Deriving Emission Factors and Estimating Direct Nitrous Oxide Emissions for Crop Cultivation in China.

Updating and refining the NO emission factors (NO-EFs) are vital to reduce the uncertainty in estimates of direct NO emissions. Based on a database with 1151 field measurements across China, the NO-EFs were established via three approaches including the maximum likelihood method, a linear regression with an intercept and a linear regression with the intercept set to 0 using 70% of the observations. The remaining 30% of the observations were then used to evaluate the predicted NO-EFs. The third method had the highest of 0.39 and the best model efficiency of 0.38 with no significant bias, showing the best calculation efficiency. The results showed that the NO-EFs varied with agroregions, crops, and management patterns. The agroregions of Huang-Huai-Hai and Yangtze River had the higher NO-EFs in maize and wheat seasons than other regions, and the highest NO-EFs of 0.66-0.92% in the rice season was found in the South and Southwest agroregions. Both fertilizer types and water regimes had the remarkable effects on NO-EFs. Based on the best estimation by the selected method, direct NO emissions from China's crop cultivation were estimated to be 194 Gg NO-N with a 95% confidence interval of 180-208 Gg NO-N in the year 2016.