Effect of C-N on Composting of Pig Manure With Sawdust

Waste Management 24 (2004) 805–813

www.elsevier.com/locate/wasman

Effect of C/N on composting of pig manure with sawdust

G.F. Huang a, J.W.C. Wong b,*, Q.T. Wu a, B.B. Nagar b

a College of Natural Resources and Environment, South China Agricultural University, PR Chinab Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong, Hong Kong

Accepted 30 March 2004

Abstract

The aim of this composting trial was to evaluate the effect of C/N on the composting process of pig manure with the purpose of

reducing the amount of sawdust normally used as co-composting materials. Two aerobic static piles were prepared consisting of pig

manure mixed with sawdust at an initial C/N of 30 (pile A) and 15 (pile B), respectively. Pile B containing larger amount of pig

manure showed a slower rise in temperature, lower maximum temperature, and shorter thermophilic phase than pile A. It also

resulted in higher pH and electrical conductivity (EC) values, and even higher contents of soluble NH4-N and volatile solids

throughout the composting period. Chemical and biological parameters including dissolved organic carbon (DOC) (4932 mg kg�1),

soluble NH4-N (371 mg kg�1), C/Nsolid (18.3), C/Naquoeus (5.8) and seed germination index (GI) (66.5%) indicated that pile A

achieved maturity after 49 days of composting. After 63 days of composting, pile B contained 5352 and 912 mg kg�1 of DOC and

soluble NH4-N content, respectively, which was much higher than the criterion of 5% and 400 mg kg�1, indicating its immature

nature. Pile B showed a relatively low GI value of 46%, which may be due to its high indigenous EC value as a result of larger

amount of pig manure. Therefore, co-composting of pig manure with sawdust at a low initial C/N would require a composting

longer than 63 days, and, the high salinity due to the large amount of pig manure would pose a potential inhibition on plant growth.

� 2004 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, intensive livestock production has

resulted in high density of animals in small areas, pro-

ducing large quantities of solid waste with relatively

insufficient nearby land for application. This has led to

environmental concerns including odor pollution,methane emissions, N and P pollution of waterways

(Tamminga, 1992). The high N and P discharges lead to

eutrophication, and adversely affect the growth and di-

versity of aquatic life (Morse, 1995; Lopez-Real and

Baptista, 1996). In Hong Kong, the pig industry has

witnessed a steady growth in recent years. In the last

decade, indiscriminate disposal of animal waste, par-

ticularly pig manure (22,000 ton annually), was reportedto account for 70% of total stream pollution in the New

Territories and about 50% of organic matter find access

to the sea. This serious environmental pollution has

* Corresponding author. Tel.: +852-3411-7056; fax: +852-3411-5995.

E-mail address: [email protected] (J.W.C. Wong).

0956-053X/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2004.03.011

called for suitable environmentally and economically

feasible technologies for animal waste treatment.

Currently, composting is used as a major treatment

technology for livestock waste generated locally (Hong

Kong Environmental Protection Department

(HKEPD), 1999), which provides an environment

friendly alternative method for disposal of solid organicwastes, because it leads to stabilization, and utilization

of organic waste. Most studies show that mature com-

post application to agronomic soils increase crop pro-

duction due to its high plant nutrient contents and

moisture retention characteristics. It also improves the

physical properties of the soil because organic materials

such as agricultural wastes and sewage sludge are de-

graded into relatively stable compost that can serve assoil conditioner (McConnell et al., 1993; Wong et al.,

1996). However, immature compost, when applied to

soils, maintains high decomposition activity, which may

retard plant growth due to nitrogen starvation, anaer-

obic conditions and phytotoxicity of NH3 and some

organic acids (Mathur et al., 1993; Fang et al., 1999a).

