Greenhouse gas emission from direct seeding paddy field under different rice tillage systems in central China

Soil & Tillage Research 106 (2009) 54–61

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Greenhouse gas emission from direct seeding paddy field under different rice tillage systems in central China Shahrear Ahmad a,b,1, Chengfang Li a,b,1, Guangzhao Dai a,b, Ming Zhan a,b, Jinping Wang a,b, Shenggang Pan a,b, Cougui Cao a,b,* a Key Laboratory of Crop Production, Physiology and Ecology Center of Agriculture Ministry of the People’s Republic of China, Huazhong Agricultural University, Wuhan 430070, PR China b College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430070, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 May 2009 Received in revised form 12 August 2009 Accepted 12 September 2009

Agricultural tillage practices play an important role in the production and/or consumption of green house gas (GHG) that contributes substantially to the observed global warming. Central China is one of the world’s major rice producing areas but a few studies have tried to characterize the mechanisms of GHG release from rice paddy field and quantify global warming (GWPs) based on GHGs emission on this region. In this study four tillage systems consisting of no-tillage with no fertilizer (NT0), conventional tillage with no fertilizer (CT0), no-tillage with compound fertilizer (NTC) and conventional tillage with compound fertilizer (CTC) applications in rice (Oryza sativa L.) cultivation were compared in terms of the carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) emissions from different tillage systems of the subtropical region of China during the rice growing season in 2008. GWPs based on CO2, CH4 and N2O’s cumulative emissions were also compared. Tillage and fertilization had no influence on CO2 emissions. No-tillage had no effect on N2O emissions but significantly affected CH4 emissions. Application of fertilizer significantly affected CH4 and N2O emissions. Higher CH4 emissions and lower N2O emissions were observed in CTC than in NTC. Cumulative CH4 emission flux in NTC was 51.68 g CH4 m2 while it was 65.96 g CH4 m2 in CTC, 28% (p < 0.05) higher than that in NTC. Cumulative N2O emission flux in CTC was 561.00 mg N2O m2, and was 741.71 mg N2O m2 in NTC, 33% (p < 0.05) higher than that in CTC. There was no significant difference in N2O emissions between NT0 and CT0 systems, but significant in CH4 emissions. GWP of CTC was 26011.58 kg CO2 ha1, which was 12% higher than that in NTC (23361.3 kg CO2 ha1), therefore our findings show that no-tillage system was an effective strategy to reduce GWP from rice paddies in central China and thus can serve as a good agricultural system for environmental conservation. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Green house gas Rice paddy field No-tillage Conventional tillage Cumulative emission Global warming potential

1. Introduction Greenhouse effect is a naturally occurring phenomenon; that the effect be intensified by the emission of greenhouse gases (GHGs) into the atmosphere as the result of human activity. On a global basis, CO2, CH4 and N2O, contribute 60%, 15% and 5%, respectively, to the anthropogenic GHG effect (Rodhe, 1990). The concept of global warming potential (GWP) was developed to compare the ability of gas to trap heat in the atmosphere relative to the amount of CO2. It is determined by multiplying its mass by its GWP coefficient. The GWP

* Corresponding author at: Key Laboratory of Crop Production, Physiology and Ecology Center of Agriculture Ministry of the People’s Republic of China, Huazhong Agricultural University, Wuhan 430070, PR China. Tel.: +86 27 87283775. E-mail address: [email protected] (C. Cao). 1 These authors have contributed equally to this work. 0167-1987/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2009.09.005

coefficient of a gas determines the amount of heat a GHG can trap relative to a unit mass of CO2 (Shine et al., 1995). GHG are through to be an integrative criterion to assess the contribution of the system to the global warming effect in terms of CO2, CH4 and N2O emission. Atmospheric concentrations of these gases have increased considerably since the industrial revolution, and are still increasing annually by 0.5%, 1.1% and 0.3% respectively (IPCC, 2001). If GHG emissions continue to increase at the present rate, the average global temperature is projected to increase by about 1.8 8C by the year 2025 and by 3.8 8C by the end of the century (IPCC, 2001). Agricultural activities contribute a large percentage of these emissions: about 60% of N2O, 39% of CH4 and 1% of CO2 in the global emissions (OECD, 2000) with China being the highest rice producing country in the world, producing 29% of the total world production of rough rice (IRRI World Rice Statistics website, updated 2009) statistically making China the most susceptible nation to succumb to the effects of GHGs.

