure (Stevenson, 1982, Bremner and

Mulvaney, 1982), soil, and ash content. Soil and ash contents were determined by

ashing manure in a muffle furnace (450C) for 16 hours. The ash was dissolved in

Fig 3.3 Experimental plots at Dufuya wetlands

36

concentrated HCl acid and separated from mineral soil by filtering. The soil was oven

dried and weighed.

Table 3.2 Selected chemical properties of cattle manure from communal and

commercial farming areas

Manure

type

Organic

C (%)

Total N

(%)

C: N

Ratio

Soil + ash

content

(%)

Soil and ash-free basis

(%)

Organic C Total N

High N

manure

22.82 1.36 16.8: 1 77.18 61.3 6.4

Low N

manure

9.13 0.51

17.9: 1 90.87 23.0 2.3

3.6 TEST CROPS AND CROP MANAGEMENT

An irrigated crop rotation system that includes tomato (September to February) and

rape (March to August) on raised plots is the most common wetland vegetable

farming practice at Dufuya wetland. For this reason, tomato (Lycopersicon

esculentum, Mill var. Heinz) and rape (Brassica napus, L var. Giant) were chosen as

the test crops grown on raised plots and in lysimeters. The first tomato crop was

transplanted on 9 September 2007 while the first rape crop was planted on the 8th of

January 2008. The second tomato crop was established in April 2008 while the second

rape crop was planted in September 2008. Weed control was done using a hand hoe.

About 35 mm of irrigation water was applied using the bucket system once a week to

maintain plant growth at field capacity moisture level during the dry season and mid-

summer season dry periods for both test crops. The water was drawn from a shallow

well (Fig 3.2). This is a common practice by farmers based on extension advice.

Wetland vegetable farmers at Dufuya do not vary the amounts of irrigation water

from crop to crop.

3.7 NITROUS OXIDE FLUX MEASUREMENT

Nitrous oxide emissions from soil were trapped using open-bottomed polythene static

chambers with a trapping area of 0.03m2 (18 cm internal diameter, 20 cm height, 1.5

mm wall thickness) (Holland et al., 1999; Meyer et al., 2001). The closed ends of the

cylinders were tightly fitted with 5 mm diameter self-sealing rubber septa (Fig 3.4).

37

There were seven and six gas sampling campaigns at 14 day interval for the tomato

and rape crops respectively. Each vegetable bed was subjected to N2O gas sampling at

intervals of two weeks after planting up to the last vegetable harvesting event.

Because soil temperature is known to affect N2O production, N2O gas sampling from

each plot was done at the same time (Denmead et al., 1979, Blackmer et al., 1982,

Conrad et al., 1983).

The open ends of the acrylic chambers fitted with chamber collars were manually

inserted into the soil to a depth of 6 cm and chamber guides were fastened to the bases

by means of two snap locks. Chamber collars were inserted into the soil a day before

the gas sampling campaigns to reduce disturbance effects. Gas sampling was done at

intervals of 0 minutes at the beginning of gas sampling to obtain the background

atmospheric concentration of N2O and 60 minutes (Mathias et al., 1980; Kaiser et al,

1996). Gas samples were extracted using a 10 ml Plastipak syringe (Becton Dickinson

and Co., Rutherford, NJ) and injected into 2 ml evacuated gas testing vials that do not

allow gaseous diffusion and exchange with the atmosphere.

The gas samples were analyzed for N2O concentration by means of a Varian Model

3400 gas chromatograph (Walnut Creek, CA) equipped with an electron capture

Fig 3.4 Static chamber for nitrous oxide gas sampling in a plot with a tomato crop.

The chambers were fitted with self-sealing rubber septa

38

detector and a stainless steel column (3.66 m long by 3.18 mm internal diameter)

packed with 80/100 Porapack-Q. Carrier gas (10% CH4, 90% Ar) flow rate was 30 ml

min-1. Air samples were emptied in 2 ml sampling loop and samples were injected

automatically via a six-port gas-actuated sampling valve. The sampling loop was

preceded by a Drierite (W.A. Hammond Drierite Co., Xenia, OH) trap for water

absorption. Other analytical conditions were: detector temperature 390C, oven

temperature 60C, and injection temperature- ambient. Nitrous oxide was quantified

by comparing sample peak area with that of a 1.17 ppmv custom standard (Matheson

Gases, Ottawa, ON; retention time was 2.07) (Mosier and Mack, 1980; Galle et al.,

2003). The laboratory analyses were performed the University of Kwazule Natal in

SA. Nitrous oxide fluxes (FN) were calculated using the Hutchinson and Livingston

(1993) model:

Fn =𝛿𝐶𝑛

𝛿𝑡 .

V

𝐴 .

Mn

𝑉𝑚𝑜𝑙 (3)

Where 𝛿𝐶𝑛/ 𝛿𝑡 is the rate of change in N2O concentration (µmol mol-1 min-1), V is the

chamber headspace volume (m3), Mn is the molecular weight of N2O (44 g mol-1), A

is the surface area (m2) and Vmol is the volume of one mole of gas at 20ºC (0.024 m3

mol-1). Further conversions were performed to calculate Fn fluxes in g ha-1 day-1 as

follows:

Fn g ha−1day−1 = N2O g hr−1 . 24 ℎ𝑟 . 𝐴

10000 (4)

Total N lost as N2O (N kg ha-1) was calculated using Equation (4):

N kg ha−1 = Fn g ha−1day−1 . 𝑇𝑑𝑎𝑦𝑠

1000 .

28

44 (5)

Where T is the number of days with similar daily N2O emissions rates and 28/44 is

the conversion ratio for converting N2O molar mass to N content. At the same time

that gas samples were collected soil temperature measurements were done 5cm from

each static chamber using a soil thermometer. At the same time that gas samples were

collected soil temperature was done 5cm from each static chamber using a soil

39

thermometer. Soil cores collected from plots were oven-dried at 105C (to constant

weight) for determination of mean gravimetric water content.

3.8 LYSIMETER EXPERIMENTS

A cluster of zero tension (free drainage) 40 x 40 x 50 cm lysimeters was established in

November 2006, about ten months before commencement of the experiments in

September 2007. The lysimeter boxes were fabricated from 1.6 mm thick galvanized

steel sheets, which is rust-resistant. The choice of galvanized steel as material for

lysimeter box fabrication was made after considering the fact that it is light, easy to

handle, relatively cheap, has good thermal conductivity and inert (Jensen, 1982; Shih

and Rosen, 1985). The depth of the lysimeters was chosen after considering the

rooting depth of the test crops (rape and tomato), which rarely exceed 40 cm

(Aboukhaled et al., 1982). To avoid sidewall flow, the lysimeters were coated with

asphalt-based waterproofing paint, which provided a rough surface. A 1 mm wire

mesh was fixed at the lysimeter outlet and covered with a 10 cm layer of gravel before

the soil was emplaced, thereby reducing the effective depth of the lysimeters to 40

cm. The gravel enhanced drainage and prevented the soil from washing into the

lysimeter outlet, which effectively stopped blockage. Each lysimeter outlet was

connected to a flexible plastic pipe laid at a slope of 2% to ensure rapid leachate flow

into collecting buckets. Repacking of the lysimeters closely followed the sequence of

the soil horizons identified during soil excavation.

Leachate samples from lysimeters were collected from leachate collectors (Fig 3.5)

Volumes of leachate were recorded in litres whenever there was a leachate break-

through. Cumulative leachate volumes were computed and recorded every fortnight

during the growing period of each crop. Cumulative annual rainfall total and

supplementary cumulative irrigation applications (35 mm applied every week during

the dry season and mid-season droughts) were plotted on the same graph in order to

establish the water balance. Representative 100 ml samples of composite leachate

were collected for nitrate N concentration analysis by colorimetric method, which

involves the reduction of nitrate (NO3) using a Cu catalyst (Keeney and Nelson,

1982).

40

A Cecil spectrometer (model CE2010) was used to measure absorbance. N loads were

calculated by multiplying the total N concentration by the volume of the leachate and

the subsequent conversions to kg N ha-1 as follows:

NO3Nleach = [NO3N] × Vol × 0.002 × Tdays (6)

Where NO3Nleach is total NO3-N leached from soil in kg N ha-1, [NO3N] is

concentration of NO3-N in leachate, Vol is mean daily leachate volume in litres for

the period, 0.002 conversion ratio after resolving mg [NO3-N] to kg N ha-1 and

converting NO3- molar mass to N content (14/62); Tdays is the number of days of

approximately similar leachate volumes.

Labeled leachate collectors in

channel

Block 1

Block 2

Block 3

Block 4

N fertilizer + manure lysimeter

experiment: Experment 1

Buried rubber pipes collecting leachate from

lysimeters to leachate collectors at 2% slope flow

torwards the leachate collectors

2 Buried 40 x 40 x 50 cm lysimeter boxes

Fig 3.5 Aerial view of the lysimeter station for the experiments

41

Leachate volumes in litres were converted leachate depth in mm using the following

model:

H =V

A x 1000 (7)

Where, H is the depth of leachate in mm, V is the volume of leachate collected in m3,

A is the base area of the lysimeters in m2 and 1000 is the conversion factor from m to

mm depth. Soil temperature was measured in each lysimeter using a soil thermometer

at intervals of 14 days when leachate volumes were computed. Soil cores collected

from lysimeters were oven-dried at 105C (to constant weight) for determination of

mean gravimetric water content fortnightly.

3.9 MINERAL N MEASUREMENTS

At the same time that gas and leachate samples were collected (n = 4), soil samples

also collected from the plots and lysimeters, respectively. The soil samples were

returned to the laboratory and on the same day extracted with 0.5 M K2SO4 (10 g soil

in 50 mL), then frozen until analysis for NO3-N and NH4-N (Mosier and Mack 1980,

Galle et al. 2003). Soil slurries were shaken for 1 minute, left to equilibrate overnight,

and reshaken for more than 1 hour before filtering. Filtrates were stored in 7 ml

scintillation vials and frozen until analysis for NH4-N and NO3-N. Both analyses were

performed using an Alpkem 3550 Flow Injector Analyzer (01 Analytical, College

Station, TX) using colorimetric techniques (Robertson et al., 1999) at the Midlands

State University Chemical Technology laboratory.

3.10 DRY MATTER YIELD AND N UPTAKE

Four randomly selected plants were chosen and labeled in each plot/lysimeter for crop

biomass sampling. All rape leaves and tomato fruits that reached horticultural

maturity were harvested from the selected plants at every harvesting event and taken

to the laboratory. The samples were rinsed; oven dried at 65C for 24 hours and kept

in a dry place. At the end of the growing season, the aboveground biomass of the

selected plants was summed up. The composite samples were then ground to pass a 2

mm sieve and analyzed for N concentration semi-micro Kjeldahl procedure (Bremmer

42

and Malvaney, 1982). Total uptake of N was determined by multiplying the N

concentration with dry matter yield as follows (Equation 8):

Nuptake kg/ha = [N].DM (8)

Where [N] is content of N in mg/g dry matter and DM is dry matter yield in Mg/ha

3.11 WEATHER CONDITIONS

Rainfall data were collected daily at 10.00 hours from a rain gauge at the study site.

Maximum and minimum daily temperatures at the study site were gap-filled using the

department of Agricultural Technical and Extension Services (AGRITEX)

meteorological data at Sogwala (1917 S; 2921 E) rural service centre located 2 km

west of the study site. The meteorological station records daily weather data.

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

Sep2

4/0

729-S

ep

7-O

ct

26-O

ct

16-N

ov

23-N

ov

4-D

ec

13-D

ec

14-D

ec

21-D

ec

22-D

ec

27-D

ec

28-D

ec

30-D

ec

Jan2/0

83-J

an

5-J

an

11-J

an

12-J

an

13-J

an

14-J

an

23-J

an

24-J

an

29-J

an

30-J

an

3-F

eb

Fe

b9/0

8M

arc

hA

pri

lM

ay

Jun

July

Aug

Sept

9-O

ct

10-O

ct

21-O

ct

26-O

ct

8-N

ov

19-N

ov

13-D

ec

19-D

ec

Dec23/0

8

Rain

fall m

m

Air

tem

pera

ture

oC

Air temperature at gas sampling and rainfall recording day

First

tom

ato

pla

ntin

g

First

tom

ato

la

st

ha

rve

stin

g

First

rap

e p

lan

tin

g

First

rap

e la

st

ha

rve

stin

g

Se

co

nd

to

ma

to p

lan

tin

g

Se

co

nd

to

ma

to la

st

ha

rve

stin

g

Se

co

nd

ra

pe

pla

ntin

g

Se

co

nd

ra

pe

la

st

ha

rve

stin

g

Fig 3.6 Daily rainfall, air temperature at the study site

43

The 2007 - 2008 rain season started at the end of September. About 98% (792 mm) of

the total rainfall (808.2 mm) was received in the first half of the season (September to

January; Fig 3.3). The 2007 – 2008 rain season had a mean maximum and minimum

temperature of 31.5°C and 24.5°C, respectively. The 2007 – 2008 winter season was

generally frost-free and had a maximum and minimum air temperature of 20°C and

15°C, respectively.

The first tomato and rape crops were grown during the 2007 – 2008 summer season.

The second tomato crop was grown during the 2008 winter season. The 2008 - 2009

rainy season started at the beginning of October (36 mm). The last quarter of the study

period occupied about half of the 2008 – 2009 summer season (October – December

2009) during which the last rape crop was grown. The first three months of the

summer season recorded 156 mm of rainfall. The summer season was characterized

by hot and humid weather with maximum and minimum temperature of 30.5°C and

26.5°C respectively.

44

CHAPTER FOUR

4.0 EFFECTS OF CATTLE MANURE QUALITY ON NITROUS

OXIDE EMISSION AND NITRATE LEACHING

ABSTRACT

The implications of increased application of N inputs to agricultural systems in Africa

for N2O emissions and nitrate leaching are still only partially understood. A field

experiment was carried out at Dufuya wetland to determine the effects of cattle

manure quality on mineral N concentration in soil, emissions of N2O, nitrate leaching,

dry matter yield and N uptake during the growing seasons of rape and tomato crops. A

cropping sequence of first tomato, first rape, second tomato, and second rape crops

grown between September 2007 and November 2008 was used in the experiment.

Two types of cattle manure were used: high N manure (1.36%N) and low N manure

(0.51%N). Each type of manure was applied in three levels of 0, 15, and 30 Mg ha-1

as a single application just before planting of the first tomato crop. The replacement

of 15 and 30 Mg low N manure ha-1 with the same rates of high N manure applied

once in the study increased the content of mineralized N in soil by 60% or 1.5 mg kg-1

and 13% or 2.0 mg kg-1, under rape and tomato crops respectively. When 15 and 30

Mg low N manure ha-1 applied once in the four cropping events was replaced with the

same rates of high N manure N2O fluxes on soil significantly (p<0.05) decreased by

41% or 2.5 g ha-1 day-1 and 50% or 3.6 g ha-1 day-1 respectively. When 15 and 30 Mg

low N ha-1 manure were replaced with the same rates of high N manure total N lost

through N2O emission increased by 58% and 31%, respectively. When 15 and 30 Mg

of low N manure were applied instead of the same rates of high N manure nitrate N

leaching significantly decreased (p<0.05) by 36% and 42% respectively. Generally,

the proportion of applied N lost as N2O emissions nitrate leaching was higher in the

rape crop than in the tomato crop. This implies that rape production has a greater

potential to pollute the atmosphere and ground water than the production of tomatoes

in wetlands when cattle manure is used as a fertilizer.

45

4.1 INTRODUCTION

In developing countries, the increasing prices of inorganic fertilizers coupled with

growing concerns for sustaining soil productivity has led to renewed interest in the

use of cattle manures as fertility-restorer inputs (Mapfumo et al., 2007; Ouedraogo et

al., 2007; Nyamangara and Nyagumbo, 2010; Masvaya et al., 2011). Manures are a

vital resource not only for supplying plant nutrients but also for replenishing organic

matter content of agricultural soils particularly in the tropics (Groot et al., 2006;

Materechera, 2010). Direct emissions of N2O and nitrate leaching from agricultural

soils have increased substantially over the last few decades, in parallel with increasing

use of manures and N fertilizers. Current emissions, which have been estimated by the

IPCC Phase II methodology, are likely to increase still more in future, as inputs of N

to agriculture continue to rise (Smith et al., 1997). The implications of increased

application of N inputs to agricultural systems in Africa for N2O emissions and nitrate

leaching are still only partially understood. For example, there are few published

studies from Africa examining the response function relating N2O emissions and

nitrate leaching to cattle manure quality (Hickman et al., 2011). Current estimates of

N2O emissions from African agriculture at the national, regional, and continental

scale are mostly based on the IPCC Guidelines, which implicitly ignore important

characteristics of African soils and management practices of smallholder farmers,

both of which could alter emissions and nitrate leaching substantially (Smith, 2012).

Management practices substantially affect N2O emissions and nitrate leaching

(Saggar, 2010). Due to the strong influence of available soil N on N2O emissions and

nitrate leaching, losses of N through emissions of N2O and nitrate leaching seem to be

an unavoidable consequence of maintaining highly productive cropland (Mosier,

2003). Given that N2O in agricultural soil is produced predominantly through the

microbial transformations of inorganic N, the potential to produce and emit N2O

increases with the increasing availability of N and consequently the concentration of

N in applied manure (Smith, 2012).

In compiling the global GHG and nitrate leaching inventories, it is necessary to use

country-specific data, where available, for the activity data and N2O emission factors.

46

The present IPCC default emission factor for N2O of 1.2 – 2% of the N applied may

stand for the time being, but there is considerable scope for the default emission factor

to change as more data becomes available from sub-tropical African regions (Snyder

et al., 2009). Consequently, a field study was conducted in central Zimbabwe in order

to determine the effect of cattle manure quality of N2O emissions and nitrate leaching.

4.2 HYPOTHESIS AND OBJECTIVE

The following hypotheses were tested:

Nitrous oxide emission and nitrate in wetland soil increased with increasing content of

N in cattle manure (manure quality). The specific objective of the study was to

determine the effect of cattle manure quality on N2O emission and nitrate leaching.

4.3 MATERIALS AND METHODS

4.3.1 Experimental cattle manure

The two types of aerobically composted cattle manure which were used in the

lysimeter study are described in section 3.5. The quality of cattle manure was based

on the content of N. The laboratory procedure for determination of N content in

manure is also described in the same section. Table 3.2 summarizes the results of the

analysis.

4.3.2 Experimental design and treatments

Two experiments were used to determine the effect of cattle manure quality on N2O

emission and nitrate leaching with three treatments for each experiment.

Experiment 1:

i. Control (unamended)

ii. 15 Mg high N manure ha-1

iii. 30 Mg high manure N ha-1

Experiment 2:

i. Control (unamended). ii.

ii. 15 Mg low N manure ha-1

iii. 30 Mg low N manure ha-1

A completely randomized block design with four replications was used in which the

slope was the blocking factor.

47

4.3.3 Nitrous oxide, mineral N and dry matter measurements

Mineral N measurements in soil in the lysimeter and plot experiments were achieved

by following procedures described in section 3.9. Dry matter yield and N uptake by

the aboveground biomass of the vegetable crops was undertaken using the method

described in section 3.10.

4.3.4 Lysimeter study

The lysimeter experiments for leachate sampling were established following the

procedure described in section 3.8. The concentration of NO3-N in leachate was

determined using the laboratory methods described in the same section. Equation 6 of

that section was used to calculate nitrate N leached per ha.

4.3.5 Statistical analysis

Treatment effects on measured variables were analyzed using Two Way ANOVA

(GenStat Discovery Edition 3, 2003). Differences between treatment means were

judged significant at p ≤ 0.05 as determined by Fisher’s protected least significant

difference test. Flux data were log-transformed if needed, to normalize the

distributions before the statistical analysis. Mean separation was performed using the

LSD since there were not more than three treatments in each set of experiment.

Statistical significance of the differences between measured variables in lysimeters

subjected to high N and low N manure applications was established by performing t-

test for unpaired samples using the GenStat package. The Pearson correlation

coefficients between measured variables and their r2 values were computed using

Microsoft Excel. Significance of correlations between selected variables was

established using a linear model GenStat analysis of correlation at 5% level.

4.4 RESULTS

4.4.1 Ammonium N concentration in soil

Table 3.2 of section 3.5 shows the selected chemical characteristics of high N and low

N cattle manure used in the study. The concentrations of C and N in the HN manure

were 2.5 and 2.7 times more than that in LN manure. The C to N ratios of the high

and low N manure were 17: 1 and 18: 1, respectively.

48

Results shown in Figures 4.1 indicate that cattle manure quality (manure N content)

had a significant effect (p<0.05) on the concentration of NH4-N in soil during the

growing seasons of the four vegetable crops.

0

5

10

15

20

25

NH

4 N

in s

oil,

mg

/kg

a) First tomato LSD 0.4mgNH4/kg soilSept --------------------Oct----------------------Nov------------------------Dec 2007

0

5

10

15

20

25

14d 28d 42d 56d 70d 84d 98d

NH

4 N

in s

oil,

mg/k

g

Days after planting

0Mg/ha Manure 15Mg/ha LN Manure 15Mg/ha HN Manure

30Mg/ha LN Manure 30Mg/ha HN Manure

d) Second rape LSD 0.4mgNH4/kg soilAug --------------------------Sept--------------------------------------------Oct/Nov 2008

0

5

10

15

20

25

NH

4 N

in s

oil,

mg

/kg b) First rape LSD 0.5mgNH4/kg soil

April --------------------May----------------------June-----------------------July 2008

0

5

10

15

20

25

NH

4 N

in s

oil,

mg/k

g

c) Second tomato LSD 0.4mgNH4/kg soilApril --------------------May----------------------June-----------------------July 2008

Fig 4.1: NH4-N concentration in soil following application of high and low N manure.

HN- High N manure, LN- Low N manure

49

However, treatment mean differences in the concentrations of NH4-N in soil became

apparent in this study 42 days of the growing period of the second tomato and rape

crops (Fig 4.1 c, d). Ammonium N concentrations in soil subjected to application of

15 and 30 Mg ha-1 of high N manure were 146% and 143% in excess of those

recorded in the control lysimeters. The concentration of NH4-N in soil amended with

15 and 30 Mg ha-1 of low N manure exceeded those recorded in control lysimeters by

53% and 84%, respectively.