Therefore, compost maturity and stability are key

mail to: [email protected]

Table 1

Selected physicochemical properties of raw materials

Parameter Pig manure Sawdust

pH 8.12a (0.08)b 5.55 (0.07)

EC (dS m�1) 2.90 (0.05) 0.02 (0.00)

Moisture content (%) 68.3 (0.17) 8.12 (0.35)

Total organic carbon (%) 36.6 (0.77) 46.5 (2.78)

Total nitrogen (%) 3.24 (0.12) 0.07 (0.00)

Total phosphorus (%) 1.72 (0.01) 0.006 (0.000)

C/N ratio 11.3 (0.58) 664 (14.6)

a Values are means of triplicates (dry weight basis).b Values in parentheses are standard error (n ¼ 3).

806 G.F. Huang et al. / Waste Management 24 (2004) 805–813

factors during application of composting process. For

achieving compost maturity, environmental factors such

as temperature, aeration, moisture and nutrients should

be appropriately controlled (Epstein, 1997). C/N is

among one of the important factors affecting compostquality (Golueke, 1977; Michel et al., 1996). It is rec-

ommended to maintain C/N at 25–30 as it is considered

as the optimum ratio for composting. Bhamidimarri and

Pandey (1996) have successfully co-composted piggery

wastes with sawdust at C/N of 25–30. They reported

that sawdust appeared to be an ideal bulking agent for

composting pig manure because of its ability to absorb

moisture, and its structure provides adequate porosity inthe compost heap. However, the effects of co-compo-

sting with sawdust at low initial C/N on the decompo-

sition process and on the quality of the mature product

are not well understood.

Due to the unavailability of sawdust locally in Hong

Kong, a lower C/N for composting was expected to re-

duce the amount of sawdust needed, hence providing

local economic incentive. The aim of the present studywas to investigate the changes in physicochemical and

biological properties of co-composting of pig manure

with sawdust at an initial C/N of 30 and 15, respectively,

so as to evaluate the effect of the initial C/N on the

composting of pig manure.

2. Materials and methods

2.1. Composting pile establishment

Pig manure and sawdust were collected from a pig

farm and a sawmill located in Taipo, Hong Kong. Two

composting piles were prepared constituting of pig ma-

nure and sawdust in a ratio of 3:2 (w/w, fresh weight)

and 4:1 (w/w, fresh weight), in order to achieve C/N of30 (pile A) and 15 (pile B), respectively. The purpose of

using sawdust was to adjust the C/N as it has relatively

high C content. Twigs were broken down into small

pieces with a hammer mill and added to the piles at 10%

(v/v) as a bulking agent for increasing the porosity.

Windrow composting piles of approximately 8 m3

each, were composted for 63 days. The heaps were

turned every 3 days using a front-end loader. Themoisture content was adjusted to about 60–70% at the

beginning of composting and then maintained at

the same level throughout the composting period. The

temperature was measured daily at a depth of 60 cm

from the top of the composting piles. Triplicate samples

were collected from each pile at day 0, 3, 7, 14, 21, 35, 49

and 63, and stored at 4 �C immediately till analysis. Sub-

samples were air-dried, ground to pass through a0.25-mm sieve and stored in a desiccator for further

analyses. The selected physicochemical properties of the

experimental materials are shown in Table 1.

2.2. Physicochemical properties of compost

The aqueous compost extracts were obtained by me-

chanically shaking the sampleswith double distilled water

(DDW) at a solid:DDW ratio of 1:10 (w/v, dry weight

basis) for 1 h. The suspensions were centrifuged at 12,000

rpm for 20 min and filtered through 0.45 lm membrane

filters. The filtrates were used for the following analyses.

pH was determined using an Orion 920 ISE pH meter;electrical conductivity (EC) by an Orion 160 conductivity

meter; soluble organic carbon by SHIMADZU TOC-

5000A Total Organic Carbon Analyzer; NH4-N by the

Indophenol Blue method (Page et al., 1982); PO4-P by the

Molybdenum Blue method and NO3-N by the Copper-

ised Cadmium Reduction method (Page et al., 1982).

Total organic carbon was determined by Walkley and

Black method, total nitrogen and total phosphorus byKjeldahl digestion method. Total soluble organic N and

organic P contents weremeasured by aKjeldahl digestion

method on the soluble extract followed by NH4-N and

PO4-P determination by methods described before. The

moisture content (oven-dried at 105 �C for 24 h) and

volatile solids content (weight loss on ignition at 550 �Cfor 16 h) were also determined (Page et al., 1982). E4/E6

ratiowas determinedon the extracts using 0.1MNa4P2O7

extraction followed by spectrometric determination at

465 and 665 nm, respectively (Page et al., 1982).

