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Effect of nitrogen fertilizationon methane and carbon dioxideproduction potential in relation tolabile carbon pools in tropical floodedrice soils in eastern IndiaKoushik Singha Roya, Suvadip Neogia, Amaresh Kumar Nayaka &Pratap Bhattacharyyaa

a Division of Crop Production, Central Rice Research Institute,Cuttack 753006, Odisha, IndiaAccepted author version posted online: 05 Feb 2014.Publishedonline: 24 Feb 2014.

To cite this article: Koushik Singha Roy, Suvadip Neogi, Amaresh Kumar Nayak & PratapBhattacharyya (2014) Effect of nitrogen fertilization on methane and carbon dioxide productionpotential in relation to labile carbon pools in tropical flooded rice soils in eastern India, Archives ofAgronomy and Soil Science, 60:10, 1329-1344, DOI: 10.1080/03650340.2013.869673

To link to this article: http://dx.doi.org/10.1080/03650340.2013.869673

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Effect of nitrogen fertilization on methane and carbon dioxideproduction potential in relation to labile carbon pools in tropical

flooded rice soils in eastern India

Koushik Singha Roy, Suvadip Neogi, Amaresh Kumar Nayak and Pratap Bhattacharyya*

Division of Crop Production, Central Rice Research Institute, Cuttack 753006, Odisha, India

(Received 5 October 2013; accepted 15 November 2013)

In an incubation experiment with flooded rice soil fertilized with different N amountsand sampled at different rice stages, the methane (CH4) and carbon dioxide (CO2)production in relation to soil labile carbon (C) pools under two temperature (35°C and45°C) and moisture (aerobic and submerged) regimes were investigated. The fieldtreatments imposed in the wet season included unfertilized control and 40, 80 and120 kg ha−1 N fertilization. The production of CH4 was significantly higher (27%)under submerged compared to aerobic conditions, whereas CO2 production was sig-nificantly increased under aerobic by 21% compared to submerged conditions. Theaverage labile C pools were significantly increased by 21% at the highest dose of N(120 kg ha−1) compared to control and was found highest at rice panicle initiationstage. But the grain yield had significantly responded only up to 80 kg ha−1 N,although soil labile C as well as gaseous C emission was noticed to be highest at120 kg ha−1 N. Hence, 80 kg N ha−1 is a better option in the wet season at low landtropical flooded rice in eastern India for sustaining grain yield and minimizingpotential emission of CO2 and CH4.

Keywords: CO2 production; CH4 production; soil labile carbon; rice; N fertilizer

Introduction

Agricultural activities accounts for ~15% of the global emissions of greenhouse gases(GHGs). The low land rice ecology, in particular contributes towards the emissions of themost important GHGs responsible for global warming viz. methane (CH4), carbon dioxide(CO2) and nitrous oxide (N2O). Soil is one of the important sources and sinks for GHGswhich lead to global warming and climate change (Janssens et al. 2003). Soils containabout 1500 Pg of organic carbon (C) in terrestrial ecosystem (Amundson 2001). Soilcontributes about 20% to the total emission of CO2 through soil and root respiration, 12%of total CH4 and 40% of total N2O emissions (IPCC 2007). The storage or release of thishuge C pool has been considered a key factor to influence the atmospheric CO2 concen-tration (Amundson 2001). A global flux of CO2 emission from soils accounts for68–75 Pg CO2–C year−1 (Mosier 1998).

The variations of soil CO2 flux are affected by agronomic management practices such asorganic and inorganic fertilization (Ding et al. 2007). Agricultural management practicesaffect soil CO2 flux by changing the soil environment such as soil aeration, pH, moisture,temperature, C/N ratio of substances, etc. These soil environmental characteristics can have asignificant impact on soil microbial activity and the decomposition processes that transform

*Corresponding author. Email: pratap162001@yahoo.co.in

Archives of Agronomy and Soil Science, 2014Vol. 60, No. 10, 1329–1344, http://dx.doi.org/10.1080/03650340.2013.869673

© 2014 Taylor & Francis

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plant-derived C to soil organic matter (SOM) and CO2 (Franzluebbers et al. 1995). Previousresearch has shown that soil CO2 flux rates are strongly related to soil temperature and soilmoisture conditions (Ren et al. 2007; Liu et al. 2008).