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As reported, the concentration of CO2, in the atmosphere has increased gradually from 280 mmol mol1 before industry revolution to 370 mmol mol1 by now, and it will be increasing by 0.5% per year (Yi et al., 2006). The farming soils are regarded as an important source of CO2 in the atmosphere where they account to around one-fourth of CO2 emission by human activities (Fang et al., 2003). The cultivation and field management practices such as stable manure amendment, seeding or transplanting of rice, water management, collection of harvest, treatment of harvest residuals, and plowing all greatly influence CO2 production (Makoto et al., 2005). Wetlands or agricultural fields also cover some parts of the land area and contribute to regional and global CO2 budgets. CH4 is one of the major green house gases with longevity; its global warming potential is thought to be 25 times greater on a mass basis in comparison to CO2 over a 100-year time horizon (IPCC, 2007). The increase in CH4 concentration in the atmosphere during the past decades has already aroused increasing concern throughout the world. The amount of methane emission from wetland paddy fields accounts for 10–20% of total methane emission, amounting to 50–100 Tg year1 (Sass et al., 1994; Reiner and Milkha, 2000). The entire process of methane emission from rice fields, including production, oxidation and transport to the atmosphere is affected by biotic and abiotic factors, such as growth status of rice, temperature, soil characteristics (e.g. Kumaraswamy et al., 2000). Schutz et al. (1989) reported a positive correlation between soil temperature in the 0–15 cm layer and CH4 emissions. Production and emission of CH4 from soil are also influenced by soil conditions (e.g. organic matter content, bulk density, porosity, tillage) and temperature (Li and Lin, 1993; Mitra et al., 2002; Elder and Lal, 2008). The concentration of N2O in atmosphere is lower than that of CO2, but its global warming potential (GWP) is 298 times greater than CO2 (IPCC, 2007). It accounts for 5% of the total greenhouse effect (Houghton et al., 1990). Agriculture is responsible for about 60% of N2O emission (OECD, 2000). It is well known that N2O is produced from paddy soil processes as a by-product of microbial nitrification and denitrification (Malla et al., 2005), which are affected by field water management and fertilizer application and so on (Xiong et al., 2007). Changes in the soil water content can directly impact nitrification and denitrification rates, and thus affect N2O production. Studies have reported no difference between CT and NT treatments (Choudhary et al., 2002) or more N2O emissions from conventionally tilled soils (Passianoto et al., 2003; Elder and Lal, 2008). Usually, no-tillage that goes with direct mulch crop residue may increase N2O emissions because crop residues induced higher microbial activity on the soil surface through nitrification and denitrification (Granli and Bockman, 1994), and the carbon fixation may decrease CO2 emissions (Angers et al., 1997). Hence, true mitigation is only possible if the overall impact of no-tillage adoption reduces the net GWP determined by GHG fluxes (Six et al., 2002). Only few studies have addressed this problem (Mummery et al., 1998; Six et al., 2002). The present paper provides results on CO2, CH4 and N2O fluxes over a rice growing period, with a comparison between tillage and no-tillage systems. This paper reports CO2, CH4 and N2O concentration profiles from a fertilized subtropical site in central China under conventional tillage and no-till management on rice. Therefore, the objectives of this study were (1) to evaluate CO2, CH4 and N2O emission from notillage and conventional tillage (puddling) system; (2) to asses the GWP of the no-tillage and conventional tillage system based on CO2, CH4 and N2O emission. The tillage system with less GHG under fertilizer application (NTC or CTC) was regarded as an effective strategy to reduce GWP from rice paddies in central China.