2.3. Cress seed germination index test

Seed germination and root length test were carried out

on water extracts by mechanically shaking the fresh

samples for an hour at a solid:DDW ratio of 1:10 (w/v,dry weight basis). About 5.0 ml of each extract was pip-

etted into a sterilized plastic petri dish lined with a

Whatman #2 filter paper. Ten cress seeds (Lepidium

sativum L.) were evenly placed on the filter paper and

incubated at 25 �C in the dark for 48 h. Triplicates were

analyzed for each pile sample. Treatments were evaluated

by counting the number of germinated seeds, and mea-

suring the length of roots. The responses were calculatedby a germination index (GI) that was determined ac-

cording to the following formula (Zucconi et al., 1981):

G.F. Huang et al. / Waste Management 24 (2004) 805–813 807

Germination index ð%Þ

¼ Seed germination ð%Þ � root length of treatment

Seed germination ð%Þ � root length of control

� 100:

2.4. Statistical analysis

All data were processed by an SAS statistical package

running on an IBM personal computer (Little and Hills,

1978). The least significant difference test at P ¼ 0:05was carried out to compare the means of the triplicates

and the Pearson correlations among the parameters

were determined as well.

7.5

8.0

8.5

9.0

pH

C/N, 30C/N, 15

3. Results and discussion

3.1. Changes in physicochemical parameters during com-

posting

3.1.1. Temperature

As shown in Fig. 1, pile A reached 50 �C and entered

the thermophilic phase on day 3 of composting, indi-

cating quick establishment of microbial activities in the

composting pile. However, pile B required about 7 days,

a comparatively longer time, to reach a temperatureof 50 �C. This was due to scarcity of available carbon

source at the beginning of composting at a low initial

C/N, which did not provide a favorable condition for

the growth and biological activity of microorganisms.

The temperature of pile A was higher than pile B

throughout composting. Maximum temperature of pile

C/N, 30C/N, 15ambient

0 7 14 21 28 35 42 49 56 63

Composting time, days

Fig. 1. Changes in temperature during co-composting of pig manure

and sawdust.

A in the thermophilic phase was 69 �C, compared to the

60 �C of pile B. The periodical short-term drop in

temperature was caused by cooling effect induced by

turning of the piles. Thermophilic phase of pile A con-

tinued for 40 days, while in case of pile B, it lasted foronly 32 days. The temperature of pile A decreased

sharply after thermophilic phase and entered a cooling

phase on day 45, while in case of pile B, it entered the

cooling phase on day 40. The shorter thermophilic phase

of pile B as well as the slower rise in temperature at the

beginning of composting was attributed to insufficient

supply of carbon source because of the low C/N.

3.1.2. pH and EC

The changes in pH for both piles followed the same

trend with a rise to pH 8.4 and 8.7 on day 14 and day 21.

The pH decreased to 7.6 and 8.0 at the end of composting

for pile with an initial C/N of 30 and 15, respectively

(Fig. 2(a)). The pH rise was induced due to production of

ammonia during ammonification and mineralization of

(a)

(b)

7.00 7 14 21 28 35 42 49 56 63


1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 7 14 21 28 35 42 49 56 63


EC

(dS

m-1

)

C/N, 30C/N, 15

Fig. 2. Changes in pH (a) and EC (b) during co-composting of pig

manure and sawdust.

25

30

35

40

45

50

14 21 28 35 42 49 56 63

Composting time (days)


Tot

al o

rgan

ic c

arbo

n (%

)C/N, 30C/N, 15

2500

5000

7500

10000

12500

15000

0 7

0 7

14 21 28 35 42 49 56 63

Dis

solv

ed o

rgan

ic c

arbo

n (m

g kg

-1)

C/N, 30

C/N, 15

(a)

(b)

Fig. 4. Changes in total organic carbon (a) and dissolved organic

carbon (b) during co-composting of pig manure and sawdust.