The use of 15 Mg of low N instead of 15 Mg ha-1 of high N manure (manure quality)

increased NH4-N content in soil by 32% (p<0.05) between 42 and 98 days after

planting. At 30 Mg ha-1 manure application rates, the substitution increased NH4-N

content in soil by 27% (p<0.05) between 42 and 98 days after planting the vegetable

crops. Application of 15 and 30 Mg ha-1 high and low N manure considerably

increased the content of NH4-N in soil at the beginning of the growing season of the

vegetable crops before gradually decreasing at the end of the season. Concentrations

of NH4-N in soil decreased on average by 17.1 and 18.1 mg kg-1 in 15 Mg and 30 Mg

ha-1 high N manure treatments at the end of the growing season compared with those

at the beginning of the season, respectively.

4.4.2 Nitrate N concentration in soil

A significant portion of fertilizer N added to agricultural soils is generally lost to the

environment via leaching and denitrification. Results shown in Figs 4.2 indicate that

the application of high N cattle manure significantly (p<0.05) increased the

concentration of NO3-N in soil when compared with the application of low N manure.

In addition, soil NO3-N concentrations during the September 2007 to November 2008

period followed a pattern that comparatively matched the rainfall events at Dufuya

(Fig 3.6, section 3.11). Higher NO3-N concentrations were recorded during the drier

periods of the growing season especially for the first tomato and rape; second rape

crops. The concentration of NO3-N in soil in lysimeters subjected to 15 and 30 Mg ha-

1 of high N manure exceeded that recorded in control lysimeters by 171% and 308%

respectively.

50

0

5

10

15

20N

O3

N in

so

il, m

g/k

g a) First tomatoLSD 0.5mg/kg soil

Sept ----------------------------Oct----------------------Nov-----------------------Dec 2007

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

NO

3 N

in s

oil,

mg/k

g

Days after planting

Control 15Mg/ha LN manure 15Mg/ha HN manure

30Mg/ha LN manure 30Mg/ha HN manure

d) Second rape

LSD 0.4mg/kg soil

Aug ----------------------------------Sept----------------------------Oct/Nov 2008

0

5

10

15

20

NO

3 N

in

so

il, m

g/k

g

b) First rape LSD 0.6mg/kg soil

Jan ---------------------------------Feb------------------------------March 2008

0

5

10

15

20

NO

3 N

in s

oil,

mg/k

g

c) Second tomato LSD 0.4mg/kg soilApril --------------------May----------------------June---------------------July 2008

Fig 4.2: NO3-N concentration in soil following application of high and low N manure.

HN- High N manure, LN- Low N manure

51

Lysimeters that received 15 and 30 Mg ha-1 of low N manure recorded higher NO3-N

concentrations in soil by 61% and 157% when compared with the control. Increasing

the rates of application from 15 to 30 Mg ha-1 of HN and LN manure significantly

(p<0.05) increased the concentration of NO3-N in soil by 50% and 59% respectively.

The substitution of 15 and 30 Mg of high N manure with 15 and 30 Mg of low N

manure ha-1 reduced soil NO3-N concentration by 36% and 25%, respectively

(p<0.05). Temporal trends in the concentrations of NO3-N in wetland soil show

increasing concentrations in all treatments towards the end of the growing periods of

each crop.

4.4.3 Nitrate N concentration in leachate

The application of high N manure to tomato and rape crops significantly increased

(p<0.05) the concentration of NO3-N in leachate when compared with the application

of low N manure (Fig 4.3). Shortly after application of high and low N cattle manures,

treatment separation from the control for both factors was comparatively small (p >

0.05; 14 – 28 days after planting). The concentration of NO3-N in leachate from

lysimeters that were subjected to 15 Mg high and low N manure ha-1 significantly

exceeded (p<0.05) that from the control lysimeters by 133% and 259%, respectively.

When 30 Mg ha-1 of high and low N manures was applied, the concentration of NO3-

N exceeded that from control lysimeters by 67% and 145%, respectively.

When 15 Mg of low N manure was substituted with 15 Mg ha-1 of high N manure the

concentration of NO3-N in leachate increased by 48%. The substitution of 30 Mg of

low N manure with 30 Mg ha-1 of high N manure increased the concentration of NO3-

N in leachate by 41%. Generally, leachate NO3-N concentrations from all lysimeters

were above the USEPA recommended 10 mg L-1. The concentration of NO3-N in

leachate from lysimeters subjected to the application of 15 and 30 Mg high N manure

ha-1 was 112% and 180% above the USEPA recommended concentration for safe

drinking water. The concentration of NO3-N in leachate exceeded the recommended

10 mg L-1 concentration by 60% and 114%, when 15 and 30 Mg low N manure ha-1

was applied, respectively.

52

Fig 4.3: NO3-N concentration in leachate following application of high and low N manure.

HN- High N manure, LN- Low N manure

53

4.4.4 Nitrous oxide fluxes from soil

The means showing N2O fluxes at the time of gas sampling for the period between 21

September 2007 and 9 September 2008 are given in Figures 4.4.

0

5

10

15

20

Nitro

us o

xid

e,

mg/h

a/d

ay

a) First tomato LSD 1.1Sept ------------------------Oct------------------------------Nov------------------------ ------------Dec 2007

Dry season Wet season

0

5

10

15

20

Nitro

us o

xid

e,

mg/h

a/d

ay

b) First rape LSD 0.8Jan --------------------------------Feb--------------------------------March2008

Wet season

0

5

10

15

20

Nitro

us o

xid

e,

g/h

a/d

ay c) Second tomato LSD 0.7

April -------------------------May-------------------------------June------------------------------------July 2008

Dry season

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

Nitro

us o

xid

e,

mg/h

a/d

ay

Days after planting

Control 15Mg/ha HN manure 15Mg/ha LN manure 30Mg/ha HN manure 30Mg/ha LN manure

d) Second rape LSD 1.2Aug --------------------------Sept----------------------------------------------Oct/Nov 2008

Dry season Wet season

Fig 4.4: Nitrous oxide fluxes on soil following application of high and low N manure. HN- High

N manure, LN- Low N manure

54

Study results show that N2O fluxes following high and low N manure application

were significant (p<0.05) throughout the study compared to the control. Higher N2O

emissions were recorded in the first gas samples collected from vegetable plots

amended with single applications of 30 Mg ha-1 high and low N manure, which was

applied a week before planting the first tomato crop. In single high and low N

applications, elevated N2O fluxes persisted throughout the 98 and 84-day period for

tomato and rape crops respectively. The substitution of 15 and 30 Mg low N manure

ha-1 applied once in the four cropping events with the same rates of high N manure

increased N2O fluxes in soil by 41%or 2.5 g ha-1 day-1 and 50% or 3.6 g ha-1 day-1

respectively.

4.4.5 Estimated total N lost in leachate following application of high and low N manure

There was a significant (p<0.05) increase in the loss of N through nitrate N leaching when

high manure was applied in comparison with nitrate leaching loss after application of low N

manure in lysimeters (Tables 4.1 and 4.2). Lowest amounts of total N lost in leachate were

regularly observed in the control lysimeters (1.3 to 9.6 kg N ha-1). However, loss of N under

the second tomato crop was generally low for the three treatments when no rainfall event was

recorded during the April to July 2008 winter season. Higher total N losses were recorded

when 30 Mg high N manure ha-1 was applied just before planting the first tomato crop (27.7

kg N ha-1) in September 2007 (27.7 kg N ha-1) and in the first rape (24.4 kg N ha-1). Both

crops experienced exceptionally wet summer seasons (Figure 3.6). Substituting 15 Mg and 30

Mg ha-1 of high N manure with 15 Mg and 30 Mg ha-1 of low N manure reduced N losses by

an average of 27% and 46% respectively.

55

Trts

First tomato crop First rape crop

Temporal

interval

(days

after

planting)

Mean

leachate

[NO3N]

mgL-1

Mean

daily

leachate

volume,

L

N

leached

kgha-1

Total N

applied

kg/ha

%

leached

N of

Applied

N

Temporal

interval

(days

after

planting)

Mean

leachate

[NO3N]

mgL-1

Mean

daily

leachate

volume,

L

N

leached

kgha-1

Total

N

applied

kg/ha

%

Leached

N of

applied

N

T1 0-49 3.1 4.0 1.2 - - 0-35 11.2 5.3 4.2 - -

50-77 8.7 6.5 3.2 - - 36-63 12.4 5.6 3.9 - -

78-98 16.4 6.6 4.6 - - 64-84 14.1 5.8 3.4 - -

Total - - - 9.1 0 0 - - - 11.5 0 0

T2 0-49 5.0 4.1 2.0 - - 0-35 11.6 4.2 3.4 - -

50-77 10.5 6.3 3.7 - - 36-63 16.2 5.7 5.2 - -

78-98 18.8 6.5 5.1 - - 64-84 20.9 5.5 4.8 - -

Total 10.8 76.5 14.1 - - - 13.4 0 0

T3 0-49 5.8 4.7 2.7 - - 0-35 15.7 5.5 6.0 - -

50-77 16.9 6.8 6.4 - - 36-63 21.0 5.8 6.8 - -

78-98 25.1 6.9 7.3 - 64-84 26.0 5.3 5.8 - -

Total - - - 16.4 153 10.7 - - - 18.6 0 0

Fpr - * NS * - - - * NS * - -

Lsd - 0.9 0.3 0.1 - - - 1.4 1.0 0.3 - -

CV - 9.4 1.4 1.6 - - - 0.9 1.8 1.4 - -

Second tomato crop Second rape crop

T1 0-63 10.7 0.4 0.5 - - 0-21 8.2 0.7 0.2 - -

64-98 16.2 0.6 0.7 - - 22-63 11.0 5.3 4.8 - -

- - - - - - 64-84 21.2 5.0 4.5

Total - - - 1.2 0 0 - - - 9.5 0 0

T2 0-63 13.4 0.6 1.0 - - 0-21 10.5 0.7 0.3 - -

64-98 18.8 0.6 0.8 - - 22-63 14.3 5.2 6.2 - -

- - - - 64-84 19.3 5.0 4.0

Total - - - 1.8 0 0 - - - 10.5 0 0

T3 0-63 15.9 0.6 1.2 - - 0-21 13.0 0.7 0.4 - -

64-98 22.9 0.7 1.1 - - 22-63 18.3 5.3 8.1 - -

64-84 24.6 5.4 5.6

Total - - - 2.3 0 0 - - - 14.1 0 0

Fpr - * NS * - - - * NS * - -

Lsd - 1.1 0.3 0.2 - - - 0.3 0.3 0.3 - -

CV - 8.2 4.6 4.9 - - 1.7 1.2 1.7 - -

Treatments: T1- (Control), T2- (15Mg low N manure ha-1), T3 - (30Mg low N manure ha-1)

Table 4.1: Estimated total N lost in leachate following application of low N manure

56

Trts

First tomato crop First rape crop

Temporal

interval

(days after

planting)

Mean

leachate

[NO3N]

Mg/L

Mean

daily

leachate

volume,

L

N

leached

kg/ha

Total N

applied

kg/ha

%

leached

N of

Applied

N

Temporal

interval

(days

after

planting)

Mean

leachate

[NO3N]

Mg/L

Mean

daily

leachate

volume,

L

N

leached

kgha-1

Total

N

applied

kg/ha

%

Leached

N of

applied

N

T1 0-49 3.3 4.1 1.3 - - 0-35 11.0 5.3 4.1 - -

50-77 8.6 6.6 3.2 - - 36-63 12.1 5.6 3.8 - -

78-98 16.4 6.7 4.6 - - 64-84 14.6 5.8 3.6 - -

Total - - - 9.1 0 0 - - - 11.5 0 0

T2 0-49 5.0 4.4 2.2 - - 0-35 15.7 5.3 5.8 - -

50-77 10.5 6.7 4.0 - - 36-63 20.1 5.6 6.3 - -

78-98 19.8 6.8 5.7 - - 64-84 26.0 5.8 6.3 - -

Total 11.9 204 6 - - - 18.4 0 0

T3 0-49 6.5 4.7 4.0 - - 0-35 21.6 5.5 8.3 - -

50-77 25.2 6.8 9.6 - - 36-63 26.4 5.8 8.6 - -

78-98 48.6 6.9 14.1 - 64-84 31.4 5.7 7.5 - -

Total - - - 27.7 408 7 - - - 24.4 0 0

Fpr - * NS * - - - * NS * - -

Lsd - 1.1 0.4 1.4 - - - 2.1 1.3 2.1 - -

CV - 7.8 3.1 2.2 - - - 2.7 1.9 1.9 - -

Second tomato crop Second rape crop

T1 0-63 10.7 0.5 0.7 - - 0-21 8.4 0.8 0.3 - -

64-98 16.0 0.5 0.6 - - 22-63 11.1 5.1 4.7 - -

- - - - - - 64-84 21.1 5.2 4.6

Total - - - 1.3 0 0 - - - 9.6 0 0

T2 0-63 14.3 0.5 0.9 - - 0-21 12.8 0.8 0.4 - -

64-98 21.2 0.5 1.2 - - 22-63 18.5 5.1 7.9 - -

64-84 24.8 5.2 5.4

Total - - - 2.0 0 0 - - - 13.7 0 0

T3 0-63 18.5 1.4 3.2 - - 0-21 16.0 0.7 0.4 - -

64-98 24.2 0.7 1.2 - - 22-63 23.0 5.2 10.0 - -

64-84 30.5 5.3 6.8

Total - - - 4.4 0 0 - - - 17.2 0 0

Fpr - * NS * - - - * NS * - -

Lsd - 1.6 0.6 0.3 - - - 1.6 0.3 1.2 - -

CV - 9.1 5.1 4.4 - - 1.9 1.0 1.9 - -

Treatments: T1- Control; T2- 15Mg high N manure ha-1; T3- 30Mg high N manure ha-1

Table 4.2: Estimated total N lost in leachate following application of high N manure

57

4.4.6 Estimated total N lost in N2O emission following application of high and

low N manure

There were significant treatment effects (p<0.05) on total N lost through emission of

N2O during the growing period of tomato and rape (Tables 4.3 and4.4). Estimated

total N lost through N2O emissions on plots subjected to applications of high and low

N manure were 121% and 134% above the emissions recorded on control plots.

Trts

First tomato crop First rape crop

Temporal

interval

(days

after

planting)

Mean

rate of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

Temporal

interval

(days

after

planting)

Mean

rate

of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

T 1 1 - 21 5.6 0.13 - - 1 - 49 6.0 0.30 - -

22 - 49 2.5 0.07 - - 50 - 84 3.9 0.13 - -

50 - 63 2.6 0.03 - - - - - - -

64 - 98 5.9 0.20 - - - - - - -

Total 0.43 0 0 - - 0.43 0 0

T 2 1 - 21 7.5 0.16 - 0.21 1 - 49 7.3 0.36 - 0

22 - 49 6.2 0.17 - 0.22 50 - 84 4.6 0.16 - 0

50 - 63 6.3 0.08 - 0.10 - - - - -

64 - 98 7.0 0.24 - 0.31 - - - - -

Total - - 0.65 76.5 0.85 - - 0.52 0 0

T 3 1 - 21 10.0 0.21 - 0.14 1 - 49 10.2 0.50 - 0

22 - 49 7.3 0.20 - 0.13 50 - 84 6.4 0.22 - 0

50 - 63 6.2 0.08 - 0.05 - - - - -

64 - 98 8.8 0.30 - 0.20 - - - - -

Total - - 0.79 153 0.52 - - 0.72 0 0

Fpr - - * - - - - * - -

Lsd - - 0.16 - - - - 0.23 - -

CV - - 13.40 - - - - 12.20 - -

Second tomato crop Second rape crop

T 1 1 - 98 3.3 0.32 - - 1 - 35 3.0 0.11 - -

- - - - - - 36 - 84 4.3 0.21 - -

Total 0.32 0 - 0.32 0 -

T 2 1 - 98 4.1 0.40 - 0.6 1 - 35 4.3 0.15 - 0

- - - - - - 36 - 84 6.0 0.29 - 0

Total 0.40 0 0 0.44 0 0

T 3 1 - 98 5.0 0.70 - 0 1 - 35 5.3 0.19 - 0

- - - - - - 36 - 84 8.4 0.40 - 0

Total - - 0.50 0 0 - - 0.59 0 0

Fpr - - * - - - - * - -

Lsd - - 0.14 - - - - 0.24 - -

CV - - 15.70 - - - - 8.80 - -

Trts: Treatments; T1: Control; T2: 15 Mg low N manure ha-1; T3: 30Mg low N

manure ha-1

Table 4.3: Estimated total N lost through N2O emission following application of low

N manure

58

Trts

First tomato crop First rape crop

Temporal

interval

(days

after

planting)

Mean

rate of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

Temporal

interval

(days

after

planting)

Mean

rate

of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

T 1 1 - 21 5.6 0.12 - - 1 - 49 6.0 0.30 - -

22 - 49 2.4 0.07 - - 50 - 84 3.9 0.13 - -

50 - 63 2.7 0.04 - - - - - - -

64 - 98 6.0 0.21 - - - - - - -

Total 0.44 0 0 - - 0.43 0 0

T 2 1 - 21 8.1 0.17 - 0.08 1 - 49 10.1 0.50 - 0

22 - 49 7.1 0.19 - 0.09 50 - 84 5.8 0.20 - 0

50 - 63 7.0 0.09 - 0.04 - - - - -

64 - 98 9.1 0.31 - 0.15 - - - - -

Total - - 0.76 204 0.37 - - 0.70 0 0

T 3 1 - 21 13.5 0.28 - 0.07 1 - 49 14.1 0.70 - 0

22 - 49 9.0 0.24 - 0.06 50 - 84 10.1 0.30 - 0

50 - 63 8.0 0.10 - 0.02 - - - - -

64 - 98 10.3 0.35 - 0.09 - - - - -

Total - - 0.97 408 0.24 - - 1.00 0 0

Fpr - - * - - - - * - -

Lsd - - 0.16 - - - - 0.23 - -

CV - - 13.40 - - - - 12.20 - -

Second tomato crop Second rape crop

Trt 1 1 - 98 3.3 0.32 - - 1 - 35 3.0 0.11 - -

- - - - - - 36 - 84 4.3 0.21 - -

Total 0.32 0 - 0.32 0 -

T 2 1 - 98 5.2 0.51 - 0.6 1 - 35 5.1 0.18 - 0

- - - - - - 36 - 84 7.4 0.36 - 0

Total 0.51 0 0 0.54 0 0

T 3 1 - 98 6.1 0.60 - 0 1 - 35 7.6 0.27 - 0

- - - - - - 36 - 84 10.0 0.48 - 0

Total - - 0.60 0 0 - - 0.75 0 0

Fpr - - * - - - - * - -

Lsd - - 0.14 - - - - 0.24 - -

CV - - 15.70 - - - - 8.80 - -

Trts- Treatments; T1- Control; T2- 15 Mg high N manure ha-1; T3- 30 Mg high N

manure ha-1

When 15 and 30 Mg low N ha-1 manure were replaced with the same rates of high N

manure applied once in the study total N lost through N2O emission increased by 59%

and 31%, respectively. When 15 and 30 Mg high and low N manure ha-1 were applied

0.4 and 0.9%; 0.2 and 0.5% of applied N was lost as N2O, respectively, during the

growing period of the first tomato crop. Generally, the proportion of applied N lost as

N2O was higher in the rape crop than in the tomato crop.

Table 4.4: Estimated total N lost through N2O emission following application of high N manure

59

4.4.7 N uptake following application of high and low N manure

Results presented in Tables 4.5 and 4.6 clearly show that there were significant

(p<0.05) manure quality treatment effects on the uptake of N by all four crops in the

study. Fertilization of the vegetable crops using 15 Mg and 30 Mg ha-1 of high N

manure significantly (p<0.05) increased N uptake by 41.4 – 84.2 and 86.1 – 125.5 kg

N ha-1 above that recorded for the control lysimeters, respectively. Plants in soil

subjected to 15 Mg and 30 Mg ha-1 applications of low N manure assimilated 31.0 -

54.2 and 78.8 - 110.0 kg N ha-1 in excess of the N uptake by plants in unamended soil.

The use of 15 Mg and 30 Mg ha-1 high N manure instead of low N manure reduced N

uptake by 4.4 – 38.6 and 1.4 – 40.8 kg ha-1 respectively.

Trts First tomato (2007 - 8) First rape (2008 - 9) Second tomato (2008 - 9) Second rape (2008 - 9)

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N

uptake

Kg/ha

T1 3.0 7.0 20.9 6.0 1.5 9.1 3.1 9.1 28.2 14.7 2.2 32.3

T2 6.8 11.7 79.6 12.5 4.6 57.5 7.8 7.1 55.7 18.5 3.2 59.2

T3 8.5 17.2 146.2 16.0 8.5 136.2 8.6 16.1 138.2 21.0 5.8 121.8

Fpr * * * * * * * * * * * *

Lsd

(5%)

0.1 0.8 0.7 1.4 1.3 0.5 0.5 1.6 1.2 1.0 1.2 0.6

CV

%

1.0 4.1 5.3 6.2 15.4 5.4 4.2 6.6 7.4 3.8 12.4 6.8

Trts- Treatments; T1- Control, T2 – 100 kg N + 15 Mg high N manure ha-1, T3 – 200

kg N + 30 Mg high N manure ha-1, DM- dry matter yield, mgN/g DM- milligrams of

N per gram dry matter

Table 4.5: Dry matter yield and N uptake by aboveground plant biomass following application

of high N cattle manure

60

Trts – Treatments; T1- Control ( 0 Mg manure ha-1), T2 – 15 Mg low N manure ha-1,

T3 – 200 kg N + 30 Mg low N manure ha-1, DM- dry matter yield, mgN/g DM-

milligrams of N per gram dry matter

4.4.8 Correlations between measured variables

Fig 4.5 – 4.8 show the regression analyses indicating the direct and the indirect effects

of soil factors on N2O emissions and NO3-N in leachate following application of high

and low N manures. The regression analysis has shown that the relationships between

soil moisture and NO3-N in leachate were significantly (p<0.05) correlated with r2

values ranginging from 0.51 – 0.71. There were significant direct relationships

between the concentration of NO3-N in soil and the concentration of NO3-N in

leachate with r2 values ranging from 0.75 - 0.99. Higher values of the coefficients of

determination of the relationships between concentration of NO3-N in soil and the

concentration of NO3-N in leachate were recorded when compared with the r2 values

for the relationship between soil moisture and NO3-N in leachate.

