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Fig. 9.3 Distribution pattern of residuals for the simulated Ordinary Least Square linear regression models. (a) Model F-D-F-1; (b) model F-D-F-2; (c) model F-D-F-M; and (d) model MSD-Akiyama. The MSD-Akiyama model simulated using the data that were shown in Table 9.2 in Akiyama et al.'s (2005) study. Fit line represents the normal distribution pattern

The model F-D-F-M yielded that N2O emission factor and background emission averaged 0.73% and 0.79kgN2O-N ha-1, respectively (Table 9.5). Stand errors of EF and background emission were estimated to be 0.11 and 0.28%, respectively. Results of t-tests suggested that both nitrogen application and background emission contributed significantly to seasonal N2O total in rice paddies under the F-D-F-M water regime (Table 9.5).

9.3.3 Quantifying Seasonal Direct N2O Emissions in 1980s and 1990s

Rice planted area in mainland China totals about 32.9 million hectare in the 1980s, which is 19% greater than in the 1990s (Table 9.6). The decrease in area mainly occurred in the IV (Middle and lower Yangtze River) and V (Southern China) crop-zone regions. In the 1980s, 18% of rice paddy is continuous flooding, 77% under the water regime of F-D-F, and 5% under the F-D-F-M, while their respective percentages have changed to be 12, 75 and 13%, respectively. The percentage of various water regime derived from the surveyed data set is generally consistent with previous estimates. Xing (1998) estimated that continuous flooding rice paddies account for 10% of the total in 1995. Li et al. (2002) reported that rice paddies with mid-season drainage contribute ~80% to the total in China in 2000. Yan et al. (2003) estimated that two-thirds of rice paddy is under intermittent irrigation or middle season drainage in China.

Chemical fertilizer application rate was, on average, estimated to be 168.21 kg N ha-1 during the rice growing season in the 1980s, which is 16% lower than in the 1990s (Table 9.6). Seasonal chemical N input was highest in the II and III crop-zone regions. In contrast, manure and crop residue nitrogen inputs were generally lower in the 1990s than in the 1980s. Seasonal organic manure nitrogen input rate, with an average of 40.65 kg N ha-1, in the 1980s was almost twice as that in the 1990s. About 31% and 24% of the total above-ground crop biomass was retained in paddy fields in the 1980s and 1990s, respectively. Thereby, crop residue N retained during the rice growing season was, on average, estimated to be 6.31 kg N ha-1 in the 1980s and 4.85kgNha-1 in the 1990s for the 5 crop-zone regions in mainland China (Table 9.6).

The emission factors and background emissions simulated under different water regimes in the present study are assumed to be applicable for N2O emissions from rice paddies in mainland China in the 1980s and 1990s. Thus, N2O emissions during the rice growing season averaged 28.68 GgN2O-N in the 1980s and 32.26 GgN2O-N in the 1990s. Seasonal N2O fluxes were estimated to be, on average, 0.88 kgN2O-N ha-1 in the 1980s and 1.00kgN2O-N ha-1 in the 1990s, which was equivalent to 0.41-0.46% of the seasonal total N input (Table 9.6). Substantial N2O emission occurred in the region of Middle and lower Yangtze River, contributing 52-53% to the national total N2O emission during the rice growing season (Fig. 9.3).

Table 9.6 Rice planted area, yield, organic and chemical nitrogen inputs (Mean ± SE) and seasonal direct N20 emissions during paddy rice growing season in 1980s and 1990s

Table 9.6 Rice planted area, yield, organic and chemical nitrogen inputs (Mean ± SE) and seasonal direct N20 emissions during paddy rice growing season in 1980s and 1990s

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