70

75

80

85

90

95

0 7 14 21 28 35 42 49 56 63


Vol

atile

sol

ids

(%

)

C/N, 30

C/N, 15

Fig. 3. Changes in volatile solids during co-composting of pig manure

and sawdust.


organic nitrogen as a result of microbial activities

(Bishop and Godfrey, 1983). The decrease in pH at the

later stage of composting was caused by the volatilization

of ammoniacal nitrogen and the Hþ-released as a result

of microbial nitrification process by nitrifying bacteria(Eklind and Kirchmann, 2000). The decomposition of

organic matter and production of organic and inorganic

acids by the activities of microorganisms in compost

would also be responsible for the decrease (Mathur,

1991). The large quantities of carbon dioxide that are

given off during the composting process might also be

responsible for the decrease in pH. pH of pile A with an

initial C/N of 30 remained lower than that with an initialC/N of 15 throughout the composting period due to the

higher amount of sawdust which was acidic in nature and

the higher volatilization of ammonia.

Both piles showed a similar pattern of change in EC

(Fig. 2(b)). EC of both piles increased from the begin-

ning of composting showing peak value on day 14,

followed by a steady decrease till the end of composting

period. The initial increase could be due to the release ofmineral salts such as phosphates and ammonium ions

through the decomposition of organic substances. The

volatilization of ammonia and the precipitation of

mineral salts could be the possible reasons for the de-

crease in EC at the later phase of composting (Wong

et al., 1995). Through out the composting process, EC of

pile A was significantly lower than that of pile B, which

can be attributed to the higher amount of pig manure inpile B. The EC value reflected the degree of salinity in

the co-compost of pig manure and sawdust, indicating

its possible phytotoxic/phyto-inhibitory effects on the

growth of plant if applied to soil.

3.1.3. Volatile solids

The content of volatile solids decreased with com-

posting time with about 4% and 5% loss for pile A andpile B, respectively, owing to the loss of organic matter

through microbial degradation (Fig. 3). The larger

amount of sawdust in pile A contained higher content of

recalcitrant decomposable compounds, such as cellulose

and lignin which may account for the insignificant lower

degree of organic matter loss as compared to pile B after

63 days of composting.

3.2. Nutrient analysis

3.2.1. Carbon decomposition

As shown in Fig. 4(a), contents of total organic car-

bon declined significantly, from initial 47% to final 34%

in pile A, and from 46% to 30% in pile B. There was no

significant difference in the loss of total organic carbon

between the two different C/N treatments. The contentsof total organic carbon of pile A were higher than that

of pile B during composting because sawdust contained

higher amount of organic carbon.


Dissolved organic carbon (DOC) decreased with an

increase in composting time for both piles (Fig. 4(b)).

Although pile A had a higher initial C/N and DOC

content, the final DOC was much lower, with a loss of

70%, as compared to the 50% loss in case of pile Bhaving lower initial C/N. This demonstrated the active

microbial decomposition of organic substrates from pig

manure and sawdust as the temperature increased, es-

pecially in pile A with a higher initial C/N. Garcia et al.

(1991a) found that the DOC in mature compost was

<0.5%. Based on that, pile A had reached maturity at

day 49, while pile B was marginally mature.

3.2.2. Nitrogen cycling

Total nitrogen contents of both piles increased

slightly after 63 days of composting (Fig. 5(a)), due to

the net loss of dry mass in terms of carbon dioxide, as

well as the water loss by evaporation caused by heat

evolved during oxidation of organic matter (Inoko et al.,

1979; Viel et al., 1987; Fang et al., 1999b). Nitrogen-

fixing bacteria might also have contributed to a lesser

0

0.5

1

1.5

2

2.5

3

3.5

4

0 7

0 7

14 21 28 35 42 49 56 63



Tot

al n

itrog

en (

%)

C/N, 30C/N, 15

0

5

10

15

20

25

30

35

40

45

50

14 21 28 35 42 49 56 63

Sol

uble

nitr

ate

(mg

kg-1

)