Soil moisture and N2O emissions from soil were correlated significantly (p<0.05)

with coefficients of determination values ranging from 0.44 – 0.59. A direct

proportional relationship was recorded between NH4-N in soil and emissions of N2O

from soil with r2 values ranging from 0.4 – 0.73. The concentration of NO3-N in soil

Trts First tomato (2007 - 8) First rape (2008 - 9) Second tomato (2008 - 9) Second rape (2008 - 9)

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N

uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

T1 3.0 6.9 20.6 10.3 1.5 15.5 3.0 9.5 28.5 10.0 4.3 43.1

T2 6.6 11.3 74.8 10.4 4.5 46.5 6.4 11.6 74.2 13.4 5.3 71.0

T3 8.1 16.1 130.2 13.4 7.1 95.4 8.1 16.9 136.9 14.5 8.4 121.8

Fpr * * * * * * * * * * * *

Lsd

(5%)

0.1 2.4 0.7 0.7 1.5 0.3 0.2 2.6 0.6 0.4 2.2 0.2

CV

%

0.9 12.1 5.2 3.5 19.8 3.7 2.0 12.0 4.5 1.9 21.0 4.4

Table 4.6: Dry matter yield and N uptake by aboveground plant biomass following application

of low N manure

61

subjected to applications of high and low N manure was significantly correlated with

the emissions of N2O from soil. The r2 values the relationships between NH4-N, NO3-

N in soil and emissions of N2O were comparatively similar.

[NO3L] = 0.5[SM] + 21.71R2 = 0.51

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in lea

cha

te,

mg

/L

Soil moisture, g/100g

a) First tomato

[NO3L] = 1.1[SM] + 12.8R2 = 0.71

0

10

20

30

40

50

60

0 10 20 30 40 50 60NO

3 in lea

cha

te,

mg

/L

Soil moisture, g/100g

b) Second rape

[NO3L] = [NO3S] - 1.1

R2 = 0.99

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in leachate

, m

g/L

c) First tomato

[NO3L]= 0.878[NO3S]+ 1.1

R2 = 0.77

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in

le

ach

ate

, m

g/L d) First rape

[NO3L] = [NO3S]+ 0.05

R2 = 0.95

0

10

20

30

40

50

60

0 10 20 30 40 50 60NO3 in soil, mg/kg

NO

3 in leachate

, m

g/L e) Second tomato

[NO3L] = [NO3S] + 0.73

R2 = 0.95

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in

leachate

, m

g/L f ) Second rape

Fig 4.5 Regression analyses showing the effects of soil factors on NO3-N in leachate

following application of low N manure. [NO3S]- Nitrate N in soil, [NO3L]- Nitrate

N in leachate

62

.

[NO3L] = 0.36[SM] + 22.03R2 = 0.51

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in

le

ach

ate

, m

g/L

Soil moisture, g/100g

a) First tomato

[NO3L] = [SM] + 10R2 = 0.61

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in lea

cha

te,

mg

/L

Soil moisture, g/100g

b) Second rape

[NO3L]= NO3S] - 0.9

R2 = 0.99

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in leachate

, m

g/L

c) First tomato

[NO3L] = [NO3S] + 5.57

R2 = 0.75

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in

leachate

, m

g/L

d) First tomato

[NO3L] = [NO3S] + 2.2R2 = 0.86

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in leachate

, m

g/L

NO3 in soil, mg/kg

e) Second rape

[NO3L] = [NO3S] + 0.34R2 = 0.99

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in leachate

, m

g/L

NO3 in soil, mg/kg

f) Second tomato

Fig 4.6 Regression analyses showing the effects of soil factors on NO3-N in leachate

following application of high N manure. [NO3S]- Nitrate N in soil, [NO3L]- Nitrate

N in leachate

63

[N2O] = 0.32[SM] + 0.54R2 = 0.59

0

5

10

15

20

25

15 20 25 30 35

N2O

, g/h

a/d

ay

Soil moisture, g/100g

a) First tomato

[N2O] = 0.32[SM] + 0.54R2 = 0.44

0

5

10

15

20

25

15 20 25 30 35

N2O

, g/h

a/d

ay

Soil moisture, g/100g

b) Second rape

[N2O]= 0.91[NH4] - 0.22R2 = 0.63

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

c) First tomato

[N2O] = 0.19[NH4]+ 2.6R2 = 0.40

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

d) Second tomato

[N2O] = 0.43[NH4] + 3.25R2 = 0.41

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

e) First rape

[N2O] = 0.1[NH4] + 8R2 = 0.42

0

5

10

15

20

25

0 5 10 15 20

N2

0,

g/h

a/d

ay

NH4 N in soil, mg/kg

f) Second rape

[N2O]= 0.81[NO3]+ 2.4664R2 = 0.52

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NO3 N in soil, mg/kg

g) Second tomato[N2O]=0.39[NO3]+ 4.36

R2 = 0.50

0

5

10

15

20

25

0 5 10 15 20

N2

O, g/h

a/d

ay

NO3 N in soil, mg/kg

h) First rape

64

[N2O] = 0.197[NO3] + 2.77R2 = 0.37

0

5

10

15

20

25

0 5 10 15 20

N2

O, g

/ha

/da

y

NO3 N in soil, mg/kg

i) Second tomato

[N2O] = 0.11[NO3]+ 8.19R2 = 0.52

0

5

10

15

20

25

0 5 10 15 20

N2

O,

g/h

a/d

ay

NO3 N in soil, mg/kg

j) Second rape

[N2O] = 0.36[SM] - 1.911R2 = 0.58

0

5

10

15

20

25

15 20 25 30 35

N2O

, g/h

a/d

ay

Soil moisture, g/100g

a) First tomato[N2O] = 0.36[SM] + 1.911

R2 = 0.53

0

5

10

15

20

25

15 20 25 30 35

N2O

, g

/ha/d

ay

Soil moisture, g/100g

b) Second rape

[N2O] = 0.59[NH4] + 1.713R2 = 0.57

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

c) First tomato

[N2O]= 0.49[NH4] + 2.41R2 = 0.58

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

d) First rape

[N2O] = 0.16[NH4] + 2.96R2 = 0.63

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

e) Second tomato

[N2O] = 0.16[NH4] + 2.96R2 = 0.73

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

f) Second rape

Fig 4.7 Regression analyses showing relationships between mineral N, N2O and soil

moisture after application of high N manure

65

4.5 DISCUSSION

4.5.1 Cattle manure quality, nitrate N leaching and nitrous oxide emissions

The quality of cattle manure, a parameter that reflects its available nutrient supplying

potential and carbon content, is perhaps one of the most important properties of

organic materials added to a soil that govern the rate of decomposition and nutrient

release patterns in organic matter decay in soils (Mafongoya and Hove, 2008; Roberts

et al., 2009; Wuta and Nyamugafata, 2012). A comparatively large body of

knowledge has been generated in Zimbabwe which points to the fact that smallholder

cattle manures are generally of poor quality due to inadequate and low quality grazing

and inappropriate handling of the manure in kraals (Zingore et al., 2008; Nyamangara

and Makumire; 2010 Wuta and Nyamugafata, 2012). In this study, two manure types

were used to test the effect of their quality and application rates on nitrate N

concentration, N uptake and N leaching under wetland vegetable production. Results

of the chemical analysis of the two types of cattle manures showed quite the opposite

of these previous findings (Table 3.2; Section 3.5). The smallholder cattle manure

[N2O] = 0.66[NO3] + 2.7R2 = 0.41

0

5

10

15

20

25

0 10 20

N2O

, g

/ha/d

ay

NO3 N in soil, mg/kg

g) First tomato

[N2O] = 0.48[NO3] + 3.47R2 = 0.53

0

5

10

15

20

25

0 5 10 15 20

N2O

, g

/ha/d

ay

NO3 N in soil, mg/kg

h) First rape

[N2O] =0.17[NO3] + 2.99R2 = 0.71

0

5

10

15

20

25

0 5 10 15 20

N2O

, g

/ha/d

ay

NO3 N in soil, mg/kg

i) Second tomato[N2O] = 0.48[NO3] + 3.47

R2 = 0.54

0

5

10

15

20

25

0 5 10 15 20

N2O

, g

/ha/d

ay

NO3 N in soil, mg/kg

j) Second rape

Fig 4.8 Regression analyses showing relationships between mineral N, N2O and soil

moisture after application of low N manure

66

collected from a homestead within the wetland community contained more N than

that of the cattle manure collected from an adjacent commercial farming area. This

was due in greater part to the perennially green grazing of high forage value around

the wetland area, which acted as an effective supplement for the poor grazing in the

dry land portions of the Dufuya community.

Cattle manure quality and rate of application exerted a dominant control on

mineralized N concentration changes in soil and leachate, and N2O emissions.

However, mineral N concentration and N2O emission differences between high and

low N manured soil during the early stages of the first crop growth were

comparatively small. The two types of cattle manure had C: N ratios (17: 1 for high N

and 18: 1 for low N manures), which were below the threshold (24: 1) of net

immobilization of mineralized N (van der Meer, 2008; Nyamangara and Makamure,

2010; Mapanda et al., 2011). The narrow C: N ratios of high and low N manures may

not necessarily mean that they readily release mineralized N upon microbial

degradation (Mtambanengwe et al., 1998; Sleshi and Mafongoya, 2007). Other

manure quality parameters such as condensed tannins; soluble C and fibre-bound N

have also been highlighted as important modifiers of N release patterns (Van Der

Ploeg et al., 2007). The materials constituting aerobically composted high N manure

contain high levels of reactive phenols and tannins, which polymerize with a range of

amino acids (containing N) from microbially degraded crude proteins in manures to

generate complexes, which are resistant to enzymatic decomposition by micro

organisms in wetland soil systems (Sardans et al., 2008). This results in slow release

of mineralized N into the soil solution (leachate).

The first season after application of high and low N manures may have encouraged a

rapid growth of microbial biomass due to an abundance of organic substrate

(Gonzalez-Chavez et al., 2009), which placed a heavy demand for N on the limited

amounts of mineralized N from slowly decomposing manure in soil. The active

uptake and assimilation of the limited reserves of mineralized N by the microbiotica

in manure fertilized lysimeters is suspected to have weakened the superior potential of

the high N manure over low N manure to supply mineralized N in soil observed at 14

days after planting after application of the two types of cattle manure.

67

The high N manure (1.36% N) with narrow C: N ratio (17: 1) consistently supplied

more nitrate N into the soil solution in soil than low N manure (0.51%; C: N 18: 1).

There is considerable argument over how manure quality determines the mineralized

N release from added organic materials. Mineralized N release patterns from organic

materials are controlled by a hierarchical set of N, lignin and polyphenol content in

the added organic material (Markewich et al., 2010; Abro et al., 2011).

In this study insignificant (p>0.05) leachate volume responses to rates and manure

quality treatments for all four crops is reported. This implied that under equal weather

conditions leachate discharges from lysimeters subjected to the three application rates

of the two types of manure would be constant for the same soil conditions. It was,

therefore, leachate nitrate N concentration responses to treatments that introduced

changes in the quantities of N lost through leaching. Variations in the volumes of

leachate between seasons determined the quantities of N lost by leaching for each

crop. As a result, test crops grown during the wet summer season had higher losses of

N in the N fertilizer and cattle manure treatments. The highest loss of N by leaching

under the second tomato crop (planted in April 2008) were five times lower than the

highest N loss recorded for the first tomato crop (planted in September 2007).

In this study, the use of low N manure reduced loss of nitrate N through leaching.

This may be attributed to high total N content (1.36% N) in the high N manure. The

higher content of N in the high N manure increased the soluble N release capacity of

this type of cattle manure upon microbial decomposition. Dufuya wetland vegetable

farmers commonly use both types of cattle manure. The manure collected from kraals

around and within the wetland poses greater danger of the ground water

contamination with nitrates from vegetable production systems. The use of manures

collected from adjacent commercial farming area poses a reduced hazard of wetland

ground water contamination by nitrates.

Applications of irrigation water and incident rain in amounts beyond field capacity of

the wetland soil caused a net translocation of dissolved nitrate N in leachate where it

posed a potential hazard of ground water contamination. Increased use of N fertilizer

and animal manures has accentuated nitrate-N contamination (Ajdary et al., 2007,

Vogeler et al., 2007), because nitrate leaching is related to N fertilization rate (Stadler

et al., 2008; Claret et al., 2011). A wetland soil environment by its very nature has

68

such permanent soil wetness that the water table rarely falls beyond 15 cm from the

surface during the rainy season and under such conditions the margins between field

capacity and gravitational soil moisture levels are very narrow indeed. This means

that wetland soil conditions are highly susceptible to leaching processes. Lower

application rates of high and low N manures (15 Mg ha-1) caused reduced losses of N

by leaching. Such farming practice in the production of wetland vegetables at Dufuya

is likely to reduce risk of ground water pollution by nitrates.

Except for the first tomato (Fig 4.3), the concentrations of nitrate N in leachate were

generally higher in the unamended lysimeters when compared with the permissible

concentrations in leachate safe for potable water. This was attributable to the fact that

the experimental soil was previous under manure and N fertilized vegetable cropping

before commencement of the experiment. The residual effects of the previous manure

applications on the concentration of nitrate N persisted during the experimental

period.

4.5.2 Regression analysis between measured variables

The influence of soil moisture on variability found in NO3-N in leachate and

emissions of N2O after application of high and low N manures was only recorded in

the first tomato and second rape crops (Fig 4.5 – 4.8 a, b). The manure organic matter

decomposition processes are oxidative and proceed only in the presence of water at

least up to field capacity moisture content. The vegetative periods of the first tomato

and second rape crops occurred over both dry (September 2007 and 2008) and wet

summer seasons (after October 2007 and 2008, Fig 3.6). This introduced a change in

wetland hydrology that influenced dynamics in the N mineralization processes over

the cropping seasons in one cropping event.

The higher r2 values recorded for the relationships between concentration of NO3-N in

soil and the concentration of NO3-N in leachate when compared with r2 values for the

relationships between soil moisture and NO3-N in leachate indicate that NO3-N in soil

exerted a greater influence on concentration of NO3-N in leachate. Nitrate N in soil

act as a reserve for the supply of NO3-N in leachate.

69

Regression analysis conducted NH4-N and NO3-N in soil and N2O emissions (Fig 4.5)

showed the influence of NH4-N and NO3-N in soil on N2O emissions were

comparatively similar. Both processes of nitrification of NH4-N and denitrification of

NO3-N are thought to contribute immensely to the emissions of N2O although the

later has been suggested to play a bigger role in the emissions (Ma et al., 2007).

In a study on woody legume fallow productivity, biological N2-fixation, and residual

benefits to two successive maize crops in Zimbabwe Chikowo et al. (2004) reported

conflicting results with those reported in this study. The researchers observed that it is

neither moisture nor tillage per se that leads to increased N2O fluxes from cropping

systems, but rather accelerated soil N cycling. For the first tomato and rape and

second rape crops the rise in the N2O emission coincided with wet weather spells.

Dobbie et al. (1999) measured emissions of nitrous oxide from extensively managed

agricultural fields over three years and found exponential increases in nitrous and

nitric oxides fluxes with increasing soil water-filled pore space, temperature and soil

mineral N. In a study on the effects of temperature, water content and N fertilization

on emissions of nitrous oxide by soils, Smith et al. (1998) confirmed that exponential

relationships between N2O flux and both water-filled pore space and temperature are

only observed when soil mineral N is not limiting. While C from manure stimulates

microbial respiration, water in a wetland soil limits O2 diffusion in soil. Flooding of

wetland soil decreases atmospheric oxygen diffusion to the soil by a factor of 105 and

sets in motion a series of unique physical, chemical and biological processes in the

transformation of N derived from applied cattle manure not found in dry land soils.

4.5.3 Effect of manure quality and application rates on soil N uptake

Optimizing the three-way pact comprising N uptake, fertility inputs, and greenhouse

gases may minimize the contribution of croplands to global warming. The uptake of N

by crops may act as a bio-sink for N that can otherwise be exposed to denitification

(Mapanda et al., 2012a).

The use of low N manure reduced N uptake by all test crops. This was attributed to

the fact that the content of N in the smallholder manure was more than double the

content of N in commercial farm manure. The elevated content of N in the

smallholder manure increased its capacity to supply mineralized N for crop biomass

70

accumulation, which in turn increased uptake of N from the wetland soil system. The

N content and C: N ratio of applied organic residues is important for N

immobilization processes after incorporation, where microbial biomass acts as a sink

for N (Nyamangara and Makumire, 2010).

While doubling the rate of application of high and low N manures increased the N2O

fluxes from soil the same practice in vegetable production increased dry matter yield

and soil N uptake by much larger percentages for all the crops. In this respect, N2O

emissions stimulated by doubling the application rates of cattle manure to vegetable

production fall far short of the elevated uptake of N by wetland vegetable crops. This

implied that increasing cattle manure application rates from15 to 30 Mg ha-1 does not

necessarily mean that the potential for atmospheric pollution and nutrient N loss by

N2O emission increases two-fold. This was attributed to the improved N availability

in the vegetable plots subjected to double doses of manure applications, which

increased biomass build-up and the associated higher demand for N from soil solution

that would have been subjected to denitrification and subsequent evolution of N2O.

The net result was a reduction in the concentration of nitrate N in soil solution, which

is the substrate for denitrification processes. Nevertheless, the potential for pollution

of the atmosphere by N2O emissions was there because of poor N use efficiency often

associated with vegetable crops.

4.6 CONCLUSIONS

The hypothesis that N2O emission and nitrate in wetland soil increased with

increasing content of N in cattle manure (manure quality) was confirmed by the study

results. The application of high quality manures to wetland vegetable production

system significantly increases the loss of N from applied cattle manure through nitrate

leaching and emissions of N2O. Applications of cattle manure with high N content

(1.36%N) enhances agriculture’s contribution to the global emissions of N2O and

nitrate leaching to terrestrial environments. The rates of nitrate N leaching losses of

applied N were similar with global average nitrate N leaching losses of 19% of

applied N (Bin-Le Lin et al.; 2001). Percentages of N applied lost as N2O-N were

generally lower than the global default value of 1.25% of N applied. Percentage losses

71

of N in N2O of applied N were also lower than the global average rates of 0.2 – 2.5%

N2O-N of applied N.

72

CHAPTER FIVE

5.0 EFFECTS OF APPLICATION RATES OF MINERAL N

FERTILIZER AND CATTLE MANURE ON N2O EMISSION AND

NITRATE LEACHING

ABSTRACT

The quantity of N2O emitted and nitrate leaching on agricultural land is dependent on

fertilizer application rate and N crop uptake among other factors. Field experiments

were established at Dufuya wetland in order to determine the effect of application

rates of mineral N fertilizer and smallholder cattle manure to tomato and rape crops

on mineralized N concentrations in topsoil; N2O emission; nitrate leaching; dry matter

accumulation and N uptake by tomato and rape crops. A randomised complete block

design with three treatments and four replications was used in each experiment. The

control (unamended); 100 kg ammonium nitrate + 15 Mg cattle manure ha-1 and 200

kg ammonium nitrate + 30 Mg cattle manure ha-1 constituted the three treatments for

experiment 1. The control (unamended); 15 Mg manure ha-1; and 30 Mg manure ha-1

constituted the three treatments for experiment 2. Treatment effects on mineralised N

concentrations in topsoil, N2O fluxes on soil surface, nitrate leaching, dry matter

accumulation and uptake of N were significant (p<0.05). Increasing the application

rates of N fertilizer and cattle manure from 100 kg N + 15 Mg manure to 200 kg N +

30 Mg manure ha-1 increased (p<0.05) the concentration of mineralized N in soil by

53% and 76% during the growing season of the tomato and rape crops respectively.

When cattle manure applications were increased from 15 to 30 Mg ha-1 the

concentration of mineralized N in soil significantly (p<0.05) increased by 44% and

52% under tomato and rape crops respectively. Increasing the application rates of N

fertilizer and cattle manure from 100 kg N + 15 Mg manure ha-1 to 200 kg N + 30 Mg

manure ha-1 significantly (p<0.05) increased leaching of nitrate N by 64% and 58%

under tomato and rape, respectively. Increasing the rate of application of manures

from 15 to 30 Mg ha-1 increased (p<0.05) the loss of nitrate N through leaching by

69% and 51%, under rape and tomato crops, respectively. Doubling the rate of

mineral N fertilizer and cattle manure increased the loss of N in N2O emissions by

40% and 45% under tomato and rape crops respectively. When the applications rates

73

of mineral N fertilizer + manure were increased from 100 kg N + 15 Mg manure to

200 kg N + 30 Mg manure ha-1 loss of N in N2O emissions per unit dry matter yield

decreased significantly (p<0.05) by 0.02 – 0.03 kg N2O-N/Mg of harvested dry

matter.

5.1 INTRODUCTION

The concept of integrated nutrient management of organic and mineral nutrient

resources has become a dominant paradigm for research in smallholder agriculture

systems in sub-tropical Africa to ensure both efficient and economic use of scarce

resources (Nyamangara et al., 2003; Materechera, 2010; Wuta and Nyamugata, 2012).

Wetland cropping in Zimbabwe commonly involves the production of leafy

vegetables where animal manures are applied in combination with mineral fertilizers

(Nyamangara and Nyagumbo, 2010; Masvaya et al., 2011). The organic and mineral

fertilizers are applied in rates far in excess of those required by the growing

vegetables in order to avoid a yield depression of the high value crops from which the

farmers derive livelihoods. N recovery rarely exceeds 70% of applied N and averages

50% for most vegetable crops (Surya and Rothstein, 2011). Research on the impacts

of increased application of N inputs to agricultural wetland systems on nitrate N

leaching in soil is scarce in African regions.

The increase in N fertilizer use is now widely recognized as a major factor in the

increase in N2O emissions indicated by increases in atmospheric concentration. The

evidence points to a further major increase from agricultural sources in the future, in

view of projections that there will be a doubling of N fertilizer use in African

developing countries by 2025 (Bouwman, 1998). Intensive use of animal manure as

fertilizer is likely to be a major feature of agricultural systems of tropical and sub-

tropical regions as livestock numbers are likely to rise (Mosier et al., 2003). Recent

atmospheric studies have evidenced the imprint of large N2O sources in African

tropical/subtropical lands. The uncertainty in the contributions of N2O emissions from

African regions to the global anthropogenic N2O emissions is largely due to the

scarcity of data and low frequency of sampling in African tropical studies (Hickman

et al., 2011). Currently, there are no published studies from Africa examining the

74

response function relating N2O emissions to incremental increases in N inputs

(McSwiney and Robertson, 2005).

