National inventories of N2O from soils would typically contain a wide range of emission estimates. Total uncertainty in emissions inventory is generally combined by uncertainties in emission factors and activity data. Similar to uncertainty estimate in the IPCC methodology (IPCC 2000), we use the error propagation equation to calculate the uncertainties in seasonal N2O emissions from rice paddies under each water regime as follow:

Where: UC is the combined uncertainty expressed as a percentage for each water regime; UA and UE are the percentage uncertainties for the activity data and emission factor, respectively. In this study, the activity data is the input data of the estimate method and UA is principally determined by the reliability of nitrogen input data. The confidence interval of parameter estimates in the simulated model was used to calculate UE .A confidence interval of 95% that is suggested by the IPCC guidelines represents a 95% probability of containing the unknown true value. UE was expressed as half the 95% confidence interval divided by the mean.

Eventually, the total uncertainty in seasonal N2O estimates during the rice growing season for each decade was calculated by the Eq. (9.2) (IPCC 2000):

_ v(Uf 'xfY + (Uf-d-f ■ Xf-d-fY + (Uf-d-f-m • Xf-d-f-mY

Where: Utotai is the total uncertainty expressed as a percentage for each decade; x, and U, (i represents the water regimes of F, F-D-F, and F-D-F-M) are the uncertain quantities and the percentage uncertainties (i.e. UC in the Eq. (9.1)) associated with them under different water regimes, respectively.

9.3 Results

9.3.1 Interannual and Spatial Variations ofN2O Emissions

It is apparent that N2O emissions varied interannually and spatially (Tables 9.2 and 9.3). Under an identical water regime of F-D-F, seasonal total N2O in 1994 averaged 1.49 kg N2O-N ha-1 when ammonium bicarbonate was applied at the rate of 191 kgNha-1, which was 57% lower than that in 1996 in Shuzhou (Table 9.2). In Wuxi, seasonal N2O emission from plots with urea applied at the rate of 250kgNha-1, on average, amounted to 2.49kgN2O-Nha-1 in the 2002 season, which was increased by 41% as compared to that in the 2001 season. In 2002, on the other hand, seasonal total of N2O emissions from plots with urea applied at the rate of 150kgNha-1 was 2.67kgN2O-N ha-1 in Nanjing, while that from plots with the identical fertilizer application and water regime was only 1.71 kgN2O-N ha-1 in Wuxi (Table 9.3). Although it varied interannually and spatially, seasonal N2O emissions generally increased with nitrogen input under the water regime of F-D-F or the F-D-F-M (Fig. 9.1b, c).

9.3.2 Modeling Emission Factor and Background Emission for N2O

A one-way ANOVA indicated that seasonal total N2O emission from rice paddies significantly varied with water regime (F2>70 = 21.7, P <0.0001), which suggests that the effect of water regime on seasonal pattern of N2O fluxes has incurred a pronounced difference in seasonal N2O amount. To accurately quantify seasonal N2O emissions and minimize its uncertainties, the data set of N2O was classified into three categories based on water regime (F, F-D-F, and F-D-F-M, Tables 9.1, 9.2 and 9.3) and seasonal N2O emissions were separately modeled under different water regimes in this study.

Probably due to limited available data and low N2O emissions, no pronounced relationship between N2O emission and nitrogen input was found in the continuous flooding rice paddy fields (Fig. 9.2a, F1>5 = 0.25, P = 0.65). Mean and stand error of seasonal N2O amounts were estimated to be 0.048 and 0.024kgN2O-Nha-1, and those of N inputs to be 278 and 60kgNha-1 for the 6 field measurements, respectively (Table 9.1). Seasonal total ofN2O was, on average, equivalent to 0.02% of the N fertilizer applied under continuous flooding.

In contrast, a significant linear relationship between seasonal N2O amount and N input in rice paddies was found under the water regime of F-D-F (Table 9.4, Fig. 9.2b). Based on simulated parameters of the model F-D-F-1 (Table 9.5), the fertilizer-induced emission factor for N2O during the rice growing season averaged 0.42%, with a stand error of 0.13%. Seasonal background N2O emission was estimated to be 0.009 kg N2O-N ha-1, with a high stand error (0.30kgN2O-N ha-1), suggesting its large uncertainty. A t-test showed that this estimated value did not

Fig. 9.2 Dependence of seasonal N2O amount on nitrogen input in rice paddies under different water regimes.

(a) Continuous flooding (F);

(b) flooding-mid-season drainage-reflooding (F-D-F); and (c) flooding-midseason drainage-reflooding-moisture intermittent irrigation but without water logging (F-D-F-M)

Fig. 9.2 Dependence of seasonal N2O amount on nitrogen input in rice paddies under different water regimes.

(a) Continuous flooding (F);

(b) flooding-mid-season drainage-reflooding (F-D-F); and (c) flooding-midseason drainage-reflooding-moisture intermittent irrigation but without water logging (F-D-F-M)

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