Rhizosphere respiration has been estimated to be 25–45% of gross primary produc-tivity and accounts for 15–71% of ecosystem respiration (Rochette et al. 1999).Understanding controlling processes on soil respiration is critical because relativelysmall changes in respiration rates may dramatically alter atmospheric CO2 concentrationsas well as rates of soil C sequestration. Soil CO2 emission integrates soil CO2 production,including rhizosphere and microbial respiration.

Rice paddy, a unique anthropogenic wetland ecosystem, is commonly believed to play acrucial role in the global C budget. As an essential agricultural measure, the application ofN fertilizers shows profound effect on the CH4 production and flux from flooded soil (Dan et al.2001; Rath et al. 2002). Methane emission varied from 14 to 375 mg m−2 d−1 in most ricegrowing areas in the world. It is affected by water regimes, soil amendments, cultivars and typeof fertilizers used. In India the mean CH4 emission from rice field range from 3.5 to 4.2 Tg yr−1.Majority of rice grown in the eastern part of India comprise an area of 23.4 million ha.

Nitrogen (N) fertilizers applied to soils influence both soil CO2, CH4 emissions, thoughtheir actual effects vary (Lee et al. 2007). The relative benefits of N fertilizers in maintainingorganic C levels in arable soils are of increasing concern (Ladd et al. 1994). The effect ofchemical fertilizers is particularly crucial for predicting the future trend of CO2 emission andpossible approaches to mitigate climatic change by agricultural practices.

Carbon mineralization in agricultural soils has been actively investigated within theresearch of C cycling of terrestrial ecosystem and global change. While both CH4 andCO2 production from soil result from C mineralization under submerged condition,production and emission of CH4 has been widely documented with regard to the effectsof soil factors such as temperature, moisture, pH and Eh (redoxpotential of soil) as well asplant cultivars (Ding et al. 2003; Kerdchoechuen 2005). The effect of differentN fertilization on CH4 emission from paddy soils has been widely studied (Cai et al.1997; Bodelier et al. 2000). However, there has been little information on the relativedominance of both CH4 and CO2 production during C mineralization under submergedconditions. To understand the biophysical processes performing at micro-scale in fieldsoils, variation of C pools, microbial activities and production of GHGs have been leaststudied (Six et al. 2000; Drury et al. 2004; Eynard et al. 2005).

However, with the rapid economic and social development, soils are subject todegradation. Therefore, it is necessary to investigate soil CO2, CH4 evolution fromtropical rice soils for better understanding the mechanisms that regulate C storage andloss processes in extensively cultivated paddy field. Furthermore, the effects ofN fertilization and rice growth on variation in CO2 emission under anaerobic conditionsfrom paddy soils are not well known. The present study was conducted with the followingobjectives: (1) to investigate CO2 and CH4 emissions under different N fertilization atdifferent temperature and moisture regimes; (2) to evaluate the response of grain yieldagainst progressive N fertilization and (3) to estimate soil C pools in flooded rice soil.

Material and methods

Experimental site

The experimental plots were located at Central Rice Research Institute (CRRI), Cuttack,(20° 27′ 10″ N, 85° 56′ 9″ E; 24 m above mean sea level) in India and the incubation study

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was conducted at the Soil Chemistry Laboratory at CRRI, Cuttack. The mean annual tem-perature and rainfall is 27.7°C and 1500 mm, respectively, at the study site. The soil of the siteis an Aeric Endoaquept in nature and its basic characteristics are described in Table 1.

Crop cultivation and treatments

The field experiment was carried under rice–rice cultivation in the wet season (kharif;July–December) of the year 2012. The field was ploughed thoroughly and flooded 2–3 daysbefore transplanting. The rice seedlings (cv. Naveen) 25 days old were transplanted at thespacing of 20 cm × 15 cm within both wet (July–December) and dry season(January–April). The size of the plot was 5 m × 5 m for each treatment. Nitrogen wasapplied in the form of urea, 50% as basal and in two equal split at maximum tillering andpanicle initiation stage. Full dose (40 kg ha−1) of P2O5 and K2O were applied as basal justbefore transplanting in the form of single super phosphate and muriate of potash. All thefield plots remained continuously flooded to a water depth of 7 ± 3 cm during the entireperiod of crop growth and were drained 10 days before harvest. The treatments included inthe study were:

T1 – Unfertilized control (without any N and other fertilizers)T2 – Low dose of nitrogen (40 kg N ha−1)T3 – Medium dose of nitrogen (80 kg N ha−1)T4 – High dose of nitrogen (120 kg N ha−1)

Soil samples collection

The soil samples (0.5–1.0 kg for each treatment) were collected within the rows (rowspacing was 20 cm) of rice plants from each replicated plots of each treatment (eachtreatment plot was replicated thrice in a randomized block design) at 0–15 cm depth atthree growth stages of rice viz. panicle initiation (PI), grain filling (GF) and harvest (H).The soil samples were then air dried in shade, sieved (2 mm) and composite(Bhattacharyya et al. 2012, 2013). The samples were dried prior to maintain the moistureconditions (aerobic and submerged) on soil dry weight basis. These samples were thenanalyzed for CO2 and CH4 production. The labile soil C pools were analyzed at theabovementioned rice growth stages.

Table 1. Characteristics of initial soil of the study.

Soil Parameters Concentration

Clay (%) 25.8Silt (%) 21.5Sand (%) 52.6Bulk density (Mg m−3) 1.42pH (1:2.5 soil:solution ratio) 6.26Electrical conductivity (dS m−1) 0.44Total C (g kg−1) 11.5Total N (g kg−1) 0.87Available N (g kg−1) 0.67Available P (g kg−1) 0.025Available K (g kg−1) 0.26

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Incubation experiment

The soil samples of different treatments, sampled at each successive growth stages viz. PI,GF and H were incubated separately for 40 days each. Separate sets of soil samples werekept for CO2 and CH4 production for 3, 10, 20 and 40 days. The samples of incubationwere destructively analyzed. The incubation was performed at two temperature regimesviz. 35°C and 45°C. There were two moisture regimes kept for the study, i.e. aerobiccondition (66% of field capacity) and submerged condition.

The dry soil samples of 20 g were placed into 100 ml Schott bottles closed withneoprene septa for CO2 production study. The submerged condition was maintained bykeeping 1:1.25 soil:water ratio. In the other set, aerobic condition was maintained as perthe moisture level of field capacity for each treatment. Scintillation vials were hangedinside the Schott bottles containing 5 ml 1N NaOH for CO2 absorption. Carbon dioxidecontent which was trapped in alkali solution was analyzed by titrimetry (Anderson 1982).

For CH4 estimation, Schott bottles were incubated by placing only 20 g of soils undertwo moisture and three temperature regimes as described previously for 3,10, 20, 40 dayswithout the alkali traps. CH4 concentration in the head space was quantified by with-drawing air samples (1 ml). Methane concentrations in the air samples collected wereanalyzed in a Chemito 2000 gas chromatograph (M/s Thermo Scientific Pvt. Ltd.,Netherlands) equipped with a flame ionization detector (FID) and Porapak Q column(183 cm long, 0.32 cm outer diameter, 80/100 mesh size, stainless steel column).Temperature of the injector, column and detector was maintained at 150°C, 50°C and230°C, respectively. The carrier gas (N2) flow was maintained at 15 ml min−1. The gaschromatograph was calibrated before and after each set of measurements by using 1.2 and1.8 µl CH4 l−1 in N2 (Chemtron Science Laboratories, India) as the primary standard toprovide a standard curve that was linear over the concentration range used in this study.

Soil labile carbon pool analysis

The labile soil C pools include microbial biomass C (MBC), readily mineralizable C (RMC),water soluble carbohydrate C (WSC), acid hydrolyzable carbohydrate C (AHC) and potas-sium permanganate oxidizable C (POC). The MBC was measured by modified chloroformfumigation–extraction method at atmospheric pressure (Witt et al. 2000). The RMC content ofsoil samples were estimated after extraction with 0.5MK2SO4 (Inubushi et al. 1991) followedby wet digestion of the soil extract with dichromate (Vance et al. 1987). The WSC and AHCwere estimated followed by the procedure of Haynes and Swift (1990). The POC wasdetermined by following the methodology of Blair et al. (1995).

Statistical analysis

The datasets were subjected to analysis of variance and means were separated by TukeyKramer’s HSD Test at the 0.05 level of probability. The correlation matrix was developedamong different C pools and selected microbial populations. All these analysis were doneusing SPSS version 20.0.