5.2 HYPOTHESIS AND OBJECTIVE

The following hypothesis was tested:

Increasing the application rates of smallholder cattle manure and N fertilizer increases

NO3-N leaching and N2O fluxes on wetland soil.

The specific objective of this study was to:

Determine the effects of application rates of smallholder cattle manure and N fertilizer

on NO3-N leaching and N2O fluxes on wetland soil.

5.3 MATERIALS AND METHODS

5.3.1 Experimental design and treatments

Two experiments were used in the study. Experiment 1 was used to determine the

effects of combined application rates of N fertilizer with smallholder cattle manure on

NO3-N leaching and N2O fluxes from wetland soil. Experiment 2 was used to

determine the effects of application rates of smallholder cattle manure on NO3-N

leaching and N2O fluxes from wetland soil.

A randomized complete block design with four replications was employed in each

experiment. The blocking factor was the slope gradient.

The following treatments were used in Experiment 1:

i. Control (unamended)

ii. 100 kg N fertilizer ha-1 + 15 Mg smallholder cattle ha-1

iii. 200 kg N fertilizer ha-1 + 30 Mg smallholder cattle manure ha-1

The following treatments were used in Experiment 2:

i. Control (unamended)

ii. 15 Mg smallholder manure ha-1

iii. 30 Mg smallholder manure ha-1

In Experiments 1 and 2, cattle manure was broadcast only once in the four cropping

seasons before planting of the first crop (tomato) in the first season in plots/

75

lysimeters. This is a common manure application practice at Dufuya wetland. The

manure (1.36%N) was evenly broadcast in the respective plots/lysimeters and then

incorporated into the topsoil a few days before transplanting the crop. In addition to

manure applications, ammonium nitrate (34.5%N) fertilizer (100 and 200 kg ha-1 N)

was applied to each crop in two equal applications (50 and 100 kg N ha-1) in

Experiment 2. The first application (basal) was broadcast evenly on the surface and

covered with soil a day before planting the first test crop. The second application was

applied a month after trans-planting the first test crop.

5.3.2 Experimental manure

The aerobically composted smallholder manure (high N manure) was collected from a

kraal of a homestead in a nearby village within the Dufuya wetland. The laboratory

procedure for the analysis of the smallholder cattle manure is described in section 3.5.

Results of the laboratory analysis of manure are shown in Table 3.2 of section 3.5.

Manure application rates were determined on a moisture free basis. The content of

moisture in manure varies depending on weather conditions.

5.3.3 Nitrous oxide and nitrate leaching measurements

Nitrous oxide measurements were performed following procedures described in

section 3.7 (Fig 3.4, Equations 3 - 5). Leachate sampling and analysis were described

in section 3.8 (Fig 3.5, Equations 6 and 7).

5.3.4 Dry matter sampling and analysis

Dry matter and N uptakes measurements were done following procedures described in

section 3.10 and Equation 8 of the same section.

5.3.5 Statistical analysis

Treatment effects on nitrate and ammonium N concentrations in soil, nitrate leaching,

N2O fluxes, nitrate leaching and N uptake by the test crops was performed using

Generalized Two Way analysis of variance (GenStat, 2003). Flux data were log-

transformed if needed, to normalize the distributions before the statistical analysis.

Differences between treatment means were judged significant at p ≤ 0.05 as

determined by Fisher’s protected least significant difference test. The Pearson

correlation coefficients between measured variables and their r2 values were

76

performed using Microsoft Excel. Significance of correlations between selected

variables was established using a GenStat analysis of correlation.

5.4 REULTS

5.4.1 Ammonium N concentrations in soil after N fertilizer and manure

application

Results presented in Fig 5.1 a - d show mean concentrations of soil NH4-N on plots

subjected to different application rates of mineral N fertilizer and cattle manure in

plots and lysimeters. Concentrations of NH4-N in soil increased significantly (p<0.05)

with increasing rates of mineral N and manure application throughout the study

period.

Except for the second crop (first rape crop) (Fig 5.4 b); NH4-N concentrations in soil

appeared to decrease as the growing seasons progressed. Temporal variations in NH4-

N concentrations in wetland soil on plots subjected to different rates of N fertilizer

and cattle manure showed decreasing concentrations as the vegetative period

progressed towards the end for the tomato and rape crops. Higher rainfall totals (Fig

3.6) appeared to coincide with reductions in the concentrations of NH4-N in soil. The

vegetative periods that experienced drier weather conditions were associated with

elevated concentrations of NH4-N in wetland soil. Results have shown that the second

split application of mineral N fertilizer was followed by an increase in the

concentration of NH4-N in soil for each crop. Increasing mineral N and manure

application rates from 100 kg N + 15 Mg manure to 200 kg N + 30 Mg manure ha-1

increased NH4-N concentrations in soil by 53% and 58% during the vegetative

periods of tomato and rape crops, respectively. When compared with the control, plots

subjected to 100 kg N + 15 Mg manure and 200 kg N + 30 Mg manure ha-1 recorded

110% and 111% higher NH4-N concentrations in soil for the tomato and rape crops,

respectively.

77

5.4.2 Ammonium N concentrations in soil after manure application

Ammonium N in soil increased significantly (p<0.05) as shown by the errors bars

(Fig 5.2) with increasing rates of application of manure. Increasing manure

Fig 5.1 NH4-N concentration in soil following application of N fertilizer and manure. AN

topdressing- 2ndapplication of ammonium nitrate fertilizer as top dressing a month after planting

each crop

78

application from 15 to 30 Mg ha-1 increased NH4-N concentrations by 32% and 43%

under tomato and rape crops respectively.

Ammonium N concentrations in soil subjected to application of 15 and 30 Mg ha-1 of

high N manure were 155% and 144% in excess of those recorded in the control

lysimeters. Except for the first tomato crop (Fig 5.2 b) concentrations of NH4-N in

soil decreased on average by 17.1 and 18.1 mg kg-1 in 15 Mg and 30 Mg ha-1 manure

0

5

10

15

20

NH

4 N

in s

oil,

mg/k

g

a) First tomato LSD 1.1mgNH4/kg soilSept --------------------Oct----------------------------Nov------------------------ ----Dec 2007

Dry season Wet season

0

5

10

15

20

NH

4 N

in s

oil,

mg/k

g

b) First rape LSD 1.0mgNH4/kg soilJan --------------------------------Feb--------------------------------March2008

Wet season

0

5

10

15

20

NH

4 N

in s

oil,

mg/k

g

c) Second tomato LSD 0.9mgNH4/kg soilApril --------------------May----------------------June-----------------------------July

2008

Dry season

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

NH

4 N

in s

oil,

mg/k

g

Days after planting

Control 15Mg/ha HN manure single 30Mg/ha HN manure single

d) Second rape LSD 0.8mgNH4/kg soilAug --------------------------Sept-------------------------------Oct/Nov 2008

Dry season Wet season

Fig 5.2: NH4-N concentration in soil following cattle manure application. HN- high N manure

79

treatments at the end of the growing season compared with those at the beginning of

the season, respectively.

5.4.3 Nitrate N concentrations in soil after N fertilizer and manure application

Results showed that N fertilizer and manure application rates had a significant effect

(p<0.05) on the concentration of NO3-N in soil (5.3 a - d). The highest NO3-N

concentrations in the soil (18.2 mg kg-1 soil) were recorded 14 after planting the first

tomato crop (Fig 5.6 a) on the 200 kg N + 30 Mg manure ha-1 plots.

A comparative analysis of the results presented in Fig 3.4 and 5.6 clearly show that

wet weather conditions induced lower NO3-N concentrations in soil normally

associated with increased nitrate leaching. The second split application of mineral N

fertilizer at 28 days after planting each crop was associated with an increase in the soil

NO3-N concentrations. For instance, plots subjected to 200 kg N fertilizer + 30 Mg

manure ha-1 recorded an increase from 13.1 to 15.6 mg NO3-N kg-1 soil at 28 and 42

days after planting the first tomato crop. Increasing the application rates of N fertilizer

and cattle manure from 100 kg N + 15 Mg manure to 200 kg N + 30 Mg manure ha-1

increased the concentration of soil NO3-N by 54% and 79% during the growing

seasons of the tomato and rape, respectively.

Generally, temporal changes in the concentration of NO3-N showed decreasing

concentrations towards the end of the growing season, except for the first rape crop

(Fig 5.6 b). Nitrate N in soil was lowest in the control plots (0.9 and 1.1 mg kg-1 soil

for second rape and first tomato at 84 and 98 days after planting. Soil NO3-N

concentrations in plots subjected to 100 kg N + 15 Mg manure and 200 kg N + 30 Mg

manure ha-1 were 1.5 and 2 – 7 times respectively in excess of those recorded in

unamended plots.

80

5.4.4 Nitrate N concentrations in soil after manure application

Results shown in Figs 5.4 indicate that the concentration of NO3-N in soil subjected to

15 Mg ha-1 manure was significantly (p<0.05) different from that recorded in soil

subjected to 30 Mg ha-1 manure application. Increasing the rates of manure

Fig 5.3: NO3-N concentration in soil following application of N fertilizer and manure. AN

topdressing- 2ndapplication of ammonium nitrate fertilizer as top dressing a month after planting

each crop

81

application from 15 to 30 Mg ha-1 increased the concentration of NO3-N in soil by

55% and 61% respectively. In addition, soil NO3-N concentrations during the

September 2007 to November 2008 period followed a pattern that comparatively

matched the rainfall events at Dufuya (Fig 3.6). Higher NO3-N concentrations were

recorded during the drier periods of the growing season especially for the first tomato

and rape; second rape crops.

The concentration of NO3-N in soil subjected to 15 and 30 Mg ha-1 of manure

exceeded that recorded in unamended soil by 182% and 246% respectively. Temporal

0

5

10

15

20

NO

3 N

in s

oil,

mg/k

g a) First tomato LSD 1.0mgNH4/kg soilSept --------------------Oct----------------------------Nov------------------------ ----Dec 2007

Single app 30 and 15T First split app 3.75Tand 7.5TDry season Wet season

0

5

10

15

20

NO

3 N

in s

oil,

mg/k

g

b) First rape LSD 0.9mgNH4/kg soilJan --------------------------------Feb--------------------------------March2008

Second split app 3.75Tand 7.5TWet season

0

5

10

15

20

NO

3 N

in s

oil,

mg/k

g

c) Second tomato LSD 0.9mgNH4/kg soilApril --------------------May----------------------June-----------------------------July 2008

Third split app 3.75 and 7.5T Dry season

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

NO

3 N

in s

oil,

mg/k

g

Days after planting

Control 15Mg/ha HN manure single 30Mg/ha HN manure single

d) Second rape LSD 0.9mgNH4/kg soilAug --------------------------Sept-------------------------------Oct/Nov 2008

Fourth split app 3.75 and 7.5TDry season Wet season

Fig 5.4: NO3-N concentration in soil following cattle manure application. HN- high N manure

82

trends in the concentrations of NO3-N in wetland soil show increasing concentrations

in all treatments towards the end of the growing periods of each crop.

5.4.5 Nitrous oxide fluxes in soil following application of mineral N fertilizer and

manure

Differences in the emissions of N2O recorded on plots that received different rates of

N fertilizer and cattle manure were statistically significant (p<0.05; Fig 5.54 a - d).

Generally, the second split application of mineral N fertilizer at 28 days after planting

each crop induced an increase in N2O emissions. increasing the rate of application of

N fertilizer and manure from 100 kg N + 15 Mg manure to 200 kg N + 30 Mg manure

ha-1 increased N2O fluxes from wetland soil by 21 (2.6 g ha-1 day-1) – 106% (7.3 g ha-1

day-1) to 29 (3.1 g ha-1 day-1) – 107% (7.1 g ha-1 day-1) for the tomato and rape crops,

respectively.

The N2O fluxes from plots that received 100 kg N + 15 Mg and 200 kg N + 30 Mg

manure ha-1 were 187%; 348% and 110%; 234% higher than those recorded on

control plots for the tomato and rape crops, respectively. Except for the second tomato

and rape, temporal changes in the emissions of N2O from wetland soil on plots

subjected to different rates of application of N fertilizer and cattle manure showed

decreasing emissions as the growing period progressed towards the end.

The highest emission of N2O was recorded on plots that received 200 kg N + 30 Mg

manure ha-1 at 14 days after planting the first tomato crop (Fig 5.5 a). Incidentally,

this increase in N2O emissions was recorded soon after application of the manure and

N fertilizer just before planting each crop. The split N fertilizer induced an increase of

N2O emissions from soil was recorded in gas samples collected not immediately after

the topdressing, but 42 days after planting each vegetable crop. In this instance, N2O

emissions increased by an average of 9.3 g ha-1 day-1 or 133% and 10.6 g ha-1 day-1 or

129% on plots subjected to 100 kg N + 15 Mg manure and 200 kg N + 30 Mg manure

ha-1 respectively.

83

5.4.6 Nitrous oxide fluxes in soil following application of manure

The means showing N2O fluxes at the time of gas sampling for the period between 21

September 2007 and 9 September 2008 are given in Fig 5.6. Results show that N2O

Fig 5.5: Nitrous oxide fluxes on soil following application of N fertilizer and manure. AN

topdressing- 2ndapplication of ammonium nitrate fertilizer as top dressing a month after planting

each crop

84

fluxes following manure application were significant (p<0.05) throughout the study

compared to the control (Fig 5.6).

Considerably higher N2O emissions were observed in the first gas samples collected

from vegetable plots amended with applications of 30 Mg ha-1 manure, which was

applied a week before planting the first tomato crop. In plots subjected to 15 and 30

Mg manure ha-1 applications, increased N2O fluxes persisted throughout the 98 and

84-day period for tomato and rape crops respectively. Though depressed, N2O fluxes

0

5

10

15

20N

itro

us o

xid

e,

g/h

a/d

ay

a) First tomato LSD 1.2mgSept --------------------Oct----------------------------Nov------------------------ ----Dec 2007

Dry season Wet season

0

5

10

15

20

Nitro

us o

xid

e,

g/h

a/d

ay

b) First rape LSD 0.9Jan -------------------------------- ---Feb--------------------------------March2008

Wet season

0

5

10

15

20

Nitro

us o

xid

e,

g/h

a/d

ay

c) Second tomato LSD 1.1April --------------------May----------------------June-----------------------------July 2008

Dry season

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

Nitro

us o

xid

e,

g/h

a/d

ay

Days after planting

Control 15Mg/ha HN manure single 30Mg/ha HN manure single

d) Second rape LSD 0.8Aug --------------------------Sept-------------------------------Oct/Nov 2008

Dry season Wet season

Fig 5.6: Nitrous oxide fluxes on soil following manure application. HN- high N manure

85

from plots subjected to higher rates of manure applications (30 Mg ha-1) were higher

than those from plots subjected to lower rates applications (15 Mg ha-1). When

manure application rates were increased from 15 to 30 Mg ha-1 N2O emissions

significantly increased by 29 and 34% for tomato and rape vegetable crops,

respectively.

5.4.7 Nitrate N concentration in leachate and leachate volume after application

of N fertilizer and manure

Application rate of mineral N fertilizer and cattle manure had a significant effect

(p<0.05) on the concentration of nitrate N in the leachate throughout the study period

(Fig 5.7). The concentrations of NO3-N in leachate samples collected from lysimeters

that received 100 kg N + 15 Mg manure and 200 kg N + 30 Mg ha-1 were 37% (3.8

mg L-1) – 200% (9 mg L-1) and 45% (6.5 mg L-1) – 487% (21.9 mg L-1) more than

those recorded in leachate samples from the control lysimeters.

When the rate of application of mineral N fertilizer and manure was increased from

100 kg N + 15 Mg to 200 kg N + 30 Mg manure ha-1 NO3-N concentration in leachate

increased significantly (p<0.05) by 20% (5.7 mg L-1) – 81% (19.8 mg L-1). Leachate

samples collected 14 days after planting each crop had comparatively low nitrate N

concentrations ranging from 3.1 – 10.8 mg L-1 in control lysimeters and 4.5 – 29.6 mg

L-1 in fertilized lysimeters.

Generally, the concentrations of NO3-N in leachate collected from lysimeters

subjected to mineral N fertilizer and cattle manure applications were above the 10 mg

NO3 N L-1 set by the United States Environmental Protection Agency (USEPA, 1990)

for safe drinking water (Fig 5.7 a - d). On average, NO3-N concentration in leachate

was 0.3 mg L-1 below the recommended safe NO3 N concentration for drinking water

in the control lysimeters for the first tomato crop (Fig 5.7a). The concentration of

NO3-N in leachate from lysimeters subjected to 100 kg N + 15 Mg and 200 kg N + 30

Mg ha-1 exceeded the recommended 10 mg L-1 for safe drinking water by 1.3 – 2 and

2.2 – 3 times, respectively. Control lysimeter concentrations of NO3-N exceeded the

safe drinking water standard by 0.4 – 0.5 times for the first rape, second tomato and

rape crops (Fig 5.7 b - d).

86

Fig 5.7: NO3-N leaching in soil following application of N fertilizer and manure. AN topdressing-

2ndapplication of ammonium nitrate fertilizer as top dressing a month after planting each crop

87

Results showed that effects of combined applications of mineral N fertilizer with

manure and cattle manure on volumes of leachate collected from lysimeters were

statistically not significant (p>0.05) during the study period.

Cumulative volumes of leachate accounted for 85 and 95% of the cumulative

precipitation received during the growing season of the first tomato and rape crops,

respectively (Fig 5.8). However, the cumulative volumes of leachate recorded during

the growing periods of the second tomato and rape crops exceeded cumulative

incident precipitation (excluding the 35 mm per week irrigation during the dry season

and mid-season droughts) by 5 and 26% at the end of the growing period,

respectively. Overally, total leachate volumes over the four cropping seasons

exceeded cumulative precipitation by 188.1 mm (16%).

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

SepSepOct OctNovDecDecJan Jan JanFebFebMarMarApr AprMayMayJun Jun Jul AugSepOct OctNovNov

Leachate

volu

me,

mm

/ p

recip

ita

tion

, m

m

Period

Cumulative leachate volume Cumulative precipitation

First tomatoFirst rape Second tomato Second rape

2007 2008

Fig 5.8: Cumulative precipitation and leachate volumes following various application rates of N

fertilizer and cattle manure to rape and tomato crops at Dufuya wetland

88

5.4.8 Nitrate N concentration in leachate after manure application

Results presented in Fig 5.9 indicate that increasing manure application rate from 15

to 30 Mg ha-1 significantly increased the concentration of NO3-N in leachate between

42 and 98 days after planting the vegetable crops. Shortly after application of cattle

manure, treatment separation from the control for both factors was comparatively

small (p > 0.05; 14 – 28 days after planting, Fig 5.9 a-d).

0

10

20

30

40

50

60

NO

3 in leachate

, m

g/L

c) Second tomato LSD 0.4mg/kg soilApril ----------------------------May--------------------------June-------------------------July 2008

Dry season

0

10

20

30

40

50

60

NO

3 N

in leachate

, m

g/L a) First tomato LSD 0.6mg/kg soil

Sept -------------------------Oct--------------------------Nov------------------------------Dec 2007Dry season Wet season

0

10

20

30

40

50

60

NO

3 N

in leachate

, m

g/L b) First rape LSD 0.7m/kg soil

Jan ---------------------------------------Feb------------------------------March 2008Wet season

0

10

20

30

40

50

60

14d 28d 42d 56d 70d 84d 98d

NO

3 N

in leachate

, m

g/L

Days after planting

0Mg/ha manure 15Mg/ha HN manure

30Mg/ha HN manure Safe NO3 concentration

d) Second rape LSD 0.5mg/kg soil

Aug ---------------------------------------Sept------------------------------------------Oct/Nov 2008Dry season Wet season

Fig 5.9: NO3-N concentration in leachate following manure application. HN- high N

manure

89

Increasing the rate of application of manure from 15 to 30 Mg ha-1 increased the

content of NO3-N in leachate by 30%. The concentrations of NO3-N in leachate from

lysimeters that were subjected to 15 and 30 Mg N manure ha-1 significantly exceeded

(p<0.05) that from unamended lysimeters by 130% and 360%, respectively.

The concentration of NO3-N in leachate from lysimeters subjected to the application

of 100 kg N + 15 Mg manure and 200 kg N + 30 Mg manure ha-1 exceeded that from

lysimeters that received 15 and 30 manure ha-1 by 88 and 76%, respectively.

Generally, leachate NO3-N concentrations from all lysimeters were above the USEPA

recommended 10 mg L-1. The concentration of NO3-N in leachate from lysimeters

subjected to the application of 15 and 30 Mg manure ha-1 was 138% and 184% above

the USEPA recommended concentration for safe drinking water.

5.4.9 Correlations between selected variables

The regression analysis of the relationships between measured variables after

application of high N manure is described in section 4.4.8 of Chapter four. Scatter

diagrams with regression equations are shown in Fig 4.6.

The content of soil moisture was significantly correlated with the concentration of

NO3-N in leachate with r2 values of 0.43 and 0.59. A large proportion of the

concentration of NO3-N in leachate (r2-values between 0.98 - 0.99, p<0.05, Fig 5.10)

could be predicted by concentrations of NO3-N in soil. Soil NO3-N was also an

important predictor of N2O emissions with r2 values ranging between 0.37 - 0.65.

Significant (p<0.05) positive linear correlations were recorded between NH4-N

concentrations in soil and N2O emissions from soil with coefficients of determination

values ranging from 0.30 – 0.56. This meant that the emissions of N2O from soil

increased with increasing accumulations of NH4-N in soil.