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2. Materials and methods 2.1. Site description, soil properties and materials The experimental site was at the experimental Farm of Zhonggui County of Dafashi Town in Wuxue City, Hubei Province, China (298550 N latitude, 1158300 E longitude, 20 m above sea level). According to metrological data collected from Wuxue city weather station from 2003 to 2008, this region has a humid mid-subtropical monsoon climate with an average annual temperature of 16.8 8C, a mean annual precipitation of 1278.7–1442.6 mm, and most of the rainfall occurs between April and August. The paddy field soil is hydromorphic paddy soil, which is silty clay loam derived from Quarternary yellow sediment. The thicknesses of plow layer and plow pan are 20 and 10 cm, respectively. The main soil properties (0–20 cm depth) of the site are as follows: pH, 6.58; total (nitrogen) N, 3.57 g kg1; organic (carbon) C, 18.29 g kg1; NO3-N, 4.37 mg kg1; NH4+-N, 2.43 mg kg1; total (phosphorus) P, 0.70 g kg1; available P, 3.65 mg kg1; available (potassium) K, 111 mg kg1; soil bulk density, 1.26 g cm3. Rice (Oryza sativa L.) variety used was Liangyoupeijiu which is a predominant local medium-rice variety and was provided by Crop Production, Physiology and Ecology Center, Huazhong Agricultural University, China. The total duration of rice growth was 128 days of which vegetative stage was 65 days, reproductive stage 35 days and ripening stage 28 days. The planting system of experimental field was rape (Brassica napus L.)–rice rotation, in which rice is directly seeded from May to October each year and rape is planted from October to next year May from 2006, and rice and rape were planted under no-tilling conditions. 2.2. Experimental design and cultural practices The experiment consists of a randomized block design (three replicates per treatments) and was conducted in 12 subplots of 45 m2. Four treatments were tested for no-tillage with no fertilizer (NT0), conventional tillage with no fertilizer (CT0), no-tillage with compound fertilizer (NTC) and conventional tillage (puddling) with compound fertilizer (CTC). Puddling is a common field preparation practice to maintain wetland condition for rice fields. Ridges of 20 cm high between plots were covered by plastic films in order to prevent exchange of water and fertilizer. Spraying 36% Glyphosate of 3 L ha1 used to control (to kill) weeds and seedlings of the previous (rap) crop on May 28, 2008, and then the field was submerged on May 30, 2008. Conventional tillage treatment was done by hoe to a depth of 8–10 cm, and subsequently mould board ploughed to 20 cm depth before sowing on May 31, 2008. Under no-tillage system after field submergence the seeds were sown directly without ploughing. Before sowing, rice seeds were soaked in water for 12 h that improve seedling quality. Then these rice seeds were sown at the rate of 22.5 kg ha1 on June 2, 2008 and harvested on October 10, 2008. A day after sowing (DAS) collection of data started on June 3, 2008 and ended a day before harvesting (DBH) on October 9, 2008. In both NTC and CTC, fertilizers were applied at the rates of 210 kg N ha1, 105 kg P2O5 ha1 and 240 kg K2O ha1. Compound fertilizer was (N:P2O5:K2O = 15%:15%:15%) with 555 kg ha1 and was used as a basal fertilizer on June 1, 2008. At the same time superphosphate ((CaH2PO4)2; as a source of 12%, P2O5) with 175 kg ha1 and potassic fertilizer (muriate of potash, KCl; as a source of 60%, K2O) with 260 kg ha1 were applied. For topdress-1 (June 16), topdress-2 (July 16) and topdress-3 (August 7) 42 kg N ha1 as urea were used for each. According to the conventional irrigation–drainage practices, once water depth decreased to 1–2 cm in the plots during rice growing season, plots were regularly irrigated to a depth of 10 cm

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(Uchida and Ando, 2007). Other agronomic managements such as irrigation, application of pesticide and herbicide were the same for the four treatments. Metrological data were collected from Wuxue city weather station, which is 1 km away from our experimental site. 2.3. Soil bulk density Just after rice harvesting, soil bulk density (BD) within 0–5 cm depth was measured by core method using metallic cores of 53 mm diameter and 50 mm height (Bao, 2000). Three soil cores were collected for each plot at 0–5 cm depth. Soil BD was determined after oven-drying at 60 8C for 48 h. 2.4. Gas sampling and measurements

Total gas emissions during the study period were calculated by integrating gas emissions on sampling days and cumulative gas emissions on the sampling days. Cumulative gas emissions were determined according to Singh et al. (1996). 2.4.3. Global warming potential (GWP) GWP is a measure of how much a given mass of greenhouse gas (GHG) is estimated to contribute to global warming. Gaseous emissions were converted to CO2 equivalents using GWP. GWP is an index defined as the cumulative radiative forcing between the present and some chosen later time ‘horizon’ caused by a unit mass of gas emitted now. It is used to compare the effectiveness of each GHG to trap heat in the atmosphere relative to some standard gas, by convention CO2. The GWP of different treatments were calculated using the following equation (Watson et al., 1996): GWP ¼ ðCO2 Þ þ ðCH4  25Þ þ ðN2 O  298Þ

