Estimations of emission factors for fertilizer-induced direct N2O emissions from agricultural soils in Japan:: Summary of available data

Agricultural fields are significant sources of anthropogenic atmospheric nitrous oxide (N2O). We compiled and analyzed data on N2O emissions from Japanese agricultural fields (246 measurements from 36 sites) reported in peer-reviewed journals and research reports. Agricultural fields were classified into three categories: upland fields, tea fields and rice paddy fields. In this analysis, data measured over a period of more than 90 days for upland fields and 209 days for tea fields were used to estimate annual fertilizer-induced emission factors (EF) because of limitations in the available data. The EF is defined as the emission from fertilized plots minus the background emission (emission from a zero-N control plot), and is expressed as a percentage of the N applied. The mean of N2O emissions from upland fields with well-drained soils was significantly lower than that from poorly drained soils. Mean (+/- standard deviation) N2O emissions measured over a period of more than 90 days from fertilized upland fields were 1.03 +/- 1.14 kg N ha(-1) and 4.78 +/- 5.36 kg N ha(-1) for well-drained and poorly drained soils, respectively. Because the ratio of the total areas of well-drained soils and poorly drained soils was different from the ratio of the number of available EF data for each soil category, we used a weighted mean to estimate EF for all upland fields. The EF was estimated to be 0.62 +/- 0.48% for all fertilized upland fields. Mean N2O emissions and the estimated EF for fertilized tea fields measured over a period of more than 209 days were 24.3 +/- 16.3 kg N ha(-1) and 2.82 +/- 1.80%, respectively. The mean N2O emission and estimated EF from Japanese rice paddy fields were 0.36 kg N ha(-1) and 0.31 +/- 0.31% for the cropping season, respectively. Significant uncertainties remain in these results because of limitations in the available data.