90

[NO3L] = 0.22[SM] + 21.8R2 = 0.43

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in

le

ach

ate

, m

g/L

Soil moisture, g/100g

a) First tomato

[NO3L] = 0.221[SM] + 20.143R2 = 0.59

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in

le

ach

ate

, L

Soil moisture, g/100g

b) Second rape

[NO3S] = [NO3S] + 1.24R2 = 0.99

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in lea

cha

te,

mg

/L

NO3 in soil, mg/kg

c) First tomato

[NO3L] = [NO3S] - 0.49

R2 = 0.98

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil, mg/kg

NO

3 in leachate

, m

g/L d) First rape

[NO3L] = 1.04[NO3S] + 0.38R2 = 0.98

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO

3 in leachate

, m

g/L

NO3 in soil, mg/kg

e) Second tomato

[NO3S] = [NO3L] + 0.38

R2 = 0.99

0

10

20

30

40

50

60

0 10 20 30 40 50 60

NO3 in soil mg/kg

NO

3 in

leachate

, m

g/L f ) Second tomato

[N2O] = 0.6061[NO3] + 1.6864R2 = 0.65

0

5

10

15

20

25

0 5 10 15 20 25

N2

O g

/ha

/da

y

NO3-N mg/kg

a) First tomato

[N2O] = 0.5998[NH4] + 3.5688R2 = 0.66

0

5

10

15

20

25

0 5 10 15 20 25

N2

O g

/ha

/da

y

NH4-N mg/kg

b) First tomato

Fig 5.10: Regression analyses showing the relationships between soil measured

variables and NO3-N in leachate after applications of N fertilizer and manure.

[NO3S]- Nitrate N in soil, [NO3L] - Nitrate N in leachate

91

5.4.10 N uptake following application of mineral N fertilizer and manure

The different application rates of ammonium nitrate fertilizer and cattle manure had

significant effect (p<0.05) on N uptake by the vegetable crops during the study period

(Table 5.1). Increasing N fertilizer and manure application rates from 100 kg N + 15

Mg to 200 kg N + 30 Mg ha-1 for the first tomato and rape crops, second tomato and

rape crops increased N uptake by 146%, 83%, 423% and 80% respectively. The

second rape crop, which was planted in September 2008, registered the greatest N

[N2O] = 0.3951[NO3-N] + 3.6969R2 = 0.44

0

5

10

15

20

25

0 5 10 15 20 25

N2O

g/h

a/d

ay

NO3-N mg/kg

c) First rape

[N2O] = 0.4873[NH4] + 4.6231R2 = 0.37

0

5

10

15

20

25

0 5 10 15 20 25

N2

O g

/ha

/da

y

NH4-N mg/kg

d) First rape

[N2O] = 1.6334[NO3] + 0.8143R2 = 0.59

0

5

10

15

20

25

0 5 10 15 20 25

N2O

g/h

a/d

ay

NO3-N mg/kg

e) Second tomato[N2O] = 1.5803[NH4] + 1.7877

R2 = 0.56

0

5

10

15

20

25

0 5 10 15 20 25

N2O

g/h

a/d

ay

NH4-N mg/kg

f) Second tomato

[N2O] = 0.3169[NO3] + 4.7322R2 = 0.38

0

5

10

15

20

25

0 5 10 15 20 25

N2

O g

/ha

/da

y

NO3-N mg/kg

g) Second rape [N2O] = 0.27[NH4] + 5.9569R2 = 0.30

0

5

10

15

20

25

0 5 10 15 20 25

N2O

g/h

a/d

ay

NH4-N mg/kg

h) Second rape

Fig 5.11: Regression analyses showing relationships between N2O flux on surface soil

and mineral N concentration in soil

92

uptake in soil subjected to 200 kg N + 30 Mg high manure (128.8 kg N ha-1) and 200

kg N + 30 Mg low N manure (121.9 kg N ha-1). The lowest levels of N uptake were

recorded in the control lysimeters (42.7 and 43.1 kg N ha-1).

Trts – Treatments; T1- Control, T2 – 100 kg N + 15 Mg manure ha-1, T3 – 200 kg N +

30 Mg manure ha-1; DM - Dry matter; mg/g- milligrams N per gram dry matter yield.

5.4.11 N uptake following application of high N manure

The uptake of N by aboveground biomass of tomato and rape crops after application

of high manure is described in section 4.4.7. Table 4.5 shows the results. Results

presented in Table 4.5 clearly showed that there were significant (p<0.05) manure

rates of application treatment effects on the uptake of N by all four crops in the study.

Fertilization of the vegetable crops using 15 Mg and 30 Mg ha-1 of manure increased

N uptake by 62.8 and 105.8 kg N ha-1 above that recorded for the control lysimeters,

respectively. Increasing application rates of cattle manure from 15 Mg to 30 Mg ha-1

increased N uptake by 58.8 kg N ha-1.

5.4.12 Estimated total N lost in leachate following application of N fertilizer and

manure

Significant differences in the total amounts of N lost as leachate NO3-N (<0.05) were

identified between treatments (Table 5.2). Total N lost through leaching consistently

increased with increasing application rates of mineral N fertilizer and cattle manure.

When N fertilizer and cattle manure application rates were increased from 100 kg N +

15 Mg to 200 kg N + 30 Mg ha-1 nitrate N leaching increased by 38 to 90% and 29 to

34% for the tomato and rape crops respectively. Lowest amounts of total N lost as

Trts First tomato (2007 - 8) First rape (2008 - 9) Second tomato (2008 - 9) Second rape (2008 - 9)

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

T1 2.9 9.8 28.4 9.9 1.0 9.9 2.9 12.3 35.7 14.8 2.2 33.1

T2 7.8 16.6 129.5 16.3 5.3 86.4 7.9 9.7 76.6 21.1 4.0 84.9

T3 9.6 32.9 315.8 18.9 8.0 151.2 9.7 17.3 167.8 25.1 7.0 175.7

Fpr * * * * * * * * * * * * Lsd

(5%)

0.1 0.3 2.3 0.9 0.8 0.7 0.1 0.7 1.3 0.2 1.0 1.3

CV% 0.5 0.8 9.1 3.1 5.6 4.8 0.9 2.5 11.1 0.6 7.9 11.0

Table 5.1: Dry matter yield and N uptake by aboveground plant biomass following

application of N fertilizer and manure

93

NO3-N were regularly observed in the control plots (1.9 to 13.7 kg N ha-1). Plots

amended with 100 kg N + 15 Mg manure and 200 kg N + 30 Mg manure ha-1 recorded

1.5 to 3 and 2.3 to 3.7 times higher losses of N as NO3-N in leachate when compared

with those on control plots.

Higher total N losses were observed for manure in combination with inorganic

fertilizer treatments in the first tomato (33.9 kg N ha-1) and rape crops (35.3kg N ha-1).

Temporal variations in the leaching losses of N appeared to have matched the patterns

of wet weather periods in summers. Generally, loss of N through leaching increased

with increasing rainfall events when increased leaching from soil was expected.

Lower total amount of N lost in leachate was recorded in the second tomato, a crop

which grew under dry weather conditions of the 2008 April to July winter season

when leachate break through was least expected. Generally, the proportion of applied

N lost in leachate was higher in the rape crop than in the tomato crop. When 100 kg N

+ 15 Mg manure and 200 kg N + 30 Mg manure ha-1 were applied to the tomato and

rape crops 3 to 8% and 11 to 27% of applied N was lost as NO3-N in leachate,

respectively.

94

Trts- Treatments; T1- Control, T2- 100 kg N + 15Mg manure ha-1, T2 - 200 kg N +

30Mg manure ha-1.

5.4.13 Estimated total N lost in leachate following application of cattle manure

Results for the loss of N in leachate following application of high N manure are

described in section 4.4.5. Results are shown in Table 4.2. Significant (p<0.05) nitrate

N leaching responses to treatments were recorded. Lowest amounts of total N lost in

leachate were regularly observed in the control lysimeters (1.3 to 9.6 kg N ha-1).

However, loss of N under the second tomato crop was generally low for the three

treatments when no rainfall event was recorded during the April to July 2008 winter

season. Higher total N losses were recorded when 30 Mg high N manure ha-1 was

Trt

First tomato crop First rape crop

Temporal

interval

(days after

planting)

Mean

leachate

[NO3N]

mgL-1

Mean

daily

leachate

volume,

L

N

leached

kgha-1

Total N

applied

kg/ha

%

leached

N of

applied

N

Temporal

interval

(days

after

planting)

Mean

leachate

[NO3N]

mgL-1

Mean

daily

leachate

volume,

L

N

leached

kgha-1

Total N

applied

kg/ha

%

leached

N of

applied

N

T1 0-21 3.1 0.7 0.1 - - 0-35 12.1 5.6 4.7 - -

22-49 7.8 3.7 1.6 - - 36-84 15.9 5.8 9.0 - -

50-98 13.9 5.4 7.4 - - - - - - - -

Tot

al

- - - 9.1 0 - - - - 13.7 0 -

T2 0-21 4.5 0.9 0.2 - - 0-35 22.7 5.9 9.4 - -

22-49 18.5 3.9 4.0 - - 36-84 31.5 5.8 17.9 - -

50-98 35.1 5.9 20.3 - - - - - - - -

Tot

al

24.5 304 8 - - - 27.3 100 27

T3 0-21 6.5 0.8 0.2 - - 0-35 31.2 5.9 12.9 - -

22-49 25.2 4.0 5.6 - - 36-84 38.8 5.9 22.4 - -

50-98 48.6 5.9 28.1 - - - - - - -

Tot

al

- - - 33.9 608 6 - - - 35.3 200 18

Fpr - * NS * - - - * NS * - -

Lsd - 0.9 0.3 0.1 - - - 1.4 1.0 0.3 - -

CV - 9.4 1.4 1.6 - - - 0.9 1.8 1.4 - -

Second tomato crop Second rape crop

T1 0-35 10.7 1.4 1.0 - - 0-35 10.6 1.1 0.8 - -

36-98 16.2 0.6 0.9 - - 36-98 15.3 5.7 8.5 - -

Tot

al

- - - 1.9 0 - - - - 9.3 0 -

T2 0-35 14.7 1.6 0.8 - - 0-35 16.0 1.1 1.2 - -

36-98 26.2 0.8 2.1 - - 36-98 26.7 5.7 14.9 - -

Tot

al

- - - 2.9 100 3 - - - 16.1 100 16

T3 0-35 24.4 1.8 3.1 - - 0-35 24.4 1.1 1.9 - -

36-98 35.1 0.7 2.4 - - 36-98 35.2 5.7 19.7 - -

Tot

al

- - - 5.5 200 3 - - - 21.6 200 11

Fpr - * NS * - - - * NS * - -

Lsd - 1.1 0.3 0.2 - - - 0.3 0.3 0.3 - -

CV - 8.2 4.6 4.9 - - 1.7 1.2 1.7 - -

Table 5.2: Estimated total N lost through leaching following application of N fertilizer and

manure

95

applied just before planting the first tomato crop (27.7 kg N ha-1) in September 2007

(27.7 kg N ha-1) and in the first rape (24.4 kg N ha-1). Both crops experienced

exceptionally wet summer seasons (Figure 3.11). Substituting 15 Mg and 30 Mg ha-1

of high N manure with 15 Mg and 30 Mg ha-1 of low N manure reduced N losses by

an average of 26.5% and 45.6% respectively.

5.4.14 Total N lost as N2O following N fertilizer and manure application

Significant differences in the total amounts of N lost as N2O (p<0.05) were identified

between treatments (Table 5.3). Total N lost as N2O consistently increased with

increasing application rates of mineral N fertilizer and cattle manure. Higher total N

losses through N2O emission were observed for manure in combination with

inorganic fertilizer treatments in the first tomato crop (1.74 kg N ha-1) and wet

summer seasons in the first (1.21 kg N ha-1) and second rape crops (1.31 kg N ha-1).

Lower total amount of N lost as N2O was recorded in the second tomato (0.77 kg N

ha-1), a crop which grew under dry weather conditions of the 2008 April to July winter

season.

Generally, the proportion of applied N lost as N2O was higher in the rape crop than in

the tomato crop. When 100 kg N + 15 Mg ha-1 manure were applied to the tomato

0.45 and 0.60% of applied N was lost as N2O emissions. The application of 100 kg N

+ 15 Mg ha-1 manure to rape was followed by 0.87 and 0.98% of applied N being lost

in N2O emission. In the application of 200 kg N + 30 Mg ha-1 manure to tomato and

rape crops, 0.30 and 0.40% of applied N was lost as N2O emission respectively This

shows that rape production has a greater potential to emit N2O into the atmosphere

than the production of tomatoes in wetlands at least for the adopted crop rotation and

fertilizer application practice.

96

Trts

First tomato crop First rape crop

Temporal

interval

(days after

planting)

Average

rate of

N2O

emission

g/ha/day

Total N

emitted

for the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O of

applied

N

Temporal

interval

(days after

planting)

Average

rate of

N2O

emission

g/ha/day

Total N

emitted

for the

period

kg/ha

Total N

applied

kg/ha

% emitted

N2O of

applied N

T 1 1 - 21 5.9 0.12 - - 1 - 49 6.1 0.30 - -

22 - 49 2.5 0.07 - - 50 - 84 4.0 0.14 - -

50 - 63 2.7 0.04 - - - - - - -

64 - 98 6.1 0.21 - - - - - - -

Total 0.44 0 0 - - 0.44 0 0

T 2 1 - 21 13.5 0.29 - 0.10 1 - 49 12.6 0.62 - 0.62

22 - 49 9.7 0.28 - 0.10 50 - 84 7.2 0.25 - 0.25

50 - 63 10.2 0.14 - 0.05 - - - - -

64 - 98 14.8 0.52 - 0.20 - - - - -

Total - - 1.24 304 0.45 - - 0.87 100 0.87

T 3 1 to 21 17.8 0.38 - 0.10 1 - 49 16.7 0.82 - 0.41

22 to 49 18.8 0.53 - 0.13 50 - 84 11.2 0.39 - 0.20

50 - 63 14.1 0.20 - 0.05 - - - - -

64 - 98 17.9 0.63 - 0.15 - - - - -

Total - - 1.74 608 0.30 - - 1.21 200 0.61

Fpr - - * - - - - * - -

Lsd - - 0.16 - - - - 0.23 - -

CV - - 13.40 - - - - 12.20 - -

Second tomato crop Second rape crop

T 1 1 - 98 3.3 0.32 - - 1 - 35 3.0 0.11 - -

- - - - - - 36 - 84 4.3 0.21 - -

Total 0.32 0 - 0.32 0 -

T 2 1 - 98 6.1 0.60 - 0.6 1 to 35 8.7 0.31 - 0.31

- - - - - - 36 - 84 13.6 0.67 - 0.67

Total 0.60 100 0.60 0.98 100 0.98

T 3 1 - 98 7.9 0.77 - 0.4 1 - 35 11.8 0.41 - 0.21

- - - - - - 36 - 84 18.2 0.90 - 0.45

Total - - 0.77 200 0.40 - - 1.31 200 0.66

Fpr - - * - - - - * - -

Lsd - - 0.14 - - - - 0.24 - -

CV - - 15.70 - - - - 8.80 - -

Trts – Treatments; T1- Control, T2 – 100 kg N + 15 Mg manure ha-1, T3 – 200 kg N +

30 Mg manure ha-1.

Table 5.3: Estimated total N as N2O after application of N fertilizer and cattle manure

97

5.4.15 Total N lost as nitrous oxide after application of cattle manure

Results for the estimated N lost in N2O emission after application of high N manure

are shown Table 4.4 and described in section 4.4.6 of Chapter four. There were

significant treatment effects (p<0.05) on total N lost through emission of N2O during

the growing period of the vegetable crops (Tables 4.4). N2O emissions consistently

increased with increasing cattle manure applications.

Estimated total N lost to the atmospheric environment through N2O emissions on

plots subjected to 15 and 30 Mg ha-1 manure applications were 42 and 55% above the

emissions recorded on control plots (Table 4.4). Higher total N losses through N2O

emission were observed in manure application treatments in the first tomato crop

(0.97 kg N ha-1). When 15 and 30 Mg high manure ha-1 were applied once in four

cropping events 0.37 and 0.24% of applied N was lost as N2O, respectively, during

the growing period of the first tomato crop. When manure applications were increased

from 15 to 30 Mg ha-1 losses of N through emissions of N2O significantly increased

by 22%.

5.4.16 Total N in N2O emission and nitrate leaching per unit dry matter

Table 5.4 shows N lost in N2O emission and nitrate leaching per unit of harvested dry

matter yield. When the application rates of mineral N fertilizer were increased from

100 kg N + 15 Mg to 200 kg N + 30 Mg manure ha-1 the emissions of N2O per unit

harvest dry matter of rape and tomato significantly decreased (p<0.05). However, the

estimated losses of N in nitrate leaching significantly increased with increasing rates

of application of mineral N fertilizer and cattle manure to wetland cropping of tomato

and rape. The estimated loss of N in N2O emissions significantly (p<0.05) decreased

by 0.02 – 0.03 kg N-N2O per Mg of harvested dry matter when mineral N fertilizer

application rates were increased from 100 kg N + 15 Mg to 200 kg N + 30 Mg

manure ha-1. Nitrous oxide emission losses per unit harvested dry matter were

significantly (p<0.05) higher in the unamended plots than on mineral N and manure

fertilized plots (Table 5.4). Increasing the rate of applications of mineral N fertilizer +

manure and manure without N fertilizer significantly (p<0.05) increased to loss of N

in nitrate leaching by 0.1 – 0.19 and 0.02 – 1.45 kg N leached per Mg of harvested dry

matter of tomato and rape.

98

NL- Nitrate leached. TDM- Tonnes of dry matter yield ha-1. T1- Control, T2- 100 kg

N + 15 Mg manure/ha, T3- 200 kg N + 30 Mg manure/ha for N fertilizer and manure

treatments. T1- Control, T2- 15 Mg manure/ha, T3- 30 Mg manure/ha for manure

treatments

5.5 DISCUSSION

5.5.1 Mineralized N concentrations in fertilized soil

Mineral N concentrations in the 0 – 20 cm soil layer of different N fertilizer and

manure treatments examined in this study varied considerably. Results clearly

indicated that plots subjected to combined application of 200 kg N + 30 Mg ha-1

manure had the highest mineral N concentrations in soil. This was attributed largely

due to the superior capacity of the larger mass of substrate for ammonification and

subsequent nitrification in plots, which received 200 kg N and 30 Mg manure ha-1.

The application of aerobically composted manure with a high content of lignified C

(Wuta and Nyamugafata, 2012) in combination with N fertilizer widened the C: N

ratio. Under such conditions, the mineralized N reserve in soil observed in this study

is a net balance after reductions associated with N precipitation in the reaction

between reactive phenols and amino-acids from manure decomposition, assimilation

into microbial cell substance, nitrate leaching, denitrification and N crop uptake

(Yates et al., 2006; Mapfumo et al., 2007; Sileshi and Mafongoya, 2007). The high

availability of N to microbes ensured that immobilization of mineralized N was kept

Mineral N fertilizer and manure application

Treatments Tomato 1 Rape 1 Tomato 2 Rape 2

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

T1 0.40 0.93 0.44 1.38 0.11 0.31 0.02 0.63

T2 0.07 1.47 0.07 1.67 0.06 0.37 0.05 0.76

T3 0.04 1.63 0.05 1.87 0.04 0.56 0.03 0.86

Fpr * * * * * * * *

LSD5% 0.02 0.20 0.01 0.20 0.02 0.03 0.01 0.01

CV 1.3 4.4 3.2 1.2 1.1 2.4 0.7 1.5

Cattle manure application

Tomato 1 Rape 1 Tomato 2 Rape 2

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

N2Okg/

TDM

NL/

TDM

T1 0.15 1.03 0.07 1.92 0.11 0.26 0.03 0.65

T2 0.16 1.75 0.06 1.47 0.07 0.37 0.04 0.74

T3 0.10 3.2 0.04 1.49 0.04 0.56 0.02 0.82

Fpr * * * * * * * *

LSD5% 0.03 0.05 0.01 0.01 0.02 0.01 0.01 0.01

CV 2.8 3.2 4.2 2.3 1.1 1.4 4.5 7.8

Table 5.4: Loss of N in N2O emission and nitrate leaching per unit matter yield

99

low (Vogeler et al., 2007; Gonzalez-Chavez et al., 2009; Woznica, 2013). The net

result was an elevated content of mineralized N in plots that received higher fertilizer

rates.

Results the regression analysis imply that the concentration of NO3-N in soil

decreased with increasing content of soil moisture under first tomato and second rape

crops. Unlike cations, which carry a positive charge, the nitrate form of N tends to

repelled by negative charges on clay colloids. It is therefore, not actively adsorbed by

the colloidal complexes in the clay fraction of the soil. The soil used in this study is a

deeply weathered course textured loamy sand topsoil over sandy loam subsoil derived

from granite with clay content of less that 10% in the 0 –20 cm soil layer (Section 3.2;

Table 3.1). The cumulative effects of the absence of nitrate adsorptive capacity of the

topsoil and the limited content of clay fraction with colloidal nutrient adsorption

properties may have caused NO3-N leaching when soil moisture content exceeded

holding capacity (gravitational moisture). In this context, the content of NO3-N in soil

decreased through leaching as the soil moisture increased. In a study on NO3-N

leaching under different land uses of clayey and sandy soils Claret et al. (2011)

observed increasing nitrate leaching with increasing moisture content in soil.

5.5.2 Nitrate leaching in fertilized cropping

The initial applications of 50 kg and 100 kg ha-1 of ammonium nitrate in the 100 kg N

+ 15 Mg ha-1 and 200 kg N + 30 Mg ha-1 lysimeters respectively introduced readily

available N. This should have increased the concentration of mineralized N in soil and

leachate under reduced N uptake by poorly developed root systems and total biomass

of seedlings. The slow release of mineralized N from manure and the rapid N-

demanding microbial biomass build-up triggered by the introduction of C-rich manure

to lysimeter soil generated a small positive balance of nitrate N in the leachate

observed in the early stages of the growth and development of the vegetable crops.

All the three treatments recorded nitrate N concentrations above the 10 mg L-1 limit

for safe drinking water for first rape crop. This implied that in the event of the

leachate leakage reaching underground water reserve by seepage the potential for

nitrate contamination was high.

100

While wetland soil conditions are regarded as generally anoxic, significant pockets of

the soil profile are well aerated. The aerated portions of the soil profile encourage

active oxidative decomposition of nitrogenous matter in manure leading to elevated

release of mineralized N observed during the vegetative period of the second crop in

this study. Applications of 100 kg and 200 kg ha-1 of mineral N is suspected to have

narrowed the C: N ratio in manure (van der Meer, 2008; Nyamangara and Makumire,

2010; Wuta and Nyamugafata, 2012) thereby creating conducive conditions for net

mineralization of N-containing organic compounds in manure by microbial action

(Markewich et al., 2010; Abro et al., 2011). The result was the generation of a net

positive balance of nitrate N in leachate from lysimeters that received N fertilizer and

cattle manure applications observed in this study.