2.4.1. Carbon dioxide (CO2) emission flux The soil CO2 flux was measured using the soil respiration method of Parkinson (1981), where a cylinder chamber of 20 cm diameter and 30 cm height was placed on the soil while increases in CO2 concentration was monitored within the chamber. Soil surface CO2 flux was measured with a LI-6400 portable photosynthesis analyzer (Li-Cor Inc., Lincoln, NE) from June 3 to October 9. Each sample was collected 1 h interval and each CO2 flux measurement from a plot was an average of three individual measurements. 2.4.2. Methane (CH4) and nitrous oxide (N2O) emission flux The static chamber technique was adopted for sampling N2O and CH4 (Crill et al., 1988) using 58 cm  58 cm  120 cm chests, made from metal and steel. Six hills of rice seedlings were covered in each sampling chamber. CH4 and N2O gas samples collection started in June 3 and stopped in October 9. First gas samples were collected 2 days after basal fertilization and just before and after top dresses 1, 2 and 3 consecutively. After topdress-3 samples were collected at an average of 2 weeks interval. Each sampling was subdivided three times in 8-min intervals. The gases in the chest were at first mixed by a fan on the top of the box, then drawn off by using a syringe, and immediately transferred into a 20-ml vacuum glass container. Gas samples were collected synchronously. Samples of three replications were taken from each plot and the average was taken as the representative value for that plot. A gas chromatograph meter (Shimadzu GC-14B), fitted with A 6–1/8-ft stainless-steel column (Porapack N; length  inner diameter: 3 m  2 mm) and a flame ionization detector and an electron capture detector, was used for measuring CH4 and N2O, respectively. For determination of methane, N2 (flow rate: 330 ml min1), H2 (flow rate: 30 ml min1), and zero air (flow rate: 400 ml min1) were used as the carrier, fuel, and supporting gas, respectively. Column, injector, and detector temperature were set at 55, 100 and 200 8C, respectively. For the determination of N2O, N2 was used as the carrier gas and the flow rate was maintained at 40 ml min1. Column, injector and detector temperatures were set at 65, 100 and 300 8C, respectively. The gas emission flux was calculated from the difference in gas concentration according to the equation of Zheng et al. (1998):   dC 273ð273 þ TÞ1 ; F ¼ rh dt where F is the gas emission flux (mg m2 h1), r is the gas density at the standard state, h is the height of chamber above the soil (m), C is the gas mixing ratio concentration (mg m3), t is the time intervals of each time (h), and T is the mean air temperature inside the chamber during sampling.

Based on a 100-year time frame, the GWP coefficients for CH4 and N2O are 25 and 298, respectively, when the GWP value for CO2 is taken as 1 (IPCC, 2007). This means that emissions of 1 million metric tons of methane and nitrous oxide, respectively, are equivalent to emissions of 25 and 298 million metric tons of carbon dioxide. 2.5. Statistical analysis of data Data in the tables and figures are presented as mean values  standard deviations. The SPSS 11.5 analytical software package was used for all statistical analyses. Statistical analysis was accomplished by standard analysis of variance (Two-way ANOVA,) and the differences among treatments were determined using Duncan’s least significant difference (LSD) test were used where significant differences occurred. 3. Results 3.1. Air temperature The air temperature varied from 16 to 32 8C during the whole growing period of rice in 2008. From June to September, the air temperature ranged from 20 to 32 8C, and several peaks occurred. From the end of September to DBH (about 15 days), the average air temperature of 20 8C is shown in Fig. 1. 3.2. Soil bulk density Soil bulk density in NT0 was 1.29 g cm3, and it was 1.25 g cm3 in CT0, which is 3.2% lower (p < 0.05) than that in NT0; soil bulk density in NTC was 1.33 g cm3 and in CTC it was 1.26 g cm3, which 5.6% lower (p < 0.05) than that in NTC (Table 1), suggesting that no-tillage led to higher soil bulk density relative to conventional tillage irregardless of application of fertilizer.

Fig. 1. Variations of daily air temperature during rice growing season.

S. Ahmad et al. / Soil & Tillage Research 106 (2009) 54–61

57

Fig. 4. N2O emission fluxes from DAS to DBH for different rice tillage systems. The arrows indicated fertilization in fertilized treatments during rice growing season. Fig. 2. CO2 emission fluxes from DAS to DBH for different rice tillage systems. The arrows indicated fertilization in fertilized treatments during rice growing season.