Results

Carbon dioxide production

The cumulative CO2 production increased up to 40 days of incubation and fitted best inpower functions irrespective of fertilization treatments, moisture and temperature at each

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successive rice growth stages viz. PI (Table 2), GF (Table 3) and H (Table 4).Significantly highest CO2 productions were observed under 120 kg N ha−1 treatmentfollowed by always lower values at 80 kg N ha−1, 40 kg N ha−1 and control treatmentsirrespective of the temperature, moisture regimes and crop growth stages (Figure 1). In PIstage, CO2 productions under submerged condition were in the range of 0.57–8.46 and0.88–9.22 mg CO2–C g−1 soil at 35°C and 45°C, respectively, while under aerobiccondition, the CO2 productions varied in the range of 0.77–8.09 and 1.14–8.17 mgCO2–C g−1 soil at 35°C and 45°C, respectively (Figure 1a–d). In GF stage, CO2 produc-tions under submerged condition were in the range of 0.80–7.63 and 1.18–8.11 mgCO2–C g−1 soil at 35°C and 45°C, respectively, while under aerobic condition, the CO2

productions varied in the range of 1.08–6.80 and 1.15–8.44 mg CO2–C g−1 soil at 35°Cand 45°C, respectively (Figure 1e–h). In H stage, CO2 productions under submerged condi-tion were in the range of 1.15–6.51 and 1.66–6.50 mg CO2–C g−1 soil at 35°C and 45°C,respectively, while under aerobic condition, the CO2 productions varied in the range of1.27–5.21 and 1.73–7.29 mg CO2–C g−1 soil at 35°C and 45°C, respectively (Figure 1i–l).Regression analysis was carried out separately for PI, GF and H stages to see the relationshipamong temperature, soil moisture, N fertilization and days of incubation with CO2 productionand the influence of each variant on carbon dioxide production (Table 5).

Methane production

Methane production in incubated samples increased up to 20 days and decreased thereafterirrespective of the temperature and moisture regimes. The CH4 production was foundhighest in 120 kg N ha−1 treatment irrespective of temperature, moisture regimes andcrop growth stages. The CH4 productions were found to decline in the order:120 kg N ha−1 > 80 kg N ha−1 > 40 kg N ha−1> control both under aerobic (Figure 2)and submerged condition (Figure 3). The CH4 productions under aerobic condition were inthe range of 6.5–24.9 and 7.1–28.2 ng g−1 soil at 35°C and 45°C, respectively (Figure 2),while under submerged condition, the CH4 productions varied in the range of 11.9–47.5 and13.3–53.1 ng g−1 soil at 35°C and 45°C, respectively, over the crop growth stages(Figure 3).

Grain and straw yield

The grain and straw yield increased significantly under different doses of N treatmentscompared to unfertilized control treatment (Figure 4). The grain and straw yield wasincreased significantly by 33%, 37% and 28%, 46% in medium dose and high dose of Ntreatment, respectively, compared to unfertilized control treatment (Figure 4). The grainand straw yield in 80 kg N ha−1 and 120 kg N ha−1 treatment was at par to each other(Figure 4).

Soil labile carbon pools

The labile pools of C viz. MBC, RMC, WSC, AHC and POC were significantly increasedunder 120 kg N ha−1 treatment followed by declined amounts under 80 kg N ha−1,40 kg N ha−1 and control treatments and found highest in the PI stage of crop growthirrespective of the treatments (Figure 5). The MBC, RMC, WSC, AHC and POC varied inthe range of 152.5–389.7 µg C g−1 (Figure 5a), 49.6–132.5 µg C g−1 (Figure 5b),25.7–85.1 µg C g−1 (Figure 5c), 174.3–445.4 µg C g−1 (Figure 5d) and

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Table2.

Empiricalrateandcoefficientof

determ

inationof

carbon

diox

ideprod

uctio

nun

derdifferentN

treatm

ents,temperature

andmoistureregimes

atpanicle

initiationstageof

crop

.