The combined application of mineral N and cattle manure is a very common farming

practice in smallholder crop production systems. This wetland farming practice at

Dufuya has the effect of increasing the concentration of available N beyond the levels

of microbial N demand, N precipitation by reactive phenols and N uptake by the crop

(Mafongoya and hove, 2008; Markewich et al., 2010; Abro et al., 2011; Woznica,

2013). This residual balance of N is subject to leaching under wetland vegetable

production systems where soil conditions are wet throughout the year. Increased use

of N fertilizer and animal manures has accentuated nitrate-N contamination (Ajdary et

al., 2007, Vogeler et al., 2007), because nitrate leaching in the ground water is related

to N fertilization rate (Stadler et al., 2008; Claret et al., 2011). This occurs because of

excessive nitrate-N accumulation in the soil profile (Wang et al., 2010) due to N

fertilization rates that exceed crop requirements, accompanied by poor soil and crop

management practices (Fan et al., 2011).

Increasing the rates of application of mineral N and manure from100 kg + 15 Mg to

200 kg N + 30 Mg manure ha-1 increased loss of N and the potential for ground water

contamination despite the low efficiency of smallholder manures. In a study on

seasonal fluctuations in the mineral N content and nitrate leaching in an alfisol, Claret

et al. (2011) reported elevated concentrations of NO3-N and nitrate leaching with

increasing N fertilizer applications.

101

Smallholder farmers commonly add maize stover in their kraals as bedding. Maize

stover is, in greater part, composed of lignified cellulose whose biodegradability is

very low (Mapfumo et al., 2007; Materechera, 2010). Solid excreta from animals are

therefore high in stabilized carbon and when used as manure its microbial degradation

is slow. Initial release of mineralized N for crop nutrition is slow and inadequate due

to immobilization by microbes and generation of ligno-protein complexes

(Mafongoya and Hove, 2008). The molecular structure of the lingo-protein complexes

are comparatively different from those of microbial enzymes involved in the

decomposition of organic matter in the soil (Yates et al., 2006). This shortcoming was

alleviated by the application of manure in combination with N fertilizer, which

increased the content of available N in the wetland soil beyond the threshold

requirements of crops and microbes, thus adding to the replenishments of available N

reserves subject to leaching and contamination of ground water. Consequently,

doubling the rates of combined applications of mineral N and manure increased the

loss of N through leaching by only 44%.

N leaching losses tended to increase with increasing amounts of rainfall received.

Dufuya wetland soils are deeply weathered loamy sands characterized by poor water

holding capacity and increased hydraulic conductivity due to high sand content and

poor soil aggregation. As a result, small external additions of water through irrigation

and incident rain tended to swiftly shift soil moisture levels from field capacity to

saturation water, which was discharged from lysimeters as leachate. The volumes of

leachate therefore tended to follow daily rainfall events in the periods preceding

leachate collection and measurements.

Increasing the rate of combined application of mineral N and cattle manure caused N

loss responses way below the large N uptake responses. While doubling the rates of

combined application of N fertilizer and manure increased N loss through leaching by

only 8 – 80% the same wetland vegetable farming practice increased N uptake by 83

– 423%. This implied that increasing N fertilizer and manure combined applications

from 100 kg N + 15 Mg manure ha-1 to 200 kg N + 30 Mg manure ha-1 does not

necessarily mean that the potential for ground water contamination by nitrates

increases two-fold. This was attributed to the improved N availability in the

lysimeters subjected to double doses of mineral N and manure, which increased

102

biomass accumulation and the appended increased demand for N from wetland soil

solution by the fertilized crop. The net result was a reduction in the concentration of

nitrate N in soil solution (leachate). This effectively reduced the risk of ground water

contamination by nitrates in doubled N fertilizer and manure application rates.

Nevertheless, the risk potential for nitrate contamination of ground water was high

because of the poor N use efficiency by vegetable crops. N recovery seldom exceeds

70% of applied N and averages 50% for most crops (Krouk et al., 2010; Liu et al.,

2010; Mishima et al., 2011; Surya and Rothstein, 2011).

5.5.3 Nitrous oxide fluxes from soil

In a review paper on methane and N oxide fluxes in tropical agricultural soils, Mosier

et al. (2003) reported that doubling the concentration of N2O in the atmosphere would

result in an estimated 10% decrease in the O3 layer and this would increase the

ultraviolet radiation reaching the Earth by 20%. Organic N in applied manure

becomes potentially damaging to the atmospheric environment through emissions of

N2O after undergoing heterotrophic microbial decomposition and mineralization

(Gonzalez-Chavez et al., 2009; Markewich et al., 2010; Kamaa et al., 2010). The

microbial degradation of nitrogenous organic substance (in manure crude protein)

may yield net mineralized N when N is turned into available/soluble forms (NH4-N

and NO3-N) or immobilized N (assimilated into microbial cell substance, and

therefore temporarily sequestrated from denitrification). However, whether organic N

in applied manure is immobilized or mineralized depends on the concentration of

available N in soil relative to the content of C in applied manure (Kamaa et al., 2010;

Mapanda et al., 2011). In related studies on dynamics of organic matter

decomposition and organic N mineralization Mtambanengwe et al. (1998) and Sileshi

and Mafongoya (2007) reported increased net N mineralization in decomposing

organic substrates with narrower C: N ratios. This implies that the application of N

fertilizer as a supplement to cattle manure in vegetable production enhances the

potential of cattle manure to release mineralized N into the soil ambience where it is

subject to N2O-releasing processes of nitrification and denitrification in soil. In this

context, the general recommendation that manure applications should be

supplemented by mineral N fertilizer amendments to improve available N supply in

soil has far reaching environmental consequences on the atmosphere. The practice

enhances the applied manure’s potential to release mineralized forms of N (Figures

103

4.3 and 4.4) by narrowing the C: N ratios (Table 3.2) into ranges favorable for net

mineralized N that is exposed to N2O-releasing microbial processes in soil. The lower

N use efficiency associated with vegetable crops (Krouk et al., 2010; Liu et al., 2010;

Mishima et al., 2011) meant that a larger pool of unused mineralized N in the soil

ambience was exposed to N2O-emitting process of denitrification in the current study.

In the current study, it was suspected that the application of manure in combination

with N fertilizer provided N for bacteria biomass synthesis and NO3-N formation in

quantities beyond losses caused by temporary immobilization, leaching, gaseous

emissions, formation of ligno-protein complexes of low biodegradability and crop

uptake. Nitrate N is a substrate in the processes leading to emissions of N2O (Rees et

al., 2006) when soil conditions become anaerobic after water saturation of the wetland

soil profile (Chirinda et al., 2010; Lesschen et al., 2011; Wang et al., 2012).

Results from the regression analysis implied that soil mineral N concentrations (NH4-

N and NO3-N) displayed significant (p<0.05) influence on the variability found in

N2O emission. Both processes of nitrification of NH4-N and denitrification of NO3-N

are thought to contribute immensely to the emissions of N2O although the later has

been suggested to play a bigger role in the emissions (Tiedje et al., 1984; Laffelar,

1986; Kuai and Verstraete, 1998; Bothe et al., 2000; Ma et al., 2007). In this study,

NH4-N and NO3-N exerted comparatively equal influence on the variability found in

N2O emissions on surface soil (r2 = 0.66 vs. 0.65 for first tomato crop; r2 = 0.37 vs.

0.24 for first rape crop; r2 = 0.56 vs. 0.59 for second tomato crop; r2 = 0.09 vs. 0.12

for second rape crop). In studies related to N transformations in soils, Berdad-Haughn

et al. (2006); Van Der Ploeg et al. (2007) and Billy et al. (2010) confirmed that

ammonification and nitrification are prior steps to loss of N by N2O emissions.

5.5.4 Total N lost as nitrous oxide and nitrate leaching

Mean ranges of total N lost as N2O from fertilized plots (0.32 to 1.74 kg N ha-1) may

appear small, but at global level, cumulative quantities of N lost as N2O are

significant because N2O has a global warming potential of 270 – 320 times compared

to CO2 (Ma et al., 2007; Snyder et al., 2009; Smith, 2012). In addition, N2O can last

approximately 100 to150 years (Grant and Beer, 2008; Munoz et al., 2010; Saggar et

al., 2010) in the atmosphere, so that a kg of N2O emitted is potentially more damaging

104

than the same unit of CO2. The relatively small amounts of total N lost per unit area as

N2O may explain why N2O is responsible for only 4 - 6% of the greenhouse effect

compared to 50% for CO2 (Grant and Beer, 2008; Smith, 2012). A number of

previous studies have indicated that 0.07 to 2.7% of applied N can be evolved as N2O

(Frimpong and Baggs, 2010). In a related study, Burke et al. (2002) reported

increased annual N2O emissions with increasing manure and N fertilizer rates.

The percentage losses of N in N2O of applied N in the h manure and N fertilizer +

manure treatments were generally lower than the global average rates of 0.2 – 2.5%

N2O-N of applied N (Mosier et al., 2003). Rates of nitrate N leaching losses against

applied N in the current study were comparably similar with global average nitrate N

leaching losses of 19% of applied N (Bin-Le Lin et al.; 2001). It is suggested that

lower rates of N losses in N2O emissions might be a result of high losses of applied N

through nitrate leaching especially under wetland conditions and lower rates of N

fertilizer applications in the sub-tropical Africa when compared with the rates in

south-east Asia and Western Europe.

High inputs of N fertilizers to rape and tomato crops are a major potential source of

nitrate leaching. The study has emphasized the importance N fertilizer and cattle

manure applications on loss of N in leachate. The loss of N in leachate was shown to

constitute an important nutrient flux, and the changes in the losses were determined

by varying application rates of mineral N fertilizer and manure.

5.5.5 Regression analysis

Results have shown that the concentrations of NO3-N in leachate were significantly

(p<0.05) dependent on the content of NO3-N in soil and soil moisture. This implies

that the concentration of NO3-N in leachate can be predicted by the content of soil

moisture and NO3-N concentrations in soil. However, the higher values of r2 in the

relationship between the concentration of NO3-N in soil and NO3-N in leachate meant

NO3-N concentration in soil is a better predictor of NO3-N in leachate. Similar values

of r2 in the relationship between NO3-N in soil and emissions of N2O when compared

with r2 values for the relationship between NH4-N in soil and emissions of N2O show

that the two independent variables exerted comparatively equal influence on the

emissions of N2O.

105

5.5.6 N uptake by the crops

N uptake by crops contributes immensely to the net balance of mineralized soil N that

is exposed to the processes of gaseous nitrous oxide emissions and nitrate leaching

when conditions conducive for these processes to occur set in. Raising the combined

application rates of N fertilizer and manure from 100 kg N plus 15Mg manure to 200

kg N plus 30 Mg manure ha-1 increased dry matter build up by 26%(2T); 18%(2.9T);

23%(1.8T) and 22%(4.6T) for the first tomato and rape; the second tomato and rape

crops respectively. In studies on N2Oemissions from rape field as affected by N

fertilizer management, Lin et al. (2011) observed increased NO3-N nutrient with

increased rates of N fertilizer. To this extent, increased uptake of N coupled with

elevated dry matter accumulations on plots that received higher manure and N

fertilizer applications acted as a biological sink for mineralized N that could have

been exposed to gaseous emission as N2O and leaching as NO3-N.

Increasing the rate of application of N fertilizer and manure from 100 kg N + 15 Mg

to 200 kg N + 30 Mg manure ha-1 increased N uptake. This was attributed to the

improved N availability in the plots subjected to double applications of mineral N and

manure, which increased biomass build-up and the appended higher demand for N

from soil solution that may have been subjected to denitrification. The net result was a

reduction in the concentration of nitrate N in soil solution, which is the substrate for

denitrification processes. Nevertheless, the potential for pollution of the atmosphere

by increased N2O emissions still remained because of poor N use efficiency by

vegetable crops. N recovery seldom exceeds 70% of applied N and averages 50% for

most crops. The N recovery rate for vegetable crops is even lower (Krouk et al., 2010;

Liu et al., 2010; Mishima et al., 2011).

5.5.7 Loss of N in N2O emission and nitrate leaching per unit dry matter

Increased dry matter accumulations on plots subjected to higher mineral fertilizer and

cattle manure applications was followed by higher uptake of N from the applied

fertilizers. Consequently, plots that were amended with higher rates of N fertilizer and

manure applications effectively sequestrated N that may be exposed to denitrification

and the associated emissions of N2O. This implies that when agronomic practices are

improved through fertilizer applications, the loss of N in N2O emissions may

significantly decrease. However, despite the enhanced N uptake in fertilizer

106

application practices, nitrate leaching losses increased. This is because of the fact that

the relationship between crop productivity and fertilizer N input is not linear, but

follows the diminishing return function (Van der Meer, 2008).

5.6 CONCLUSIONS

In this study it was hypothesized that increasing the application rates of N fertilizer

and smallholder cattle manure increases NO3-N leaching and N2O fluxes on wetland

soil. Results of the current studies have confirmed this hypothesis. There was an

increase in N2O emission from the soil and nitrate leaching when application rates of

N fertilizer and cattle manure were increased. The study has emphasized the

importance N fertilizer and cattle manure applications on loss of N as N2O and nitrate

leaching. The loss of N in N2O emission and nitrate leaching was shown to constitute

an important nutrient flux.

The average losses N in N2O emission were lower than the global default value of

1.25% of the mineral N applied, and it is suggested that this might be a result of the

generally lower fertilizer application rates in this study when compared with those

applied in developed countries of the north. Nitrate N leaching losses of applied N in

the current study were generally similar with global average nitrate N leaching losses

of 19% of applied N. The lower rates of N losses in N2O emissions were a

consequence of high losses of applied N through nitrate leaching especially under

wetland conditions.

The loss of N in emissions of N2O expressed per unit mass of harvested dry matter

yield of rape and tomato crops decreases significantly with increasing fertilizer

application, dry matter yield and N uptake. Improved agronomic practices for

increased crop productivity can be used as a mitigation factor for reducing the

contribution of agriculture in the global emissions of N2O.

107

CHAPTER SIX

6.0 THE EFFECTS SEASONAL SPLIT APPLICATION OF

CATTLE MANURE ON NITROUS OXIDE EMISSION

ABSTRACT

Precise cattle manure application timing increases the amount of N used by the crop

and thus reduces loss of N in emissions of N2O. A field experiment was carried out at

Dufuya wetland to determine the effects of cattle manure timing of application on

mineral N concentration in soil, emissions of N2O, dry matter yield and N uptake

during the growing seasons of rape and tomato crops. A cropping sequence of first

tomato, first rape, second tomato, and second rape crops grown between September

2007 and November 2008 was used in the experiment. Cattle manure collected from

the smallholder Dufuya community was used in the experiments. Two field

experiments were established. In the first experiment the manure was applied in three

levels of 0, 15, and 30 Mg ha-1 as a single application just before planting of the first

tomato crop. In the second experiment the 15 and 30 Mg ha-1 manure application rates

were divided into four split applications of 3.75 and 7.5 Mg ha-1 respectively. The

3.75 and 7.5 Mg ha-1 was applied to each of the four cropping events. Single

applications of 15 and 30 Mg ha-1 manure once in four cropping events had higher

concentrations of mineral N in the topsoil, N2O emissions, N uptake, and

aboveground dry matter yield than those recorded on plots that received split

applications of 3.75 and 7.5 Mg ha-1 manure at least up to the second test crop.

Thereafter, the concentrations of mineral N in soil, N2O emissions, N uptake, and dry

matter yield on plots subjected to split applications of manure were higher or equal to

those recorded in plots that received single basal applications of 30 Mg ha-1 applied a

week before planting the first crop. When 15 Mg manure ha-1 were applied as a single

application N2O fluxes significantly increased by 1.8 (36%) and 2.7 g ha-1 day-1

(43%) above those recorded from plots subjected to the first and second split

application of 3.75 Mg manure ha-1 applied a week before planting the first crop for

the tomato and rape crops, respectively. Single applications of 30 Mg manure ha-1

increased N2O emissions from wetland soil by 2.5 (38%) and 3.1 g ha-1 day-1 (34%)

108

above those recorded on plots that received the first and second split application of

7.5 Mg manure ha-1.

6.1 INTRODUCTION

Manure is a valuable source of N for crop production, but gaseous losses of manure N

as N2O reduce the amount of N available to the crop and, therefore, its economic

value as fertilizer (Lesschen et al., 2011). These N losses can also adversely affect air

quality, contribute to eutrophication of surface waters via atmospheric deposition, and

increase greenhouse gas emission (Mapanda et al., 2011). The longer the manure is in

the soil before plants take up its nutrients, the more those nutrients, especially N, can

be lost through volatilization, denitrification, leaching and erosion. The precise timing

of manure applications increases the amount of N used by the crop and thus reduces

loss of N in gaseous emissions of N2O (Saggar, 2010).

In general, few measurements of N2O emissions from soils have been made in sub-

tropical African agricultural systems, and many of those that have been made provide

an incomplete picture of the effects of various types of agricultural management on

trace N gas emissions (Hickman et al., 2011). Consequently, a study was carried out

in central Zimbabwe in order to determine the effect cattle manure application timing

on emissions of N2O on soil under field tomato and rape.

6.2 HYPOTHESIS AND OBJECTIVE

The following hypothesis was tested: Seasonal split application of aerobically

composted cattle manure increases the emission of N2O from wetland soil under

tomato and rape crops.

The main objective of the study was to determine the effect of seasonal split

application of aerobically composted cattle manure on N2O emission from wetland

soil under tomato and rape crops.

6.3 MATERIALS AND METHODS

6.3.1 Experimental manure

The aerobically composted smallholder manure (high N manure) used in this study is

described in section 3.5 of Chapter three.

109

6.3.2 Experimental design and treatments

Two experiments were used in the study. Experiment 1 is described in section 4.3.2 of

Chapter four under experiment one of that section. Experiment 2 had three treatments:

i. Control (unamended)

ii. 15 Mg high N manure ha-1 seasonal split application

iii. 30 Mg high N manure ha-1 seasonal split application

A randomized complete block design with four replications was employed. The

blocking factor was the slope gradient. The 15 and 30 Mg ha-1 manure rates,

applications were divided into four split seasonal applications over the study period in

which two tomato and two rape crops were planted. For the 15 Mg ha-1 cattle manure

treatments, the first application of 3.75 Mg ha-1 was done by evenly applying manures

in planting rows on the raised plots and then incorporating it a few days before

planting the first tomato crop. The balance of three applications of 3.75 Mg ha-1 was

applied to each of the remaining three crops in the study by applying into the planting

furrows and covering with soil before planting each crop. The same procedure was

repeated for the 30 Mg ha-1 high and low N manure treatments, which was divided

into four applications of 7.5 Mg ha-1 for each of the four crops.

A basal application rate of 1000 kg ha-1 compound S (5%N, 7.9% P, 16.6% K, and

8% S) was used in all treatments before planting each crop to capture common

fertilizer application practice at Dufuya wetland.

6.3.3 Gas sampling and analysis

Nitrous oxide flux and mineral N measurements were undertaken using the methods

described in sections 3.7 and 3.9, respectively. Dry matter and N uptakes

measurements were done following procedures described in section 3.10.

6.3.4 Statistical analysis

Treatment effects on nitrate and ammonium N concentrations in soil, N2O fluxes on

surface soil and N uptake by the crops was performed using Generalized One Way

ANOVA in completely randomized block design (GenStat, 2003). Flux data were log-

transformed if needed, to normalize the distributions before the statistical analysis.

Differences between treatment means were judged significant at p ≤ 0.05 as

110

determined by Fisher’s protected least significant difference test. The Pearson

correlation coefficients between measured variables and their r2 values were

performed using Microsoft Excel. Significance of correlations between selected

variables was established using GenStat analysis of correlation at 5% level.

6.4 RESULTS

6.4.1 NH4-N concentrations in soil following single and split application of

manure

Mineralized N concentrations in soil which were monitored at two-week internals for

each treatment and estimated over 98 and 84 days for tomato and rape crops

respectively are shown in Figures 6.1. The concentration of ammonium N in soil

subjected to single application was significantly (p<0.05) higher than that in soil

subjected to seasonal split applications during the growing period of the first tomato

and rape crops (Fig 6.1 a, b). However, only rates of high and low N manure

applications had a significant (p<0.05) effect on the differences in the concentrations

of NH4-N during the growing seasons of the second tomato and rape crops.

The effect of single and split application of cattle manure on NH4-N concentration

was not significant (p>0.05) during the growing period of the second tomato and rape

crops (Fig 6.1 c, d). Except for the first rape crop, NH4-N concentrations decreased

steadily towards the end of the growing period for each crop.

Single applications of 15 Mg of manure increased the concentration of NH4-N in soil

by 2.3 (30%) and 2.0 mg kg-1 soil (27%) above those recorded on plots amended with

the first and second split application of 3.75 Mg manure ha-1 for the first tomato and

rape crops, respectively. Single applications of 30 Mg manure ha-1 increased NH4-N

concentration in soil by 2.9 (29%) and 2.3 mg kg-1 soil (21%) above those recorded in

plots subjected to the first and second split 7.5 Mg ha-1 manure applications for the

first tomato and rape crops.

111

Fig 6.1: NH4-N concentration in soil following single and split application of manure.

HN- High N manure, app- application

112

6.4.2 NO3-N concentration in soil following single and split application of manure

Effects of single and split applications of manure on NO3-N were significant (p<0.05)

only up to the second split application while their effects became insignificant

(p>0.05) in the third and fourth split applications (Fig 6.2).

Generally, there were significant temporal variations in the concentrations of NO3-N

in soil from planting up to the cessation of the growing period of each vegetable crop.

Fig 6.2: NO3-N concentration in soil following single and split application of high N

manure. HN- High N manure, app- application

113

Application of 15 Mg ha-1 manure once in four cropping events significantly

increased (p<0.05) the content of NO3-N in wetland soil by 2.4 (40%) and 1.6 mg kg-1

soil (27%) above those recorded on plots amended with the first and second split

application of 3.75 Mg manure ha-1 for the first tomato and rape crops.

Single applications of 30 Mg manure ha-1 significantly (p<0.05) increased NO3-N

concentrations in soil by 2.4 (27%) and 1.8 mg kg-1 soil (21%) above those recorded

in plots amended with first and second split 7.5 Mg ha-1 manure for the first tomato

and rape crops, respectively. When a single application of 15 and 30 Mg ha-1 manure

were used instead of 3.75 and 7.5 Mg ha-1 applied as a third split application mean

NO3-N concentration differences between the two treatments approached similar

levels and were insignificant in the second tomato and rape crops (third and fourth

split applications). The mean differences in the concentrations of NO3-N in wetland

soil between plots amended with single basal applications at the beginning of the

experiment and split applications before planting the successive test crops

progressively became narrower towards the end of the experiment.