(34.24 g CH4 m2), whereas the cumulative emission of CH4 by CTC treatment was (65.96 g CH4 m2) 28% higher than NTC (51.68 g CH4 m2) (Table 1). 3.5. N2O emission During the rice growing season variations of N2O emission fluxes from different rice tillage systems are presented in Fig. 4. Fertilization led to greater N2O emission fluxes and several emission peaks in NTC and CTC were observed. In addition, in the first 2 weeks of DAS, N2O emissions were higher in NTC than in CTC. The cumulative N2O emissions were significantly affected by fertilization. Cumulative N2O emission from NTC was 741.71 mg N2O m2 and it was 561.00 mg N2O m2 from CTC, which was 33% lower (p < 0.05) than that from NTC. On the other hand tillage, there was no significant difference in cumulative N2O emissions between NT0 and CT0 (Table 1).

Fig. 3. CH4 emission fluxes from DAS to DBH for different rice tillage systems. The arrows indicated fertilization in fertilized treatments during rice growing season.

3.3. CO2 emission

3.6. GHG emission and GWP

CO2 emission flux increased gradually, and kept relatively high level in 1 week of DAS. It decreased rapidly 1 month of DBH but increased rapidly with just 15 days of DBH (Fig. 2). Tillage and fertilization both had no effect on CO2 emission. The cumulative CO2 emission flux in NT0, CT0, NTC and CTC were 762.45, 764.00, 823.10, 784.98 g CO2 m2, respectively. Cumulative CO2 emissions did not differ significantly (Table 1).

GHG emission monitored from four rice tillage systems (NT0, CT0, NTC and CTC) are presented in Table 2. GHG contribution to GWPs were decreased in the following order, CH4 > CO2 > N2O. The GWPs of NT0, CT0, NTC and CTC were 16409.25, 18837.44, 23361.3 and 26011.58 kg CO2 ha1, respectively. Contribution of reduction to GWPs, between NT0 and CT0 had no significant difference but NTC significantly contributed to reduction 12% of GWP compared with CTC (Table 2).

3.4. CH4 emission 4. Discussion As shown in Fig. 3, CH4 emission fluxes in all treatments were low initially, but increased gradually, and then peaks mid-July. Thereafter CH4 emission fluxes declined gradually and kept relatively low levels since harvesting, and the fluxes were lowest at DBH. Cumulative emissions of CH4 between no-fertilized and fertilized treatments differed (p < 0.05), and CT0 treatment (44.09 g CH4 m2) was 29% higher than NT0 treatment

4.1. CO2 emission from different tillage systems The CO2 emissions can be taken as indicators of soil tillage effects on the soil ecosystem, because CO2 emissions are closely connected to the microbial turnover and the physical accessibility of organic matter to microbes (Paustian et al., 2000). A flux of CO2

Table 1 Cumulative emission of GHG and soil bulk density (0–5 cm depth) from four rice tillage systems. Treatments

CO2 (g CO2 m2)

CH4 (g CH4 m2)

N2O (mg N2O m2)

Soil bulk density (g cm3)

NT0 CT0 NTC CTC Source of variation Tillage Fertilization Tillage  fertilizer

762.45  27.51 764.00  29.26 823.10  29.36 784.98  28.91 Significance NS NS NS

34.24  3.88 44.09  5.45 51.68  6.26 65.96  7.12

75.42  10.37 a 58.71  9.74 a 741.71  100.39 b 561.00  81.21 c

1.29  0.06 bc 1.25  0.09 a 1.33  0.07c 1.26  0.09 ab

*

NS

***

*

***

NS

NS

NS NS

a a a a

a b c d

Common letters in a row are not significantly different at the level of 5%. ‘NS’’ means not significant by Duncan’s multiple range test; NT0 = no-tillage with no fertilizer; CT0 = conventional tillage with no fertilizer; NTC = no-tillage with compound fertilizer; CTC = conventional tillage with compound fertilizer. * Significant level of p < 0.05. *** Significant level of p < 0.001.