Aerob

iccond

ition

Sub

mergedcond

ition

35°C

45°C

35°C

45°C

Treatments

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

T1

y=0.19

6x0.891

0.94

y=0.39

5x0.778

0.96

y=0.32

2x0.641

0.92

y=0.60

7x0.654

0.96

T2

y=0.38

2x0.800

0.99

y=0.51

0x0.696

0.99

y=0.36

7x0.743

0.99

y=0.63

2x0.669

0.99

T3

y=0.41

3x0.808

0.97

y=0.54

9x0.760

0.99

y=0.42

8x0.789

0.99

y=0.68

5x0.641

0.97

T4

y=0.66

6x0.703

0.94

y=0.72

0x0.700

0.98

y=0.68

2x0.674

0.98

y=0.85

0x0.600

0.99

Note:

T1:

unfertilizedcontrol(w

ithoutanyN

andfertilizers),T2:

low

dose

ofnitrogen

(40kg

Nha

−1),T3:

medium

dose

ofnitrog

en(80kg

Nha

−1),T4:

high

dose

ofnitrog

en(120

kgN

ha−1).

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Table

3.Empiricalrate

andcoefficientof

determ

inationof

carbon

diox

ideprod

uctio

nun

derdifferentN

treatm

ents,temperature

andmoistureregimes

atgrain

filling

stageof

crop

.

Aerob

iccond

ition

Sub

mergedcond

ition

35°C

45°C

35°C

45°C

Treatments

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

T1

y=0.59

0x0.604

0.98

y=0.63

7x0.594

0.98

y=0.38

8x0.722

0.98

y=0.63

4x0.604

0.98

T2

y=0.65

2x0.572

0.99

y=0.71

9x0.587

0.94

y=0.55

8x0.653

0.98

y=0.55

3x0.710

0.99

T3

y=0.65

8x0.618

0.99

y=0.66

0x0.664

0.98

y=0.61

3x0.626

0.96

y=0.80

7x0.601

0.95

T4

y=0.79

9x0.600

0.98

y=0.78

6x0.676

0.97

y=0.59

9x0.700

0.97

y=0.78

0x0.659

0.97

Note:

T1:

unfertilizedcontrol(w

ithoutanyN

andfertilizers),T2:

low

dose

ofnitrogen

(40kg

Nha

−1),T3:

medium

dose

ofnitrog

en(80kg

Nha

−1),T4:

high

dose

ofnitrog

en(120

kgN

ha−1).

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Table4.

Empiricalrateandcoefficientof

determ

inationof

carbon

diox

ideprod

uctio

nun

derdifferentN

treatm

ents,temperature

andmoistureregimes

atharvest

stageof

crop

.

Aerob

iccond

ition

Sub

mergedcond

ition

35°C

45°C

35°C

45°C

Treatments

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

Pow

erequatio

nR2

T1

y=0.67

0x0.503

0.96

y=1.04

7x0.460

0.99

y=0.59

0x0.537

0.97

y=1.01

5x0.471

0.99

T2

y=0.90

0x0.468

0.98

y=1.19

5x0.428

0.99

y=0.69

1x0.489

0.97

y=1.55

0x0.363

0.96

T3

y=1.23

1x0.392

0.95

y=1.3x

0.451

0.99

y=1.28

4x0.401

0.98

y=1.56

4x0.372

0.99

T4

yy=1.57

0x0.307

0.95

y=1.79

9x0.366

0.96

y=1.21

7x0.432

0.97

y=1.72

0x0.359

0.99

Note:

T1:

unfertilizedcontrol(w

ithoutanyN

andfertilizers),T2:

low

dose

ofnitrogen

(40kg

Nha

−1),T3:

medium

dose

ofnitrog

en(80kg

Nha

−1),T4:

high

dose

ofnitrog

en(120

kgN

ha−1).

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227.7–581.9 µg C g−1 (Figure 5e), respectively. The highest value of labile C was foundin the PI stage of crop development and in the 120 kg N ha−1 treatment.

Discussion

Effects of N fertilization on CO2 production

The application of N fertilizers can affect mineralization rates of SOM and contribute toincreases in SOM content by increasing residue input with increased crop production

Table 5. Regression equation for CO2 production in relation to treatments, temperature andmoisture.

Equations R2

Panicle initiation (PI) stage:CO2 production = 0.458Tr + 2.011D + 0.605 Temp – 0.274M – 2.77 0.89Grain filling (GF) stage:CO2 production = 0.401Tr + 1.698D + 0.581 Temp – 0.023M – 2.32 0.92Harvesting (H) stage:CO2 production = 0.291Tr + 1.263D + 0.899 Temp – 0.027M – 1.48 0.90

Note: Tr, treatment; D, days of incubation; Temp, temperature; M, moisture.