6.4.3 Nitrous oxide fluxes from soil following single and split application of

manure

Results show that N2O fluxes following single manure application were significant

(p<0.05) throughout the study compared to the control (Fig 6.3). Nevertheless, split

application of manure exerted significant (p<0.05) effect on N2O emissions within the

growing periods of the first tomato and rape crops only (Fig 6.3 a, b) when compared

with the control. Thereafter, the rates of manure applications rather than the factors of

single and split manure applications had significant effect (p<0.05) on the emission of

N2O from soil. Considerably higher N2O emissions were observed in the first gas

samples collected from vegetable plots amended with single applications of 30 Mg ha-

1 manure, which was applied a week before planting the first tomato crop. In single

manure applications, elevated N2O fluxes persisted throughout the 98 and 84-day

period for tomato and rape crops respectively. In split applications of manure, N2O

fluxes remained constant or gradually decreased despite additions of cattle manure

before each planting event.

114

0

5

10

15

20N

itro

us o

xid

e,

g/h

a/d

ay a) First tomato LSD 1.2g/ha/day

Sept --------------------Oct----------------------Nov------------------------ Dec 2007Single app 30T and 15T First split app 3.75T and 7.5T

0

5

10

15

20

Nitro

us o

xid

e,

g/h

a/d

ay

b) First rapeLSD 0.9g/ha/day

Sept --------------------Oct----------------------Nov------------------------ Dec 2007 Single app 30T and 15T First split app 3.75T and 7.5T

0

5

10

15

20

Nitro

us o

xid

e,

g/h

a/d

ay

c) Second tomato

LSD 0.7g/ha/daySept --------------------Oct----------------------Nov------------------------ Dec 2007

Single app 30T and 15T First split app 3.75T and 7.5T

0

5

10

15

20

14d 28d 42d 56d 70d 84d 98d

Nitro

us o

xid

e,

g/h

a/d

ay

Days after planting

Control 15Mg/ha HN manure single

15Mg/ha HN manure split 30Mg/ha HN manure single

30Mg/ha HN manure split

d) Second rape LSD 1.0g/ha/day Sept --------------------Oct----------------------Nov------------------------ Dec 2007

Single app 30T and 15T First split app 3.75T and 7.5T

Fig 6.3: Nitrous oxide fluxes following single and split application of high N manure.

HN- High N manure, app- application

115

Single applications of 15 Mg manure ha-1 increased N2O fluxes by 1.8 (36%) and 2.7

g ha-1 day-1 (43%) above those recorded from plots subjected to the first and second

split application of 3.75 Mg manure ha-1 applied a week before planting the first crop

for the tomato and rape crops, respectively. The same practice at 30 Mg manure ha-1

application levels increased N2O fluxes on wetland soil by 2.5 (38%) and 3.1 g ha-1

day-1 (34%).

6.4.4 Soil factors - N2O emission relationships

Regression analysis between measured variables after single application of

smallholder cattle manure is described in section 4.4.8 (Fig 4.7). The regression

analysis between selected measured variables after seasonal split application of cattle

manure is shown in Fig 6.4. Results show significant correlations (p<0.05) between

NO3-N; NH4-N in soil and emissions of N2O. Coefficients of regression in the

correlations between soil moisture and N2O emissions varied between 0.26 and 0.69

(Fig 6.4 e, f) when manure was split applied. The coefficients of regression (r2) values

for the positive linearity in the relationships between NH4-N concentrations in soil

and N2O emissions ranged from 0.41 and 0.78 after split manure application. The

coefficients of determination in the relationships between NO3-N in soil and N2O

fluxes on soil varied between 0.37 and 0.63.

[N2O] = 0.37[NH4] + 2.879R2 = 0.55

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

a) First tomato

y = 0.3231x + 3.5312R2 = 0.57

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

b) First rape

[N2O] = 0.24[NH4] + 2.5364R2 = 0.78

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

c) Second tomato [N2O] = 0.035[NH4] + 7.64R2 = 0.41

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NH4 N in soil, mg/kg

d) Second rape

116

6.4.5 Total N lost as nitrous oxide

The description of results on estimated loss of N through N2O emissions after single

applications of manure was done in section 4.4.6 of Chapter four (Table 4.4). Table

6.1 show estimated losses of N in N2O emissions following seasonal split application

of manure to rape and tomato. The effect of split applications of manure on emissions

of N2O were significant (p<0.05) during the growing period of the first tomato and

[N2O]= 0.42[SM] - 0.46

R2 = 0.26

0

5

10

15

20

25

10 15 20 25 30 35Soil moisture, g/100g

N2

O, g

/ha

/da

ye) First tomato

[N2O] = 0.28[SM] + 0.47R2 = 0.69

0

5

10

15

20

25

15 20 25 30 35

N2O

, g

/ha/d

ay

Soil moisture, g/100g

f) Second rape

[N2O] = 0.4[NO3] + 3.33R2 = 0.53

0

5

10

15

20

25

0 5 10 15 20

N2O

, g

/ha/d

ay

NO3 N in soil, mg/kg

g) FIrst tomato

[N2O] = 0.38[NO3] + 3.84R2 = 0.37

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NO3 N in soil, mg/kg

h) First rape

[N2O] = 0.21[NO3] + 2.85R2 = 0.63

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NO3 N in soil, mg/kg

i) Second tomato[N2O] = 0.05[NO3] + 7.634

R2 = 0.59

0

5

10

15

20

25

0 5 10 15 20

N2O

, g/h

a/d

ay

NO3 N in soil, mg/kg

j) Second rape

Fig 6.4 Regression analyses showing relationships between mineral N, N2O

and soil moisture after split application of manure

117

rape crops only. Thereafter, the rates of application and quality rather than the factors

of single and seasonally split manure applications had significant effect on total N lost

through N2O emissions. Losses of N through N2O emission on plots amended with

split applications of manure were 23 – 138% above the losses recorded on the control

plots. Amongst the manure amended plots, lower N losses of N2O emission were

recorded in the second tomato, a crop which grew under dry weather conditions of the

2008 April to July winter season.

Trts

First tomato crop First rape crop

Temporal

interval

(days

after

planting)

Mean

rate of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

Temporal

interval

(days

after

planting)

Mean

rate

of

N2O

emission

g/ha/day

Total

N

emitted

for

the

period

kg/ha

Total N

applied

kg/ha

%

emitted

N2O

of

applied

N

T 1 1 - 21 5.4 0.11 - - 1 - 49 6.0 0.30 - -

22 - 49 2.5 0.07 - - 50 - 84 3.9 0.13 - -

50 - 63 2.8 0.04 - - - - - - -

64 - 98 6.4 0.22 - - - - - - -

Total 0.44 0 0 - - 0.43 0 0

T 2 1 - 21 5.6 0.12 - 0.24 1 - 49 9.3 0.45 - 0.88

22 - 49 5.7 0.15 - 0.29 50 - 84 4.6 0.15 - 0.29

50 - 63 5.7 0.07 - 0.14 - - - - -

64 - 98 7.2 0.24 - 0.47 - - - - -

Total - - 0.58 51 1.14 - - 0.60 51 1.18

T 3 1 - 21 7.5 0.15 - 0.15 1 - 49 12.7 0.62 - 0.61

22 - 49 6.8 0.16 - 0.16 50 - 84 7.2 0.24 - 0.24

50 - 63 6.5 0.08 - 0.08 - - - - -

64 - 98 9.4 0.31 - 0.15 - - - - -

Total - - 0.70 102 0.69 - - 0.86 102 0.84

Fpr - - * - - - - * - -

Lsd - - 0.16 - - - - 0.23 - -

CV - - 13.40 - - - - 12.20 - -

Second tomato crop Second rape crop

T 1 1 - 98 3.3 0.32 - - 1 - 35 3.0 0.11 - -

- - - - - - 36 - 84 4.3 0.21 - -

Total 0.32 0 - 0.32 0 -

Trt 2 1 - 98 5.1 0.50 - 0.98 1 - 35 5.2 0.18 - 0.35

- - - - - - 36 - 84 7.3 0.35 - 0.68

Total 0.50 51 0.98 - - 0.53 51 1.04

T 3 1 - 98 5.9 0.58 - 0.57 1 - 35 7.7 0.27 - 0.26

- - - - - - 36 - 84 10.2 0.49 - 0.48

Total - - 0.58 102 0.57 - - 0.76 102 0.75

Fpr - - * - - - - * - -

Lsd - - 0.14 - - - - 0.24 - -

CV - - 15.70 - - - - 8.80 - -

Trt- Treatments; T 1- Control; T 2- 15 Mg high N manure ha-1 split into four 3.75 Mg

ha-1 applications per crop; T 3- 30 Mg high N manure ha-1 split into four 7.5 Mg ha-1

applications per crop

Table 6.1: Estimated total N lost through nitrous oxide emission following seasonal split application

of manure

118

When 15 Mg ha-1 of manure were applied once in the four cropping events N2O

emission increased by 31 and 38% above those recorded from plots subjected to the

first and second split application of 3.75 Mg manure ha-1 applied a week before

planting the first tomato and rape crops, respectively. Mean differences in total N lost

as N2O emission between plots amended with a single basal application of 30 Mg of

manure and those amended with the first and second seasonally split application of

7.5 Mg manure ha-1 were 39 and 13% for the first tomato and rape crops respectively.

As the study approached the last cropping event, mean differences in the loss of N

through N2O emissions between plots amended with single basal applications and

those that received seasonally split applications became progressively smaller and

insignificant.

When 15 and 30 Mg manure ha-1 were applied once in four cropping events 0.4 and

0.9% of applied N was lost as N2O, respectively, during the growing period of the

first tomato crop. When 15 and 30 Mg of manure were split applied into four

applications of 3.75 and 7.5 Mg ha-1 to every crop total N losses in N2O emission

represented 0.9 and 0.9% (for the rape crop); 0.8 and 0.6% (for the tomato crop) of

applied N. Generally, the proportion of applied N lost as N2O was higher in the rape

crop than in the tomato crop.

6.4.6 N uptake and aboveground dry matter yield

The dry matter yield and N uptake by rape and tomato crops after application of high

N manure is described in section 4.4.7 of Chapter four (Table 4.5). Dry matter yield

and N uptake following seasonal split application of manure are shown in Table 6.2.

The effects of single and split applications of manure on N uptake were significant

(p<0.05) for all vegetable crops (Table 6.2). N uptake was lowest in the control plots

and higher in plots that received 30 Mg of high N manure as a single application.

Plots amended with split applied high N manure recorded substantial reductions in N

uptake when compared with those recorded on plots amended with single manure

applications.

119

When 15 and 30 Mg high N manure ha-1 were applied once N uptake increased by

48.3 kg ha-1 or 59% and 102 kg N ha-1 or 67% in excess of those recorded in plots

amended with the first split applications of 3.75 and 7.5 Mg manure ha-1, respectively.

The second tomato crop experienced increase of N uptake of 63.4 kg ha-1 or 51% and

76.0 kg ha-1 or 43% in plots subjected to single applications of 15 and 30 Mg high N

manure ha-1 in comparison with those observed in plots amended with the third split

applications of 3.75 and 7.5 Mg manure ha-1 respectively.

Trts First tomato First rape Second tomato Second rape

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

DM

yield

T/ha

mgN/g

DM

N uptake

Kg/ha

T1 3 9.8 29.3 9.9 1.6 14.9 3.1 12.9 40.3 10.5 2.9 29.9

T2 3.3 10.4 33.6 11.2 2.9 32.8 4 15.5 62.0 13.3 6.2 82.8

T3 3.6 14.1 50.6 12.7 7.7 98.0 5.8 17 100.2 15 9.9 148.5

Fpr * * * * * * * * * * * *

Lsd

(5%)

0.1 3 3.9 0.2 0.2 1.4 0.1 2.3 2.7 0.2 0.5 5.6

CV% 0.9 15.3 5.9 1.2 2.7 1.6 1.8 10.4 2.3 1 4.2 4.5

T1- Control, T2 – 15 Mg high N manure ha-1 split into four 3.75 Mg ha-1 per

crop, T3 –30 Mg high N manure ha-1 split into four 7.5 Mg ha-1 per crop, DM-

dry matter yield, MgN/g DM- milligrams of N per gram dry matter

While N uptake responses to single applications of 15 and 30 Mg high N manure ha-1

were 37 – 67% above those in plots subjected to split applications of 3.75 and 7.5 Mg

manure ha-1 for the three previous crops, the same soil fertilization practice could

increase N uptake by only 3.4 kg ha-1 or 4% and 10.5 kg ha-1 or 7% respectively for

the last crop in the study.

Single and split applications of high and low N manure had a significant effect

(p<0.05) on the yield of aboveground dry matter throughout the study period.

However, the differences in dry matter yield between plots subjected to single

applications and those amended with the first split applications were larger than those

Table 6.2: Aboveground dry matter yield and N uptake after split application of manure

120

recorded between single manure applied plots and the plots amended with the fourth

split application of manure.

Single applications of high N manure at 15 Mg ha-1 stimulated an increase of 51, 11,

42, and 19% in dry matter yield in excess of those recorded on plots subjected to the

first, second, third and fourth split applications of high N manure. The application of

30 Mg manure ha-1 once during the study period caused an increase in dry matter

yield of 58, 23, 23, and 9% during the first, second, third and fourth split application

of 7.5 Mg ha-1 manure.

6.5 DISCUSSION

6.5.1 Effect of seasonal split manure application on mineral N and N2O emission

The cumulative residual effect of the split applications of 3.75 and 7.5 Mg ha-

1manure on the content of mineralized N surpassed or at least significantly narrowed

the mean differences in the content of mineral N between plots subjected to single

basal and split applications during the subsequent cropping events. This was attributed

to the case that the microbial decomposition of manure in a soil is a process that

occurs over a couple of seasons (Sardans et al., 2008; Materechera, 2010;

Nyamangara) and Nyagumbo, 2010). The residual effects of a previous season split

application of manure on the mineral N release potential of a manure-amended soil

system effectively increased the content of mineral N in vegetable plots subjected to

seasonal split applications of high and low N manures by the third and fourth

cropping events.

Plots subjected to single applications of manure had higher emissions of N2O than

those recorded on plots that received split applications for two consecutive cropping

events. This trend in the N2O flux responses to the treatments clearly suggested that

the one-off applications of 15 Mg and 30 Mg manure ha-1 provided substantially

higher masses of organic substrate (Groot et al., 2006; Mafongoya and Hove, 2008)

for the general purpose microbial degradation processes in aerated macro-pores of the

soil profile. In studies related to N transformations in soils Van Der Salm et al. (2006)

and Markewich et al. (2010) reported significant accumulations of mineralized N in

microbial organic matter decomposition. Upon flooding of the macro-pores with soil

121

water (Johnson et al., 2005, Bedard-Haughn et al., 2006) the nitrate N is subjected to

denitrification (Soren et al., 2006, Ma et al., 2007; Nyamadzawo et al., 2012).

Evidently, the contamination of the environment with nitrate has the potential of

increasing the contribution to atmospheric N2O emissions (Gonzalez-Chavez et al.,

2009; Markewich et al., 2010; Kamaa et al., 2011).

The mean differences in N2O emissions between plots amended with single basal

applications and those that received split applications became progressively smaller

and insignificant towards the last test crop. The decline in the potential of the

vegetable plots that received single basal manure amendments to evolve elevated

amounts of N2O over those that received split manure amendments is, quite clearly,

attributed to the rapid decrease in the capacity of the plots that received single manure

amendments to supply NO3-N. This decline in the content of mineralised N is a

consequent of successive N uptake without replenishments (Zotarelli et al., 2009;

Masvaya et al., 2011), precipitation in ligno-protein complexes of humus formation

(Yates et al., 2006; Mafongoya and Hove, 2008), N2O gas evolving denitrification

(Lin et al., 2011), and migration of NO3-N to ground water resources (Min et al.,

2010; Mapanda et al., 2012).

The content of easily decomposable simple proteins for deamination and subsequent

oxidation to generate NO3-N, which is a substrate for denitrification in manures

applied once during the study period progressively declined towards the last test crop.

In studies related to N conversions in soils Berd-Haughn et al. (2006); Yates et al.

(2006) and Van Der Ploeg et al. (2007) observed that nitrification is essentially a prior

step to the emission of N2O. Flooded soils have aerobic and anaerobic zones (Johnson

et al., 2005; Bedard-Haughn et al., 2006), allowing both nitrification and

denitrification to take place simultaneously. In fact, Snyder et al. (2009) concluded

that N2O emissions to the atmosphere are highest where conditions are sub-optimal

for nitrification and denitrification. Since the first process produces the substrate for

the second, N losses can be very high when the two processes are associated. As

much as 60 to 70% of fertilizer N applied to wetland crop may be volatilized as

oxides of N.

122

The magnitude and intensity of denitrifying reduction processes on wetland soil is

controlled by the amount of easily degradable organic matter (cattle manure quality),

their rate of decomposition (Nyamangara and Makumire, 2010) and the amount of

reducible nitrates (Ma et al., 2007). The net result was the narrowing of the mean

differences in the N2O fluxes between vegetable plots that received applications of

single basal manure amendments and those that were subjected to split applications.

Addition of manure to wetland soil enhances the potential for denitrification

(Mapanda et al., 2012a) and increased emissions of N2O gas (Ma et al., 2007) through

a general stimulation of microbial respiration, causing rapid oxygen consumption and

consequently an increase of anaerobic conditions for accelerated denitrification (Van

Groenigen et al., 2005). While C from manure stimulates microbial respiration, water

in a wetland soil limits O2 diffusion in soil (Van Groenigen et al., 2005; Bedard-

Haughn et al., 2006).

6.5.2 Regression analysis

The concentrations of NH4-N and NO3-N in soil are important predictors of N2O

fluxes in soil. The fact that the coefficients of regression for the relationships between

mineral N concentrations in soil and fluxes of N2O from soil are comparatively equal

(Fig 6.4) implies that the emissions of N2O can be equally predicted by the

concentrations of mineral N in soil.

6.5.3 Effect of single and seasonal split application of manure on soil N uptake

The NUE by the wetland vegetable crops could conceivably limit the loss of N

through denitrification. The differences between uptake of N by the crops subjected to

single basal applications and those receiving split applications of manure was smaller

at the end of the experiment. This trend in the uptake of N by the crops is attributable

to the initial abundance of N-rich easily decomposable organic compounds in the

manure.

Available forms of N became abundant in the wetland soil upon microbial

decomposition of the nitrogenous compound pools (Takaya et al., 2003), which

boosted root growth for the rapid uptake of N. The net positive balance of mineralized

N was a result of the excess N that remained in soil after immobilization by

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assimilation into the microbial cell substance (Vitten and Smith, 1993; Killham,

1994), precipitation by the formation of ligno-protein complexes (Nyamangara et al.,

2001, Uchimara et al., 2002), evolution into the atmosphere by denitrification

(Grenon et al., 2004, Van Der Salm et al., 2006) and nitrate migration to ground water

by leaching (Hallberg et al., 1985; Stanley et al., 1990; Ferguson et al., 1991). In

addition to that, the active uptake of the N by root systems acted as a trigger factor in

the rapid growth and development of the aboveground plant biomass in plots that

received subsidies of single basal applications of 15 and 30 Mg of manure per ha-1.

The vegetable plots amended with split applications at every cropping event had

initially insufficient N due to the limited quantities of manure added to create a larger

net balance of mineralized N for uptake by the poorly developed root systems of the

crops. The introduction of easily degradable C-rich materials in soil may have

triggered a burst of microbial growth and activity that placed a burden on the limited

quantities of mineralized N thereby depleting it significantly. Despite the

comparatively narrow C: N ratio, the quantities of N which was limited by the mass of

manure applied in small doses may have been insufficient to introduce relatively large

net balances of mineralized N after immobilization by microbes, precipitation by

reactive phenols from lignin degradation, emissions by denitrification and N loss by

nitrate leaching. The net result was a greater uptake of N in plots that received single

basal applications of 15 and 30 Mg low N manure over that in vegetable plots that

were subjected to split applications of 3.75 and 7.5 Mg low N manure observed in this

study.

However, as the residual effects of the slow decomposition of organic substances in

split added manure particularly in wetland oxygen limited soil profile conditions

became more apparent towards the end of the field experimental study. The

cumulative residual potential of the split applied manure to release mineralized N

equaled or surpassed that of the manure applied once at the beginning of the study

during the growing period of the last crop. As a result the difference in the uptake of

N between the test crops subjected to single basal applications and those amended

with split applications of manure became progressively smaller towards the end of the

experiment. It is noted however that despite the reported higher NUE by plants

subjected to applications of manure, our results conflict these findings for very clear

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reasons. Firstly, the plant biomass build-ups for the accelerated N assimilation were

limited by the small doses of manure for every cropping season. Secondly, wetland

vegetable production systems have an array of N leakage pathways that include

leaching by excess water and emissions by denitrification from wet anaerobic soil

which effectively depleted the mineralized N from manure applied in split doses.

6.6 CONCLUSIONS

Seasonal split application of aerobically composted cattle manure reduced the

emission of N2O on wetland soil under tomato and rape crops. Single applications of

30 and 15 Mg ha-1manure once in four cropping events increased concentrations of

mineral N in the topsoil, N2O emissions, N uptake and aboveground dry matter yield

at least up to the second crop over those recorded on plots that received split

applications of 3.75 and 7.5 Mg ha-1 manure. Thereafter, the concentration of mineral

N in soil and N2O emissions on plots subjected to split applications of manure were

higher or equal to those recorded in plots that received single basal applications of 15

and 30 Mg ha-1 applied a week before planting the first crop.

The use of split applications of manure as a measure to reduce emissions of N2O, is

only effective during the first two cropping seasons. The residual organic matter from

split applied manure in third cropping season and onwards may enhance the potential

of the soil system to evolve N2O into the atmospheric environment and therefore

increase the contribution of agriculture to the global warming problem.