C/N, 30C/N, 15

(a) (b

(c) (d

Fig. 5. Changes in total nitrogen (a), soluble NH4-N (b), soluble NO3-N (c) an

sawdust.

degree to the increase in total N in the later stage of

composting (Bishop and Godfrey, 1983). Pile B con-

tained significantly higher total nitrogen content than

pile A throughout composting because of the higher

amount of pig manure used in this treatment.The changes in concentrations of NH4-N and NO3-N

followed the typical trends for these two forms of nitro-

gen during aerobic composting (Fig. 5(b) and (c)). Dur-

ing the first 7 days of composting, NH4-N contents of

both piles increased significantly and reached peak values

due to ammonification with an increase in temperature

and pH, as well as the mineralization of organic-N

compound (Fang et al., 1999a; Mahimairaja et al., 1994).After an initial increase, NH4-N contents decreased by

volatilization loss and immobilization by microorgan-

isms. The final NH4-N contents of pile A was 316 mg

kg�1, while pile B was 912 mg kg�1. This was mainly due

to the higher N content in the pile with a lower C/N. It has

been noted that the absence of or decrease inNH4-N is an

indicator of both good composting and maturation

process (Hirai et al., 1983; Riffaldi et al., 1986). An

0 7

7



0

500

1000

1500

2000

2500

3000

3500

4000

0 14 21 28 35 42 49 56 63

Sol

uble

am

mon

ia n

itrog

en (

mg

kg-1

)

C/N, 30C/N, 15

500

600

700

800

900

1000

1100

1200

1300

1400

1500

14 21 28 35 42 49 56 63

Sol

uble

orga

nic

nitr

ogen

(m

g kg

-1)

C/N, 30C/N, 15

)

)

d soluble organic nitrogen (d) during co-composting of pig manure and

6000

7000

8000

9000

10000

11000

12000

0 7 14 21 28 35 42 49 56 63Composting time (days)



Tot

al p

hosp

horu

s (m

g kg

-1)

C/N, 30

C/N, 15

0

100

200

300

400

500

600

700

800

900

0 7 14 21 28 35 42 49 56 63

Sol

uble

inor

gani

c ph

osph

orus

(m

g kg

- 1)

C/N, 30C/N, 15

0

20

40

60

80

100

120

0 7 14 21 28 35 42 49 56 63

Sol

uble

org

anic

pho

spho

rus

(mg

kg- 1)

C/N, 30C/N, 15

(a)

(b)

(c)

Fig. 6. Changes in total phosphorus (a), soluble inorganic phosphorus

(b), and soluble organic phosphorus (c), during co-composting of pig

manure and sawdust.


NH4-N content of 400 mg kg�1 was recommended as the

maximum content in mature compost (Zucconi and de

Bertoldi, 1980). Therefore, pile A reached maturity after

composting for 49 days, but pile B required further

composting to become mature.Nitrate was almost absent at the beginning and started

to increase after the thermophilic phase (Fig. 5(c)). Ni-

trate content of pile A remained at a low level before

increasing at day 35 due to nitrification, while that of pile

B started to increase at day 21. The high temperature and

excessive amount of ammonia inhibited the activity and

growth of nitrifying bacteria in the thermophilic phase

(Morisaki et al., 1989). No significant difference in NO3Ncontent between the two piles was noted except for higher

NO3-N content in pile B than pile A at day 63. This was

simply due to the higher initial content of nitrogen in pile

B with larger amount of pig manure.

Soluble organic N of the two piles decreased with the

composting time. Pile A decreased from 915 to 722

mg kg�1, and pile B from 1357 to 1015 mg kg�1

(Fig. 5(d)). The soluble organic N was either mineralizedinto NH4-N, which vaporized or was assimilated into

organic N by the microorganisms in the compost

(Morisaki et al., 1989). Higher concentration of soluble

organic N in the pile B indicated that more organic

nitrogen compounds were synthesized in the pile with a

lower initial C/N.

