Jianwen Zou, Yao Huang and Yanyu Lu
Nitrous oxide (N2O) is one of key greenhouse gases that cause global warming. It continues to rise at a rate of approximate 0.26% per year and has reached a concentration of 319ppb (10-9 mol mol-1) in 2005 (IPCC 2007a). Agriculture accounts for about 60% of global anthropogenic N2O emissions. Globally, agricultural N2O emissions have increased by nearly 17% from 1990 to 2005 (IPCC 2007b), and are projected to increase by 35-60% up to 2030 due to increased nitrogen fertilizer use and increased animal manure production (FAO 2003). The emissions of N2O that result from anthropogenic N inputs, occur through a direct pathway (i.e. directly from soils to which the N is added), and through two indirect pathways: volatilization of compounds, such as NH3 and NOX and subsequent redeposition, and through leaching and runoff. Relative to the indirect pathways, the direct emission contributes most to the agricultural N2O sources (Zheng et al. 2004). Thus, a good estimate of direct N2O emission from agricultural fields will help assess its global source strength.
The United Nations Framework Convention on Climate Change (UNFCCC) obligates all signatory parties to periodically provide national inventories on emissions and/or removals of greenhouse gases that are not controlled by the Montreal Protocol, such as N2O releases from crop production. Accordingly, the Intergovernmental Panel on Climate Change (IPCC) developed Revised 1996 IPCC guidelines for national and Greenhouse Gases Inventories (IPCC 1997) and Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2000). The present IPCC guidelines for estimating direct N2O emission from fields include a default emission factor (EF) of 1.25% (0.25-2.25%) for fertilizer-induced emission plus a background emission (B) rate of 1kgN2O-N ha-1 yr-1 (IPCC 1997). Most global extrapolations are based on the IPCC default. Nevertheless, cropping-specific and country-specific emission factors should be used where
College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing,
210095, P.R. China e-mail: [email protected]
S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.9, © Springer-Verlag Berlin Heidelberg 2009
possible, in order to reflect the specific conditions of the country and the agricultural practices involved (IPCC 2000).
China is the most important rice producing country in the world. Rough rice production in China contributes ~30% to the world total (International Rice Research Institute (IRRI 2004)). Its planting area accounts for about 20% of the world total and 23% of all cultivated land in China (Frolking et al. 2002). Of which, ~93% is irrigated rice paddies; ~5% is distributed in rain fed lowlands and ~2% in uplands (IRRI 2004). Various water management regimes are currently practiced in China's rice paddies, such as seasonal continuous flooding (F), flooding-midseason drainage-frequent water logging with intermittent irrigation (F-D-F), and flooding-midseason drainage-reflooding-moist intermittent irrigation but without water logging (F-D-F-M) (Gao and Li 1992; Huang et al. 2004). An episode of mid-season drainage for 7-10 days rather than continuous flooding is commonly employed in China to inhibit ineffective tillers, remove toxic substances and improve roots activities.
Water regime often incurs a sensitive change in N2O emission in rice paddies (Akiyama et al. 2005). It is well documented that mid-season drainage in rice paddies triggers substantial N2O emission in contrast with continuous flooding (Cai et al. 1997; Zheng et al. 2000; Jiang et al. 2003). In addition, N2O fluxes during intermittent irrigation periods depend strongly on whether or not water logging is present in paddy fields, which often begets a significant difference in seasonal total of N2O emissions between the water regimes of F-D-F and F-D-F-M (Zou et al. 2005a).
Some studies have gone into quantifying fertilizer-induced N2O emission and its background emission from rice paddies at the regional and global scales (Yan et al. 2003; Zheng et al. 2004; Akiyama et al. 2005). Yan et al. (2003) estimated N2O emission factors and background emissions in irrigated paddy fields during the rice growing season, but they did not differentiate N2O emissions under different water regimes. In contrast, Akiyama et al. (2005) recently reported that the EFs averaged 0.22% for the continuous flooding paddies and 0.37% for the fertilized paddies with mid-season drainage. In the data set employed to estimate N2O emission factors by Akiyama et al. (2005), only 5 field studies were carried out in China (Cai et al. 1997; Chen et al. 1997; Hou et al. 2000; Zheng et al. 2000; Xiong et al. 2002). In addition, some N2O measurements from rice paddies under the F-D-F-M water regime (Xing and Zhu 1997; Zheng et al. 2000) were treated as statistical outliers, and thus they were excluded by Akiyama et al. (2005) as well.
Consequently, the estimates of emission factor and background N2O emission in previous studies may not truly reflect N2O emission from rice paddies in China where various water regimes are practiced. Here, we compiled and statistically analyzed available field measurements of N2O from rice paddies in China. County-level agricultural survey data for China in the 1980s and 1990s were acquired from the database of National Greenhouse Gases Inventories of Agriculture. The objective of this study was to quantify the cropping-specific direct emission factor and background emission for N2O during the rice growing season, and thereafter, to estimate direct N2O emissions from paddy rice production in mainland China in the 1980s-1990s.
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