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CHAPTER SEVEN

7.0 OVERALL CONLUSIONS, DISCUSSIONS AND RECOMMENDATIONS

7.1 NITROUS OXIDE EMISSIONS FROM WETLAND SOIL AMENDED

WITH CATTLE MANURE AND N FERTILIZER

7.1.1 Gaseous losses of N following N fertilizer and cattle manure application

Several studies have reported considerable emissions of N2O following the

breakdown of N compounds in manures that have been applied to soil. Soils

contribute substantially to N2O emissions, especially when fertilized. There is

increasing interest in measures that decrease production of N2O from agricultural

soils. To decrease N2O emissions effectively, a good knowledge of all its sources and

controlling factors is essential (IPCC, 2001; Vasileidou et al., 2011).

Study results have confirmed increased N2O emissions with increasing N fertilizer

and manure application rates to tomato and rape crops (Fig 5.5 and 5.6). It can be

concluded that increased application rates of mineral N fertilizer and cattle manure

increases N2O fluxes in soil. Generally, the proportion of applied N lost as N2O was

higher in the rape crop than in the tomato crop. When 100 kg N + 15 Mg ha-1 manure

were applied to the tomato and rape crops grown under wetland conditions 0.50% and

0.90% of applied N was lost as N2O emission respectively. The application of 200 kg

N + 30 Mg ha-1 manure to tomato and rape crops resulted in 0.30%, and 0.63% of

applied N being lost in N2O emission respectively (Table 5.3). It can therefore be

concluded that rape cropping in wetlands is potentially more damaging to atmospheric

environment by N2O emissions than tomato production at least for the current crop

rotation system and wetland soil fertility management.

It can generally be concluded that the additions of mineral N fertilizer and animal

manures to wetland soil in the sub-tropical Zimbabwe can be recognized as one of the

major drivers of N2O emissions into the atmosphere with global warming

consequences. However, the percentages of N applied lost as N2O-N when high and

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low quality manure; and mineral N fertilizer rates were applied to tomato and rape

crops under wetland conditions were generally lower than the global default value of

1.25% of N applied. In addition, percentage losses of N in N2O of applied N in the

high and low manure and N fertilizer + manure treatments were also lower than the

global average rates of 0.2 – 2.5% N2O-N of applied N computed from several studies

on N2O emissions in temperate agricultural systems (Mosier et al., 2003). It is

suggested that this might be a result of high losses of applied N through nitrate

leaching especially under wetland conditions and lower rates of N fertilizer

applications in the sub-tropical Africa when compared with the rates in south-east

Asia and Western Europe.

Although supplementation of cattle manure with mineral N fertilizer at the studied

rates is a generally recommended practice as it substantially increases N availability

to vegetable crops in the short term (Mugwira and Murwira, 1997, Nyamangara et al.,

1999), this study clearly showed that the practice in wetland vegetable production

increases gaseous loss of N to the atmosphere thereby contributing to the global soil

emissions of GHG.

Highest N2O emissions were recorded during the early growth stages of the first

tomato crop when cattle manure was recently applied despite the presence of dry

weather and field capacity soil moisture conditions that usually encourage lower N2O

emissions (Fig 5.8 and 5.6). The dry weather conditions meant that there was no

excessive soil moisture to reduce the O2 diffusion rate in soil. The elevated evolution

of N2O at the beginning of the growing season of the first tomato crop was not a result

of water saturated soil conditions but rather the increased soil oxygen-depleting

microbial respiratory processes and also the presence of available carbon. Soil oxygen

depletion coinciding with highest loss of N as N2O was caused by increased microbial

biomass respiration induced by the presence of freshly applied cattle manure a few

days before planting the first tomato crop. The higher losses of N in N2O emissions

under the first tomato were most likely associated with denitrification. Soil conditions

that have depleted oxygen generally encourage denitrification when organic carbon

and nitrate supplies are comparatively not limiting (Chirinda et al., 2010; Lesschen et

al., 2011; Wang et al., 2012).

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7.1.2 Effects of cattle manure quality on N2O emissions

Generally, it can be concluded that the use of 15 and 30 Mg low N manure ha-1

applied once in the four cropping events instead of the same rates of high N manure

decreases N2O fluxes from soil (Fig 4.4, Table 4.3 and 4.4). Contrary to the general

concept that smallholder cattle manures are of lower quality than commercial farming

area manures (Mugwira, 1984; Murwira, 1995; Mugwira and Murwira, 1997), manure

collected from the Dufuya community was found to be of higher quality than that

collected from adjacent commercial farming area on the basis of total N content

(Table 3.2). The concentrations of C and N in the high N manure were 2.5 and 2.7

times more than that in lower N manure. This was probably due to the high forage

value of the grazing around the wetland area.

Vegetable production practice that employ increased applications of both high and

low N cattle manure at Dufuya increased loss of N through N2O emission from the

wetland soil. The application of higher rates of high N manure was accompanied by

increased losses of N through N2O emissions when compared with the losses

associated with applications of low N manure. The applications of lower rate of high

and low N manures were followed by lower emissions of N2O, a result that is

favorable to the objective of lowering the contribution of agricultural sources to the

global greenhouse gas emissions. However, the same practice significantly reduced

dry matter yield (vegetable yield) (Tables 4.5 and 4.6). Smallholder farmers are

engaged in wetland vegetable farming to obtain higher yields for increased vegetable

sales when compared with dry land cropping. In this situation, there is a conflict of

local smallholder farmer goals and the global objective of reducing emissions from

agricultural sources. The use of low N cattle manure from the adjacent Vungu

commercial farming area might be a way to reduce emissions of N2O from wetland

vegetable production at Dufuya.

7.1.3 Effects of seasonal split application of cattle manure on N2O emission

In this study, the residual effects of a previous season split application of manure on

the mineral N potential release of a manure-amended soil system effectively increased

the content of mineral N in vegetable plots subjected to split applications of manures

by the third and fourth cropping events. Generally, it can be concluded that seasonal

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split applications of aerobically composted cattle manure to wetland vegetable crops

can reduce mineral N accumulation in soil and emissions of N2O at least up to the

second seasonal split application. By the third seasonal split application of manure,

residual mineral N in soil and emissions of N2O were not significantly lower than

those recorded on soils subjected to single applications once in four cropping seasons.

In the current study, the application of 15 and 30 Mg of high N ha-1 split applied into

four applications of 3.75 and 7.5 Mg ha-1 to every crop, had total N losses in N2O

emission representing 0.9 and 0.9% (for the rape crop); 0.8 and 0.6% (for the tomato

crop) of applied N. Generally, the proportion of applied N lost as N2O was higher in

the rape crop than in the tomato crop. It can be concluded that rape and possibly other

similar leafy vegetables production has a greater potential to emit N2O into the

atmosphere than the production of tomatoes in wetlands when cattle manure is used as

a fertilizer.

Based on the study results, it can be concluded that the estimated loss of N as N2O

emissions per unit harvested dry matter yield decreases when mineral N fertilizer

application rates are increased. The loss of N from applied fertilizer per unit harvested

dry matter yield decreases significantly with increasing harvested dry matter yield.

This implies that when agronomic practices are improved and dry matter production is

increased, the relative loss of N in N2O emissions may significantly decrease.

The most recent IPCC methodology (IPCC, 1997) for estimating direct N2O emission

from mineral fertilizers and manure applied to agricultural soils is based on

Bouwman’s (1996) work in which a large data set on the estimations of N2O

emissions were generated in temperate regions. It assumes the emission to be a fixed

percentage, 1.25±1 %, of the N applied. Outside Europe and North America all these

data are scant especially in sub-tropical regions of Africa. In compiling national GHG

inventories, it is good practice to use country-specific data, where available, for the

activity data and N2O emission factors. There is considerable scope for more data

sources to be considered in the IPCC emission factors from African regions. This

might introduce significant changes in the estimation of direct N2O emission from

agricultural soils by IPCC. The current study is of important significance in providing

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a more complete data set from sub-tropical Africa to IPCC for incorporation into the

predictive models for estimations of N2O emissions at the global scale.

7.2 NITRATE LEACHING IN WETLAND SOIL AMENDED WITH CATTLE

MANURE AND N FERTILIZER

7.2.1 Nitrate N leaching following application of N fertilizer and cattle manure

High inputs of nitrogenous fertilizers are a major potential source of nitrate

contamination of groundwater. The loss of N in leachate was shown to constitute an

important nutrient flux, and the change in the losses was determined by varying

application rates of mineral N fertilizer and manure. Generally, it can be concluded

that total N lost through leaching consistently increased with increasing application

rates of mineral N fertilizer and cattle manure.

The application of mineral N fertilizer as a supplement to cattle manure application is

a recommended practice in wetland vegetable production by resource-poor

smallholder farmers in the sub-tropics of Africa (Mutsamba et al., 2012). The

practice substantially increases the potential of the wetland vegetable production

system to increase nitrate leaching through enhanced net mineralization of organic N.

The practice enhances the manure applications to contaminate the aquatic and

terrestrial environments with nitrate overloads by narrowing the C: N ratios into

ranges for net mineralization of organic N in which nitrate form of N is generated.

The lower N use efficiency associated with vegetable crops meant that a larger pool of

unused nitrate N in soil was exposed to nitrate leaching process.

Dufuya wetland farmers are inclined to increase rates of manure and mineral N

fertilizer in order to reduce the risk of yield loss of the high value vegetable crops.

Reduced application rates of manure and N fertilizer will gradually lead to reduction

in ground water pollution with nitrates. It can be concluded that the production of rape

in wetlands is potentially more damaging to groundwater pollution than the

production of tomato crop at least for the current crop rotation system and wetland

soil fertility management. There is need to optimize agronomic practices so that

higher yields can be achieved without necessarily increasing application rates through

use of more efficient crop varieties.

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7.2.2 Nitrate N leaching from wetland soil following application of high and low

N manure

The results have clearly shown that the use of high N cattle manure significantly

increased the loss of N through nitrate leaching. The application of high N cattle

manure under vegetable production at Dufuya increases the risk of loss of N by

leaching and the associated potential of ground water contamination by nitrate N

overloads beyond safe drinking water nitrate concentration limits (10 mg NO3-N/L;

USEPA, 1990) for the Dufuya wetland community.

This research has demonstrated that the application of high and low N manure without

N fertilizer to tomato and rape crops reduced the total amount of N leached from soil

by 33 – 46% when compared with amount of N leached from soil fertilized with

combined application of mineral N fertilizer and manure. This implies that the

application of cattle manure without mineral N fertilizer may reduce ground water

pollution. However, application of high and low N manure to the vegetable crops

instead of combined applications of N fertilizer and manure reduced dry matter yield

ha-1 by 26 – 44%. While manure applications without N fertilizer may be a

recommended option from the environmental standpoint, this trade off has economic

disadvantages for the smallholder vegetable farmers who will have yield reductions.

The applications of lower rates of N fertilizer and cattle manure is recommended as

mitigation against increasing nitrate leaching in vegetable production at Dufuya.

Results from the current study have shown that 0.5% of applied N was lost as N2O

emissions against 6.9% of applied N lost in NO3-N leaching for the 100 kg N + 15

Mg ha-1 cattle manure applications for the tomato crop. When 200 kg N + 30 Mg ha-1

manure was applied to tomato crop 0.3% of applied N was lost as N2O emissions

against 4.9% of applied N lost in NO3-N leaching. The application of 100 kg and 200

kg ha-1 N fertilizer to rape crop had 0.9% and 15% of applied N lost as N2O-N

emissions and NO3-N leaching respectively. When 15 Mg ha-1 high and low manure

was applied 1 and 23%; 0.8 and 18% of applied N was lost in N2O emissions and

nitrate leaching respectively. The application of 30 Mg ha-1 high and low N cattle

manure had 0.8 and 18%; 0.6 and 13% of applied N lost as N2O emissions and nitrate

leaching respectively. It can be concluded from these results that more N in applied

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fertilizers is lost in nitrate leaching than in N2O emissions. When mineral N fertilizer

and cattle applications were increased the loss of N as N2O emissions per unit

harvested dry matter decreased significantly with improved N uptake. Therefore

agronomic practices that encourage more N uptake and therefore higher dry matter

yield should be promoted instead of just increasing N loading rates as is the case with

higher manure and N fertilizer rates.

There are no studies on nitrate leaching in wetland soils under vegetable crops carried

out in Zimbabwe. Previous studies have focused on nitrate leaching in dry land

conditions mostly on the maize crop. In a study on fertilizer use efficiency and nitrate

leaching in a tropical sandy soil in Zimbabwe, Nyamangara et al. (2003) reported

47% of applied N fertilizer lost in leachate under dry land maize. Related studies

carried out by Kamukondiwa and Bergstrom (1994a) reported NO3-N leaching losses

of up to 39 kg N ha-1 yr-1 from a field under maize and up to 18.6 kg N annually from

a field under irrigated winter wheat on deeply weathered sandy soil at Grasslands

Research Station. About 54% of applied N was reported to have been lost through

leaching by Hagmann (1994) and Vogel et al. (1994) on sandy soils when heavy rains

preceded N fertilizer application in Zimbabwe. In the current study nitrate N losses in

all treatments were below 27% of applied N in wetland soil under field tomato and

rape. The previous studies focused on nitrate leaching from manure and N fertilized

dry land cropping of maize. The current study focused on nitrate leaching in wetland

soil under rape and tomato production. The maize crop has a longer growing period

than the vegetable crops. In addition, the wetland ecosystem has higher water content

in soil which enhances nitrate leaching when compared with the dry land condition.

This probably explains why there is a wide difference in nitrate leaching rates

between the two cropping systems. However, rates of nitrate N leaching losses against

applied N in the current study were comparably similar with global average nitrate N

leaching losses of 19% of applied N (Bin-Le Lin et al.; 2001).

From the study results, it can be concluded that despite the enhanced N uptake with

increasing fertilizer applications, nitrate leaching losses per unit harvested dry matter

yield increased. This is attributed to the fact that the relationship between crop

productivity and fertilizer N input is not linear, but follows the diminishing return

function (Van der Meer, 2008).

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7.3 IMPLICATIONS FOR FARMERS

Cattle manure is the major source of nutrients for plant growth in the smallholder-

farming sector in the tropics and sub-tropics of Africa. However, the low capacity of

cattle manures as a source of N has prompted farmers to supplement the manures with

inorganic N fertilizer (Zingore et al., 2008). Vegetable production systems in the

tropics and elsewhere are mostly intensive because vegetables are high-value crops

(De Lannoy, 2001). Wetland gardeners apply manures in combination with mineral

fertilizers at rates far in excess of those employed in commercial agriculture in order

to avoid yield reductions (Andreini et al., 1995). The consequences to the terrestrial

and atmospheric environment of the larger inputs of N applied to wetland vegetables

are worsened by the fact that vegetables have a low N recovery rate (Lin et al., 2011).

Results from this study have shown that increasing the combined application rates of

N fertilizer and manure as a means to avoid vegetable yield decrease increases the

loss of N through gaseous emission of N2O and nitrate leaching. The elevated

emission of N2O and nitrate leaching from high input wetland vegetable production

systems does not only deprive the high value cash crops of nutrient N but increases

the risk of groundwater contamination and atmospheric pollution by N2O emissions

with deleterious consequences of global warming (Munoz et al., 2010; Saggar et al.,

2010). It is therefore advisable for wetland farmers to apply reduced doses of N

fertilizer and manure to vegetable production systems.

In view of the potential damage that nitrate leaching could cause to the ecology of

public water bodies through eutrophication and the deleterious effects of nitrate

overloads in ground water, Dufuya wetland gardeners are advised to reduce fertilizer

and manure application rates to vegetable production systems. Previous research

concluded that farmers in the central part of the Dufuya wetlands have abandoned

their domestic water wells because of nitrate contamination of ground water to

concentration levels beyond the 10 mg L-1 permissible for safe drinking. While the

use of cattle manure as a nutrient source for vegetable production reduces the risk of

groundwater contamination it is associated with lower yields of vegetables. The

Dufuya smallholder farmers derive most of their livelihoods from wetland market

133

gardening and any production vegetable production system that compromises crop

yields is likely to meet resistance in its adoption by the farmers.

Some farmers at Dufuya commonly apply manure in small doses in each cropping

event while others apply large doses once in 3 – 4 cropping events. Results in this

study have clearly demonstrated that small dose manure application per crop can only

reduce losses of N by nitrate leaching and gaseous emissions of N2O and therefore the

associated contamination of ground water by nitrates and the atmosphere by N2O at

least up to the second cropping event. Farmers are encouraged to apply lower manure

and fertilizer rates in split applications in combination with sound agronomic

practices that stimulate dry matter production and therefore increased nutrient uptake.

7.4 SUGGESTED FURTHER RESEARCH

Data drawn from temperate and tropical agricultural production systems demonstrate

that N2O is emitted in response to N fertilization (Mosier et al., 2003). There has been

much research in the past to determine the extent of anthropogenic sources of N2O

emissions. Difficulties in quantifying these sources have arisen due to large spatial

variability, year-to-year fluctuations, and seasonal variations of flux (Smith, 2012).

Further research on the emissions of N2O in N fertilized wetland cropping systems in

sub-tropical Africa using the continuous gas flux sampling methods should be

employed in order to capture the episodic emissions from the wetland cropping. This

may improve our estimation of the contribution of N2O to the total anthropogenic

greenhouse gas emissions from agriculture in the sub-tropical African regions.

Results of this study have shown that low combined application rates of mineral N +

cattle manure and application of manure without mineral N fertilizer are associated

with reductions in soil NO3-N and emissions of N2O. Declining concentrations of

NO3-N in soil will not only reduce the vegetable cropping system potential to

contaminate the ground water resource but also remove the substrate for

denitrification that lead to N2O emissions. In this context, the recommendation to

reduce fertilizer application rates in the vegetable production system is reasonable

from an environmental point of view. The recommendation has far reaching economic

consequences in the associated reduction in crop yields. The study could not provide

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quantitative economic analysis of low-fertilizer-use vegetable production systems.

There is a need to carry out further studies quantifying the economic consequences of

trading off reduction in vegetable yields for environmental protection.

The effect of N uptake in the sequestration and fixation of N that could otherwise be

exposed to loss by N2O emission nitrate leaching cannot be underestimated. Efficient

uptake of N by vegetable crops acts as a sink for the N that could otherwise

participate in the denitrification process and nitrate N migration to ground water with

deleterious effects on the environment. In this study, it could not be established

whether the N that was lost to the atmosphere and ground water originated solely

from the added fertilizer and cattle manure. As a result, future studies should employ

isotope labelling of N. Further studies in which NUE is established by producing a

nutrient balance of tagged N from fertilizer and manures should be undertaken.

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APPENDICES

APPENDIX 1: Summary of the results on N2O emissions and nitrate leaching

First tomato crop First rape crop

Total N

applied

kg/ha

Total N

lost as

N2O,

kgha-1

% N lost

of

applied

N

Total N

lost in

leachate

kgha-1

% N

lost

of

applied

N

Total N

applied

kg/ha

Total N

lost as

N2O,

kgha-1

% N

lost

of

applied

N

Total N

lost in

leachate

kgha-1

% N lost

of applied

N

Effect of N fertilizer and cattle manureapplication on N2O emission and nitrate leaching in wetland soil

Control 0 0.44 0 9.1 0 0 0.44 0 13.7 0

100N+15T 304 1.24 0.41 24.5 8 100 0.87 0.87 27.3 27

200N+30T 608 1.74 0.30 33.9 6 200 1.21 0.61 35.3 18

Second tomato crop Second rape crop

Control 0 0.32 0 1.9 0 0 0.32 0 9.3 0

100N+15T 100 0.60 0.60 2.9 3 100 0.98 0.98 16.2 16

200N+30T 200 0.77 0.40 5.5 3 200 1.31 0.66 21.6 11

Effect of cattle manure quality and application rate on N2O emission and nitrate leaching in wetland soil

First tomato crop First rape crop

Control 0 0.44 0 9.1 0 0 0.43 0 11.5 0

15T HN 204 0.76 0.37 11.9 6 0 0.70 0 18.4 0

30T HN 408 0.97 0.24 27.7 7 0 1.00 0 24.4 0

15T LN 76.5 0.65 0.85 10.8 5 0 0.52 0 13.4 0

30T LN 153 0.79 0.52 16.4 4 0 0.72 0 18.6 0

Second tomato crop Second rape crop

Control 0 0.32 0 1.3 0 0 0.31 0 9.6 0

15T HN 0 0.51 0 2.0 0 0 0.54 0 13.7 0

30T HN 0 0.60 0 4.4 0 0 0.75 0 17.2 0

15T LN 0 0.40 0 1.8 0 0 0.44 0 10.5 0

30T LN 0 0.50 0 2.3 0 0 0.59 0 14.1 0

Effect of quality, rate and seasonally split application of cattle manure on N2O emission in wetland soil

First tomato crop First rape crop

Control 0 0.44 0 - - 0 0.43 0 - -

153

3.75THN

Split 1

51 0.58 1.14 - - - - - - -

3.75THN

Split 2

- - - - - 51 0.60 1.18 - -

Second tomato crop Second rape crop

Control 0 0.32 0 - - 0 0.32 0 - -

3.75THN

Split 3

51 0.50 0.98 - - - - - - -

3.75THN

Split 4

- - - - - 51 0.53 1.04 - -

First tomato crop First rape crop

7.5 T HN

Split 1

102 0.70 0.69 - - - - - - -

7.5 T HN

Split 2

- - - - - 102 0.86 0.84 - -

Second tomato crop Second rape crop

7.5 T HN

Split 3

102 0.58 0.57 - - - - - - -

7.5 T HN

Split 4

- - - - - 102 0.76 0.75 - -

First tomato crop First rape crop

3.75T LN

Split 1

19.1 0.47 1.14 - - - - - - -

3.75T LN

Split 2

- - - - - 19.1 0.45 2.36 - -

Second tomato crop Second rape crop

3.75T LN

Split 3

19.1 0.41 2.15 - - - - - - -

3.75T LN

Split 4

- - - -- - 19.1 0.44 2.30 - -

First tomato crop First rape crop

7.5 T HN

Split 1

38.2 0.54 1.41 - - - - -- - -

7.5 T HN

Split 2

- - - - - 38.2 0.50 1.31 - -

Second tomato crop Second rape crop

7.5 T HN

Split 3

38.2 0.50 1.31 - - - - - - -

7.5 T HN

Split 4

- - - - - 38.2 0.60 1.57 - -

HN- high N manure, LN- low N manure, Split 1, Split 2, Split 3, Split 4- first, second,

third and fourth seasonally split manure applications.

154

APPENDIX 2: JOURNAL PUBLICATIONS