Different water regimes caused a sensitive change in N2O emission in rice paddies. Under continuous flooding, N2O emissions were generally pronounced only when fields were drained before rice harvesting (Chen et al. 1995; Lu et al. 1997). In contrast to continuous flooding, mid-season drainage triggered substantial N2O emission from rice paddies under the F-D-F water regime (Xiong et al. 2002; Jiang et al. 2003; Zou et al. 2005a). Based on the results of this study, we predict that seasonal N2O emissions will amount to 0.03kgN2O-N ha-1 when nitrogen is applied at the rate of 150 kg N ha-1 in the continuous flooding rice paddies, which is similar to the results of earlier studies at other regions (Smith et al. 1982; Granli and B0ckman 1994). However, seasonal total N2O will be, on average, up to 1.87kgN2O-N ha-1 under the water regime of F-D-F-M, which is threefold as much as that under the water regime of F-D-F.
Primarily, N2O is produced in soils via the biogeochemical processes of nitrification and denitrification that are greatly influenced by soil water status. In contrast to paddies with the water regime of F-D-F, or the seasonal continuous flooding paddies, the dry-wet alteration after mid-term drainage created a favorable soil environment to both nitrification and denitrification processes, which contributed greatly to higher N2O emissions under the water regime of F-D-F-M. Under continuous flooding, a large proportion of N2O produced from denitrification would be further reduced to N2 before leaving the soil (Firestone and Davidson 1989). On the other hand, water regime might influence the availability of nitrogen, labile C compounds, and O2 in paddy soils that are key factors to N2O production in general denitrification models (Firestone and Davidson 1989). The mid-season drainage and dry-wet alteration are able to improve root activities and accelerate soil organic C decomposition, which might produce more available C and N for soil microbes, and thereby favor N2O emissions.
Water management pattern in rice production has been greatly changed in mainland China. Since the 1980s, mid-season drainage has been commonly adopted to increase rice productivity. Due to water resources scarcity and cultivation technique development, the water regime of F-D-F-M as a water-saving irrigation technology has been increasingly practiced in China's rice production. For example, water is especially scarce in the North China Plain that contains 26% of the China's cultivated land, 30% of its irrigated land, and 24% of its total grain production (Geng et al. 2001). The water regime of F-D-F-M and aerobic rice paddies, instead of anaerobic paddies, have been suggested as potential options for rice production in this area. However, a process-based model estimated that shifting water management from continuous flooding to mid-season drainage increased N2O emissions from Chinese rice paddies by 0.13-0.20 TgN2O-N yr-1 (Li et al. 2004b) or 0.15TgN2O-N yr-1 (Li et al. 2005). In addition, N2O emissions have been shown to be extremely higher from aerobic rice paddies compared to anaerobic paddies (Xu et al. 2004). Therefore, these options would increase N2O emissions from rice production in China. Indeed, how to reconcile increasing N2O emissions and scarcity of water resources with the development of rice production, has become a real challenge in mainland China.
9.4.5 Contribution of Rice Production to Total N2O Emission from Croplands
Using a precipitation-rectified emission factor model and the IPCC uncertainty estimate methodology, we estimated EF of N2O in China's uplands to be 1.14% in 1997 with an uncertainty of 29% (Lu et al. 2006). Xing (1998) reported that direct N2O emissions from paddy fields totaled 88 Gg N2O-N, consisting of 35 Gg N2O-N emitted during the rice growing season and 53 Gg N2O-N during the upland crop seasons. Direct N2O emission from croplands in China was estimated to be 275 Gg N2O-N yr-1 in the 1990s by Zheng et al. (2004), or 398GgN2O-N in 1995 by Xing (1998). These estimates suggest that rice production occurring on 23% of the cultivated land accounts for 7-11% of the total N2O emission from croplands in China. Therefore, due to rice planting area increase in the past decades and lower emission factor, paddy rice relative to upland crop production could have greatly contributed to mitigating N2O emissions from agriculture in China.