CO

2 p

ro

du

ctio

n (

mg

CO

2–

C g

–1

so

il)

Days of incubation

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(b)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(c)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(d)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(f)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(h)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(g)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(e)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(i)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(j)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(k)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4(l)

0

2

4

6

8

10

12

3 10 20 40

T1 T2 T3 T4

(a)

Figure 1. Carbon dioxide production in (a) PI stage at 35°C temperature under aerobic condition;(b) PI stage at 45°C temperature under aerobic condition; (c) PI stage at 35°C temperature undersubmerged condition; (d) PI stage at 45°C temperature under submerged condition; (e) GF stage at35°C temperature under aerobic condition; (f) GF stage at 45°C temperature under aerobic condi-tion; (g) GF stage at 35°C temperature under submerged condition; (h) GF stage at 45°C tempera-ture under submerged condition; (i) H stage at 35°C temperature under aerobic condition; (j) H stageat 45°C temperature under aerobic condition; (k) H stage at 35°C temperature under submergedcondition; (l) H stage at 45°C temperature under submerged condition in different N treatments. PI,GF and H represent panicle initiation, grain filling and harvest stage, respectively. T1: unfertilizedcontrol (without any N and fertilizers), T2: low dose of nitrogen (40 kg N ha−1), T3: medium dose ofnitrogen (80 kg N ha−1), T4: high dose of nitrogen (120 kg N ha−1).

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(Iqbal et al. 2009). In this study, the highest CO2 flux was observed in 120 kg N ha−1

treatment probably due to the efficient use of C for microbial respiration in response to theapplication of fertilizers (Fisk & Fahey 2001). Microbial biomass tends to be dynamic insoil with limited nutrient availability (Fontaine et al. 2004). In this study, the soil also hadlower available N content and medium P status. Under such conditions, net soil carbonloss increases due to the enhancement in soil organic C mineralization. The CO2 flux in120 kg N ha−1 treatment was higher due to higher turnover rates of the more stablemicrobial biomass. The labile C source supports the growth of microbial biomass, whichis dynamic and promotes the priming effect of SOM resulting into higher CO2–C flux(Singh et al. 2009; Bhattacharyya et al. 2012, 2013).

Temperature is the key factor in influencing microbial activity and thereby soilrespiration. Productions of CO2 in all N fertilization treatments were significantly higherat 45°C than at 35°C, both at aerobic and submergence. This could be due to higherbiological activity, microbial cell metabolism and abundance of microbial mass(Bhattacharyya et al. 2012, 2013). Within a certain temperature range (35–37°C), theCO2 evolution from soil was greater due to microbial respiration, which is directlycorrelated with soil temperature (Gaumont-Guay et al. 2006).

The CO2–C production was more under aerobic condition than submerged conditiondue to less dissolution of CO2 and higher mineralization rate of organic matter underaerobic condition (Sahrawat 2003). Positive relationship was found with N doses, tem-perature and days of incubation, whereas, negative relationship was observed with soilmoisture. It clearly indicated that N fertilization (up to 120 kg ha−1), days of incubation(up to 40 days) and increase of temperature (up to 45°C) increased the cumulative CO2

production while submerged conditions reduced the CO2 production.

Effects of N fertilization on CH4 production

In the present study, the application of N fertilizer resulted in an increase in CH4 emissionand the highest CH4 flux was observed in N treatment of 120 kg N ha−1. Undersubmerged conditions, emission of CH4 resulted from soil organic carbon (SOC) miner-alization, which has been enhanced by fertilization (Kimura et al. 2004). Microbial

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biomass and enzyme activity increased due to SOC mineralization but the predominantpathway (methanogenesis with acetate as the C substrate) of CH4 formation was generallyconsidered to be unchanged after different fertilizer treatments (Kimura et al. 2004). Thecapacity of CH4 production was likely to depend on the available C resources formicrobes. Application of N fertilizers has been shown to enhance the bioavailable poolof organic C and, in turn, to promote the CH4 production by utilization of readilybioavailable organic C by methanogenic microbes (Zhang et al. 2004; Zheng et al.2007). The higher methane flux in the panicle initiation stage was also reported byGogoi et al. (2005).

