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Fig. 9.1 Spatial distribution of seasonal total N2O emissions from paddy fields during the rice growing season in mainland China in 1980s and 1990s. The rice paddies assigned to the 5 crop-zone regions: I, North and Northeast, covering Heilongjiang and Inner Mongolia, Liaoning and Jilin provinces; II, North China Plain and Central and west, covering Henan, Hebei, Tianjin, Beijing, Ningxia, Shaanxi, Shanxi, Shandong, Xingjiang and Gansu provinces; III, Southwest, covering Guizhou, Sichuan, Chongqing and Yunnan provinces; IV, Middle and lower Yangtze River, covering Anhui, Hubei, Hunan, Jiangsu, Jiangxi, Shanghai and Zhejiang provinces; V, Southern China, covering Fujian, Guangdong, Guangxi and Hainan provinces to lack of available data, we could not distinguish between organic- and inorganic nitrogen fertilizer-induced N2O emissions. Thus, seasonal total nitrogen input was used to quantify the fertilizer-induced emission factor of N2O in this study.

In all employed measurements, N2O fluxes were measured in situ by static chamber-gas chromatograph method. They were measured by manual sampling system, except that an automatic system was used to continuously measure N2O fluxes in one study in Shuzhou (Zheng et al. 2000). In general, gas samples were taken twice weekly (Cai et al. 1997; Cao et al. 1999; Chen et al. 1999; Zou et al. 2005b) or once a week (Yang et al. 1996; Cai et al. 1999; Xiong et al. 2002) over the entire rice growing season, except for once a day during the mid-season drainage (Xiong et al. 2003b; Zou et al. 2005a).

9.2.2 Rice Production Data Collection

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 database contains county statistics on paddy rice planting area and yield, water regime, percentage of crop residue retained, manure and synthetic nitrogen fertilizer use during the rice growing season for counties in mainland China. No data were available for Taiwan, Hong Kong or Macao. The paddy rice yield reported by IRRI was used to check the quality of the data set since only paddy rice yield can be obtained at the IRRI dataset and no nitrogen input data were available at national or international statistics dataset. Provincial rice yield derived from this surveyed (YS) county-scale data set is comparable to that officially reported (Y¡) by the IRRI (Ys = 1.046Y/, r2 = 0.635, P < 0.0001). This suggests that water regime and nitrogen input information in the data set could also have represented the actual status of paddy rice production in mainland China. In order to estimate the nitrogen input of crop residue and organic manure, we assumed that harvest biomass of crop residue is 4.51 ha-1, and the nitrogen content of crop residue and organic manure was derived from the IPCC guidelines, with an average of 0.45 and 0.71%, respectively (IPCC 2000).

9.2.3 Statistical Analysis

The relationship between N application and N2O emission established by Eichner (1990) and Bouwman (1996) motivated the concept of fertilizer-induced emission factor (EF). EF is defined as N2O emission from nitrogen fertilizer plots minus the emission from unfertilized control plots (all other conditions being equal to those of the fertilized plots) expressed as a percentage of N applied. This IPCC methodology suggests that linear regression models could be used to quantify N2O emissions. In the present study, we used a linear regression model (N2O-N = EF-N + B + s) with the character of Ordinary Least Squares (OLS)

to fit N2O emissions (N2O-N) by nitrogen inputs (N). In this model, e denotes the error term; EF and B are the simulated parameters that represent N2O emission factor and background emission, respectively. A t-test was used to examine statistic significance of the parameter estimates. An analysis of variance (ANOVA) F-test partitioned the total variation of N2O-N into the linear relationship with N and the part not explained by the relationship. The model fitness to the data was examined by both residual distribution pattern and power analysis. We also used a one-way ANOVA to test whether seasonal N2O amount depended on the water regime. The statistic analyses were conducted using JMP IN 5.1 (SAS INC. 2003).

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