9.4.6 Uncertainties in Quantifying Direct N2O Emission
In the present study, we did not find a significant relationship between N2O emission and nitrogen input in the continuous flooding rice paddy fields. Besides, the scanty measurements and low N2O emission may be another important cause. In contrast to continuous flooding, N2O emissions were significantly higher in paddy fields with mid-season drainage and thereby relationship between N2O emission and nitrogen input became pronounced. Under the water regime of F-D-F, fertilizer input and background emission in the simulated regression model can only explain 29% of the variability in the 27 observed seasonal average N2O fluxes. Under the water regime of F-D-F-M, however, up to 56% of the variability in the 38 observed N2O measurements can be explained by fertilizer together with background emission in the simulated regression model (Table 9.4).
Obviously, some factors other than water regime may also be important to N2O emission factor in rice paddy fields. Besides fertilizer amount, fertilizer type has been recognized as another factor influencing N2O emissions in agricultural fields
(Bouwman et al. 2002). Although seasonal N2O emissions generally increased with fertilizer input, it varied with the type of fertilizer as well in rice paddy fields. Compared with urea, application of ammonium sulphate or ammonium bicarbonate induced higher N2O emission under an identical water regime of F-D-F or F-D-F-M (Cai et al. 1997; Zheng et al. 2000). In contrast to pure chemical fertilizer application, organic manure and crop residue amendments increased seasonal N2O emissions in some studies (Zheng et al. 2000; Zou et al. 2004, 2005b), while they decreased N2O emissions in other studies (Xiong et al. 2003b).
Difference in frequency of N2O measurements may also contribute to its estimate for uncertainty. Ideally, N2O emissions should be measured frequently enough to capture its peak fluxes. Sharp peaks of N2O fluxes in paddy fields were observed in a study using an automated monitor system (Zheng et al. 2000). Relative to measurements once a week, measurements twice weekly showed more peak fluxes of N2O, particularly after nitrogen fertilizer was applied in rice paddies (Zou et al. 2005a). However, most studies in the data set measured N2O flux only once a week. As a consequence, some flux peaks might have been missed and seasonal N2O emissions could have been underestimated in these studies.
It is note worthy that this study only estimated direct N2O emissions during the rice growing season, but did not count those during the following non-rice seasons in paddy fields. Although water regime has distinguished N2O emissions in rice paddies from upland crops, some agricultural practices, such as water management and organic incorporation during the rice growing season, may have a substantial effect on the following seasonal N2O emissions (Zou et al. 2005b). Results of our previous study in a paddy rice-winter wheat rotation system indicated that compared with the water regime of F-D-F, continuous flooding in the rice season significantly increased N2O emissions from the winter wheat growing season. As well, wheat residue incorporation before rice transplanting had a far-lasting effect on N2O emissions during the winter wheat growing season (Zou et al. 2003). Therefore, annual total of N2O emissions in rice paddies would be underestimated by extrapolating N2O data during the rice growing season.
During rice growing season, N2O emissions depended significantly on water regime in paddy fields. Seasonal total N2O was, on average, equivalent to 0.02% of the nitrogen input in the continuous flooding rice paddies. The emission factor of fertilizer for N2O averaged 0.42% and 0.73% under the F-D-F and the F-D-F-M water regimes, respectively. N2O background emission during the rice growing season was not pronounced under the water regime of F-D-F, but it amounted to 0.79 kg N2O-N ha-1 under the F-D-F-M water regime. Seasonal N2O emissions amounted to 32.26GgN2O-N in the 1990s and 28.68GgN2O-N in the 1980s, accounting for 7-11% increase of the reported estimates of annual total emission from croplands in mainland China. Relative to upland crop production, paddy rice development in the past decades could have greatly contributed to mitigating N2O emissions from agriculture in China.
Acknowledgments This study was supported by the Nanjing Agricultural University and National Natural Science Foundation of China (40431001).
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