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Submergence favored CH4 production in all the fertilization treatments as it providedanaerobic condition and congenial condition to methanogenic bacteria for CH4 produc-tion. The CH4 production in alluvial soil was negligible at –1.5 MPa during 20 daysincubation but increased progressively with an increase in the moisture level to–0.01–0 MPa (saturated) and flooded condition (Rath et al. 2002). The CH4 productionunder aerobic condition suggests that anaerobic microsites may exist even under non-flooded conditions and that methanogenesis may occur at these microsites. Highertemperature enhanced CH4 production due to higher rate of C mineralization and highermetabolic activity of microorganisms (Zheng et al. 2007). In this study, all labile poolslike MBC, RMC, WSC, AHC and POC in the soil showed significantly higher values inthe 120 kg N ha−1 treatment compared to the other treatments which obviously promotedthe growth of methanogens and produced higher CH4 emissions.

Effects of N fertilization on soil carbon pools

The applications of N fertilizers have been reported to significantly affect soil organicC and its fractions due to the significant increase in C input after their application (Maet al. 2011). Soil MBC regulates SOM decomposition and nutrient cycling, and thus playsa key role in maintaining function and sustainability of terrestrial ecosystems. MBC hasbeen included in current soil monitoring concepts due to its rapid response and highsensitivity to management practices and environmental changes (Bhattacharyya et al.2012, 2013). The application of N fertilizers leads to a high degree of gaseous C fluxdue to availability of greater labile C pools compared to non-fertilized treatments andaccumulation of total nitrogen in the soil (Bhatia et al. 2005). The soluble C fraction is animportant pool with respect to SOM turnover in agricultural soils, as it acts as a readilydecomposable substrate for soil microorganisms and as a short-term reservoir of plantnutrients (García-Orenes et al. 2010). Application of N fertilizers could contribute morelabile C that can act as a source of energy and nutrients (Manna et al. 2007). Blair et al.(2006) also reported that inorganic fertilizer significantly increased soil labile C poolscompared to non-fertilized treatments and higher dose of N provide greater amount of soillabile C.

Grain and straw yield

In this study, the grain and straw yield was significantly increased in different N treat-ments but the yield was at par in medium and high dose of N treatments. The differentamounts of N were selected in this study to evaluate the efficacy of rice variety in itsresponse on increased N amounts and transformation in yield. Grain yield was increasedsignificantly up to the medium dose of N application but beyond this optimal level theyield increase was not significant. This could be due to the prevalent environmentalcondition during the wet season rice cultivation in eastern India. The average photosyn-thetic active radiation (PAR) during wet season (June–October) was 709 µmol m−2 sec−1

(observatory data taken from CRRI meteorological station, unpublished), which is com-paratively low for rice to have greater N use efficiency at higher dose of N application.Furthermore, the native N status of soil was in the medium range (0.87 g kg−1). Theadditional amount of N applied beyond the medium level of N dose, despite increasedlabile C pools in the soil with mineralization potential and release of nutrients, could notcontribute to significant increase in grain yield and also led to higher CO2 and CH4

production.

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Conclusions

Nitrogen fertilization had a significant effect on soil respiration and hence CO2 and CH4

production in rice soils managed both under aerobic and submerged condition. A gradualincrease in cumulative CO2 production with days of incubation both at 35°C and 45°Cwere observed. Aerobic and submerged condition promoted CO2 and CH4 production,respectively. The high dose of N treatment (120 kg ha−1) although supported higher labileC pools and CO2, CH4 production but did not respond significantly in increasing grainyield compared to that of medium dose of N treatment. Hence, the medium dose ofN (80 kg ha−1) treatment was considered as judicious N management treatment in lowland tropical flooded rice ecology in eastern India for sustaining grain yield and mini-mizing gaseous C emissions.

FundingSome portions of the results are the part of PhD work of Mr. Koushik Singha Roy. Mr. Koushik SinghaRoy was supported by the fellowship from the projects, ‘Soil organic carbon dynamics vis-à-visanticipatory climatic changes and crop adaptation strategies’ [grant number ICAR- NAIP/Component-4/2031] and ‘National Initiative on Climate Resilient Agriculture’ (NICRA).

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