3.2.3. Phosphorus turnover

The change of total P followed the same trend as total

N with a gradual increase throughout the composting

period, which was due to the net loss of dry mass

(Fig. 6(a)). Total P of pile B was higher than that of pile

A because of the comparatively higher amount of pig

manure added for achieving a lower initial C/N. Soluble

organic P and soluble PO4-P of pile B dropped signifi-

cantly in the first 14 days and then decreased to levelssimilar to that of pile A (Fig. 6(b)). The decrease of pile

A was less than pile B for both forms of soluble P. The

loss of soluble organic P and soluble PO4-P is likely due

to the mineralization of organic phosphorus and the

consumption by microorganisms.

3.3. Maturity evaluation

3.3.1. E4/E6 ratio

The ratio of optical densities of humic acids and

fulvic acids at 465 and 665 nm, respectively (E4/E6), has

long been considered to reflect the degree of condensa-

tion of the aromatic nucleus of humus, indicating its

maturity (Schnitzer et al., 1993). However, E4/E6 ratio is

not a universal indicator as it varies with the raw ma-

terials used for composting. E4/E6 ratio of the two pilesdecreased slowly during composting (Fig. 7(a)), which

suggested that composting produced more polycon-

densed humic acids (Garcia et al., 1991b). In particular,

the E4/E6 ratio is inversely related to the degree of

condensation of the aromatic network in HA, i.e., a low

E4/E6 ratio would be indicative of high degree of aro-

matic constituents condensation, whereas a high ratio

( c)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 7

0 7 0 7

0 714 21 28 35 42 49 56 63Composting time (days) Composting time (days)

Composting time (days) Composting time (days)

E4/

E6

C/N, 30C/N, 15

0

10

20

30

40

50

60

70

80

90

100

14 21 28 35 42 49 56 63

Ger

min

atio

n in

dex

(%)

C/N, 30C/N, 15

5

10

15

20

25

30

35

14 21 28 35 42 49 56 63

solid

C/N

rat

io

C/N, 30C/N, 15

2

4

6

8

10

12

14

16

18

14 21 28 35 42 49 56 63

Aqu

eous

C/N

rat

io

C/N, 30C/N, 15

(a)

(c)

(b)

(d)

Fig. 7. Changes in E4/E6 ratio (a), C/Nsolid (b) and C/Naqueous (c), and cress seed germination index (d) during co-composting of pig manure and

sawdust.


reflects a low degree of aromatic condensation and the

presence of high proportions of aliphatic structures

(Quatmane et al., 2002). Therefore, composting im-

proved the quality of pig manure and sawdust compostthrough humification process.

3.3.2. C/N ratio

As shown in Fig. 7(b), with an increase of composting

time, there was a decrease in C/Nsolid for both piles. Pile

A decreased from 30 to a final C/Nsolid of 17, and from

15 to 9 for pile B. The smaller decrease for pile B indi-

cated the poorer decomposition when the initial C/Nsolid

is lower. C/N in solid phase cannot be used as an ab-

solute indicator of compost maturation due to the large

variation depending on the starting materials, but a

value of around or below 20 can be considered satis-

factory when the initial value is between 25 and 30

(Hirai et al., 1983). Therefore, pile A had reached ma-

turity at day 49 according to this C/Nsolid criterion.

The C/Naqueous decreased to 5 for both piles irre-spective of their initial C/Naqueous, during the compo-

sting process (Fig. 7(c)). Chanyasak and Kubota (1981)

suggested the use of C/N in aqueous phase as an indi-

cator of compost maturity since the composting reaction

is a biochemical decomposition of organic matter oc-curring mainly in the aqueous phase. Compost with

C/Naqueous of 5–6 was suggested to have achieved ma-

turity. Therefore, it can be concluded that pile A reached

maturity after 49 days of composting. As shown in

Table 2, C/Nsolid and C/Naqueous did not correlate sig-

nificantly with seed germination index in pile B as shown

in the next section, which indicated that it may not be

appropriate to use C/Nsolid and C/Naqueous as indicatorfor evaluation of compost maturity at low initial C/N.