S. Ahmad et al. / Soil & Tillage Research 106 (2009) 54–61

58 Table 2 GWPs (kg CO2 h1) of four tillage systems. Treatment

CO2

NT0 CT0 NTC CTC

7624.50  275.10 7640.00  292.60 8231.00  293.60 7849.80  289.10

a a a a

CH4

N2O

GWP

8560.00  970.00 a 11022.50  1362.50 b 12920.00  1460.00 c 16490.00  1847.50 d

224.75  30.90 a 174.94  29.03 a 2210.30  299.16 b 1671.78  242.01 c

16409.25  1276.00 18837.44  1684.13 23361.30  2052.76 26011.58  2378.61

a a b c

Common letters in a row are not significantly different at the level of 5%. GWP, global warming potential = (CO2 in rice paddy field) + (CH4 in rice paddy field  25) + (N2O in rice paddy field  298); NT0 = no-tillage with no fertilizer; CT0 = conventional tillage with no fertilizer; NTC = no-tillage with compound fertilizer; CTC = conventional tillage with compound fertilizer.

from agricultural soil is the result of complex interactions between climate and several biological, chemical and physical soil properties. Tillage systems may affect these soil properties and therefore influence the release of CO2 gas (Robertson et al., 2000). Moisture content in the paddy field soil was high (Zheng et al., 2000) however it made no difference (Harada et al., 2007). Increased surface roughness and larger voids produced by soil disturbances increased CO2 flux in moldboard/disking (Reicosky, 1997; Ball et al., 1999). But, in our study there was no significant difference in CO2 fluxes that occurred among the treatments of NT0, CT0, NTC and CTC (Table 1). After rice seed showing, CO2 emission increased and kept relatively high level (Fig. 2). This may be due to the germinated seeds (roots) respiration and decomposition of residues of rape. Our result accorded with Reicosky and Lindstrom (1995) who observed that CO2 fluxes result from decomposition of organic material by microorganisms and root respiration. In our study CO2 emission flux decreased rapidly 1 month of DBH time (Fig. 2). It may be the de-oxidization of soil organic material which lessened the heterotrophic microorganisms (Robertson et al., 2000). But CO2 fluxes increased rapidly just 15 days of DBH for all treatments. It might be possible that drained paddy fields resulted into aerobic condition and at that time air temperature was at its peak (Fig. 1). In our study, cumulative CO2 emissions was lower than those from no-tillage rice fields reported by Harada et al. (2007), who obtained 1330–1360 g CO2 m2 of cumulative CO2 emissions from different rice tillage systems at Ogata farm, Japan. Al-Kaisi and Yin (2005) measured three times more CO2 emission from CT than from NT during the 6 h period following tillage, while the CO2 emissions measured by Reicosky and Lindstrom (1993) were 20 times more. In this study the accumulated CO2 emission were not measured during the first 6 h period of tillage systems. This probably might be the reason for the less CO2 fluxes. 4.2. CH4 emission from different tillage systems Different factors effecting CH4 emission flux have been extensively explained in literatures. Several authors (e.g. Weier, 1999; Guo and Zhou, 2007) have reported that tillage, nitrogenous fertilization and irrigation influence emissions of CH4 in soils. In our study, as shown in Fig. 3, CH4 emission fluxes in all treatments were initially low and increased drastically immediately after drainage the field. Several authors have speculated that in the first 2 weeks after rice seed sowing the level of methanogenic bacteria are generally low, thus no higher CH4 emission occurs (Nouchi et al., 1990; Ko and Kang, 2000). Another reason might be the high redox potential and high pH 6.58 measured in our study before flooding. CH4 production starts at below pH 6.1 and redox potential (Eh) of a soil below 150 mv (Jugsujinda et al., 1996). The Eh of soil gradually decreases after flooding (Takai et al., 1956). This is due to a decrease in the activity of the oxidized phase and increased activity of the reduced phase. Another reason at that moment just rice plant started growing (Yamane and Sato, 1963; Yoshida, 1978). The higher CH4 emission correlated with the higher at tillering stage. Jia et al. (2001) demonstrated a higher CH4 emission at

