GHG Emission from Postapplication Field.
The GHG emissions caused by N fertilizer applied to croplands are mainly in the form of N2O, including direct and indirect emissions. We classified Chinese agricultural land into two groups: upland fields and paddy fields. We compiled all published field measurements in China (a total of 853) and summarized the results using a meta-analysis method to derive direct and indirect N2O emission factors. Direct emission factors for upland fields and paddy fields were obtained from a study by Gao et al. (38), which includes 456 N2O emission measurements in China (195 paddy fields and 261 upland fields). Indirect emissions include N2O resulting from N deposition (associated with NH3 volatilization) and NO3− leaching. We summarized 397 published field measurements (138 paddy fields and 259 upland fields) from 47 literature sources. We used IPCC (37) values for the proportion of those losses emitted as N2O (Table S8). Then, we calculated the GHG emission factors for paddy fields and upland fields, respectively (Table S3).