3.3.3. Cress seed germination index

As shown in Fig. 7(d), theGI values of pile A decreased

froman initial level of 14% to the lowest value of 3%at day

14, while that of pile B was 0% before day 35. This may be

attributed to the release of toxic concentrations of am-

monia and lowmolecular weight short chain volatile fattyacids, primarily acetic acid (Vleeschauwer De et al., 1981;

Table 2

Correlation coefficients between physiochemical parameters and GI

values

Parameters C/N, 30 C/N, 15

pH )0.8134�� )0.5392EC )0.6653 )0.6782Volatile solids )0.7790� )0.7185�

E4/E6 )0.7074 )0.4297DOC )0.8627�� )0.7148�

Total organic carbon )0.7146 )0.5825Soluble PO4-P )0.8114�� )0.5298Soluble organic P )0.1959 )0.4829Total soluble P )0.8299�� )0.5283Total P 0.8965�� 0.7712�

Soluble NH4-N )0.8660�� )0.7197�

Soluble NO3-N 0.9922�� 0.9801��

Soluble organic N )0.9192�� )0.6723Total nitrogen 0.9781�� 0.7344�

C/Naqueous )0.8655�� )0.7609�

C/Nsolid )0.9360�� )0.6362

GI, germanian index.* P < 0:05.** P < 0:01.


Wong, 1985; Fang et al., 1999b). As the compostingprocess progressed, the GI values of pile A increased

significantly to 62% at day 49, and reached 85% at the end

of the composting time, while pile B only reached 46% at

day 63. A germination index of 50% has been used as an

indicator of phytotoxin-free compost (Zucconi et al.,

1981). Jodice (1989) reported that a GI of 50–70% indi-

cates a low level of phytotoxic substances and compost

maturity. It can be concluded that pile A was stabilizedenough at day 49, whereas pile B required further com-

posting for longer time than 63 days to reach stabilization.

The GI, which combines the measure of relative seed

germination and relative root elongation of cress seed

(Lepidium sativum L.), is an integrated biological indi-

cator, which is regarded as the most sensitive parameter

used to evaluate the toxicity and degree of maturity of

compost (Zucconi et al., 1981). It reflects the cumulativepotential effects of all chemical factors that may do

harm to the plants even though this factor is not related

to humification like for example, EC. The correlation

coefficients between chemical parameters and GI values

are shown in Table 2. Most of the chemical parameters

in pile A correlated with GI values significantly, while

only soluble NO3-N in pile B had significant correlation

with GI values Low C/Nsolid and C/Naqueous along with alow GI value at the end of composting in case of pile B

can be explained by the high EC values throughout the

composting period, and is one of the major factors in-

hibiting the seed germination in this study. Maturity

evaluation of compost is a complex job, as it is affected

by many potential factors. Multi-indicators instead of a

single chemical indicator should be recommended for

evaluating compost maturity.

4. Conclusions

Co-composting of pig manure with sawdust at an

initial C/N of 30 resulted in the compost reaching ma-

turity after 49 days of composting. It was demonstratedthat under the windrow composting trial conditions

described here, with manual turning, could yield stable

compost that could be used for organic farming or as a

soil amendment. However, treatment at a low initial

C/N of 15 affected the behaviors of a number of im-

portant parameters significantly during co-composting

of pig manure and sawdust. High DOC and soluble

NH4-N content of pile B indicated immaturity after 63days of composting. Although pile B had a relatively

low C/Nsolid and C/Naquoeus at the end of composting

period, the GI value remained lower than 50%, which

may be due to its high EC value affecting seed germi-

nation. Therefore, co-composting of pig manure with

sawdust at low initial C/N can reduce the amount of

sawdust used, but it would require a composting period

of more than 63 days. In addition, the high EC value ofthe resulting compost has to be reduced to levels that

would not exert an inhibition on plant growth.

Acknowledgements

This work was financially supported by the Rocke-

feller Brothers Fund Ltd. The authors thank Mr. K.KMa of Hong Kong Baptist University for his excellent

technical assistance throughout the project.

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Effect of C-N on Composting of Pig Manure With Sawdust