tillering stage is due to the lower rhizospheric CH4 oxidation and more effective transport mediated by rice plants. CH4 emissions increased gradually and reached at the peak in mid-July (Fig. 3). Possibly due to the fact that rice was at the highest growing stage, the available organic carbon in the form of root exudates increased (Singh et al., 1997; Kumaraswamy et al., 2000), which led to an increase of methanogen numbers. Moreover air temperature varied from 25 to 32 8C (Fig. 1), which is suitable for methanogens activities (Hou et al., 1997; Bergman et al., 1998). Therefore, at that time CH4 emissions were very high. At later growth stage of rice, CH4 emission fluxes decline gradually (Yamane and Sato, 1963; Yoshida, 1978) and CH4 was kept at relatively low levels until harvesting, and the CH4 emission fluxes were lowest at harvesting. This was due to drainage from the fields as a result increase in the activities of the oxidized phase and decreased activity of the reduced phase, and Eh of soil gradually increased (Takai et al., 1956). NT0 showed significantly lower CH4 emissions than CT0 (Table 1 and Fig. 3) because in NT0, there was no disturbance of soil which resulted significantly in higher bulk density (Table 1). Furthering this higher bulk density reduces the volume fraction of large pores. Moreover soil organic matter does not come out easily due to the cause of tillage operation in NT0. These factors influenced as mechanism of CH4 emission, thus emission reduced in NT0. For the same causes NTC also reduced lower emission than CTC. But application of fertilizer enhanced emission in NTC and CTC. It may be after fertilizer application in flooded field pH suddenly decreased which influenced CH4 emission. Khalil and Shearer (2006) observed 20–40 mg m2 h1 seasonal average CH4 emissions, whereas we found much less CH4 emission flux and that was only 10–20 mg m2 h1 as an average. It was due to the chemical N fertilizer application, different soil properties and temperature. CH4 production and oxidation can act simultaneously in soils, the increased CH4 emissions by N fertilization may result from either an increased production or a decreased oxidation of CH4 (Chu et al., 2007). Moreover CH4 emissions decreased by 29% in NT0 than in CT0, 28% in NTC than in CTC. Our findings accord with Hanaki et al. (2002) and Harada et al. (2007) but they found more reduced CH4 emissions in their study. Possibly because of the differences in soil properties and other local or environmental factors (e.g. pH, soil bulk density, fertilization, temperature) (Jiang et al., 2006). 4.3. N2O emission from different tillage systems N2O produced in soils is mainly by the microbial processes of nitrification and denitrification (Granli and Bockman, 1994). Tillage may affect biological, chemical and physical soil properties and therefore influence the release of the greenhouse gases (Oorts et al., 2007). But there is a large uncertainty associated with the estimates regarding the influence the tillage practice have on N2O emissions (Chatskikh and Olesen, 2007). Some studies have reported higher N2O emissions from no-tillage than conventional tillage soils (Ball et al., 1999; Vinten et al., 2002). There are also indications that this effect of tillage practice on N2O emissions

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diminishes after long-term practice of no-tillage (Yamulki and Jarvis, 2002; Elmi et al., 2003; Six et al., 2004). In our study, tillage did not result with significant differences for N2O emissions in NT0 and CT0, but under fertilized condition tillage significantly affected N2O emissions in NTC and CTC. Presumably, different conditions, such as the method of fertilizer application, quantity and application time, soil status and climates resulted in the differences of N2O emission. In present study, fertilization resulted in 6–8-folds of emissions of N2O compared with no fertilization, which had been documented that application of N fertilizer boosted N2O emission (Xiong et al., 2007). In the first 2 weeks after rice seeding, higher N2O emission fluxes occurred in NTC and CTC treatment (Fig. 4). The reason could possibly be due to more nitrified nitrogen resulted from large quantities of N fertilizers applied during that period (as basal). Moreover rice plants were smaller and had an inefficient root system for absorption of applied N (Dhyani and Mishra, 1992), from NTC and CTC treatment, as a result both treatment showed higher emission. In this regard NTC was significantly higher than CTC. This could possibly be due to tillage effects applied N fertilizers which did not mix with soil in the NTC and as a result, large quantities of the applied fertilizers in the NTC treatments were directly exposed to air and sunlight, and thus were nitrified easily. Nitrous oxide is emitted as a result of denitrification in anaerobic soil and nitrification in aerobic soil with the anaerobic production considered more important (Ball et al., 1999; Mkhabela et al., 2008), which is dependent on soil bulk density, large aggregates, mineral N concentration and available C etc. (e.g. Six et al., 2002; Oorts et al., 2007). No-tillage resulted in higher soil bulk density at soil surface, documented by our study results (Table 1), and thus decreased the total pore volume, especially that of large pores. This in turn decreased aeration, resulting in more anaerobic sites where denitrification can occur, which results in increases in the production of N2O from the soil applied N fertilizer (Yamulki and Jarvis, 2002). Although Arah et al. (1991) thought that higher soil bulk density restricted opportunities for gas escape, thus decreasing N2O emissions, Ball et al. (1999) pointed out that the low diffusivity near the no-tilled soil surface did not hamper the rapid emission of N2O, because the sites of production were close to the soil surface. In addition, Six et al. (2002) reported that large aggregates formed hot spots of denitrification due to anaerobic conditions inside the aggregates, leading to larger N2O fluxes. No-tillage induced more large aggregates in paddy soils (Xu et al., 2000), and therefore more N2O production resulted more N2O emissions from applied N fertilizer via denitrification in no-tillage rice fields. Moreover, more rape residues in no-tillage rice fields remained in the soil surface, providing more available C that can sustain potential of denitrification (Rochette et al., 2000; Liu et al., 2006); meanwhile, N fertilizer was applied at the soil surface, leading to more N2O production in the NTC. Accordingly, in our study, higher N2O emissions from the NTC were observed than from the CTC. However, there was no significant difference in N2O emissions between NT0 and CT0. Although more rape residues provided more available C at soil surface in NT0 than in CT0, N fertilizer was not applied. As a result, a little N2O is produced, and thus no significant differences in N2O emissions occurred between NT0 and CT0. Soil moisture was between 90% and 108% WFPS in the rice paddy fields (Zheng et al., 2000). N2O showed small amount emissions in paddy fields (Table 1) because the high soil moisture favors the reduction of N2O to N2 or the high WFPS inhibit the N2O exchange between soil and the atmosphere (Pathak et al., 2002). Effect of temperature is clear in our study. Although July and August showed several peak temperatures in our study (Fig. 1), higher emission occurred only in NTC and CTC, which might be due to application of fertilizer (Fig. 4, arrow shows). So our findings were not accorded with Hayakawa

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et al. (2009) who observed increased N2O emission in positive correlation to the recorded temperature. 4.4. GWP from different tillage systems Fluxes of CO2, CH4 and N2O between terrestrial ecosystems and the atmosphere are of highest importance as these GHGs contribute substantially to the observed global warming (IPCC, 2007). Agricultural land use, land use changes and forestry play an important role in the production and/or consumption of these gases. Tillage practices and climate affect the release of GHG but only a few integrated studies have tried to quantify gas released or to characterize the mechanisms involved in their release (IPCC, 2001). In this regard our different tillage system for rice paddy field is explainable. The tillage systems of NT0 and CT0 had no significant differences for GWP. Grace et al. (2003) estimated GWPs of 12,805.86–16,142.86 kg CO2 h1 of irrigated rice–wheat system in the IGP depending on crop management practices. But we found significantly more GWP in our study in NTC and CTC. However, they included only CH4 and N2O emissions but not CO2 emission in their estimation. But our study showed relatively higher amount contributions of CO2 to GWPs, which was for NTC and CTC (though between treatments no significant difference could be seen). Agricultural activities which contribute nitrogen to the soil (fertilizer application and N fixation) are the main cause of about 75% of global N2O emissions (Ruser et al., 2006) whereas our study showed a small contribution of N2O emission which was only 9.46% for NTC and 6.43% for CTC from rice paddy field. Wetland rice agriculture is a major anthropogenic source of atmospheric methane and this source has increased in recent years due to the expansion of rice cultivation (Conrad and Rothfuss, 1991; Kumaraswamy et al., 2000). Although CH4 contribution to GWP was highest among the GHG, we were able to reduce this CH4 emission by NTC tillage system. Moreover we achieved a good result from NTC tillage system for reduction (12%) GWP from rice paddy field in central China in compared to CTC. 5. Conclusions Rice cropping system in the central China could be a major source of atmospheric CO2, CH4 and N2O because of its large area and high use of agricultural inputs. So from our research with rice paddy field in terms of measurements of GHG and its contribution to integrated GWP from four tillage systems of NT0, CT0, NTC and CTC the following conclusion can be drawn: i. The four tillage systems NT0, CT0, NTC and CTC contributed equally to the CO2 emissions from the paddy fields. ii. It was observed that emissions were significantly higher in CT0 than in NT0 and was also higher in CTC than in NTC in cumulative CH4 emissions. iii. N2O emission was equal from NT0 and CT0 but significantly in CTC lower than in NTC. iv. Moreover, GWPs based on these GHGs emissions were lower in NT0 than in CT0 (p > 0.05), and in NTC than in CTC (p < 0.05). CTC had highest contribution to GWPs followed by NTC and least was NT0. It is apparent that NTC reduced a significant amount of GWPs (12%) compared to CTC. It can be concluded that no-tillage system is an effective strategy to reduce GHG from rice fields in central China and will contribute to alleviating global warming. Acknowledgements This research was funded by National Technology Project for high food yield, China (No. 2006BA520A02) and Agricultural

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