Direct emission factor for N2O from rice–winter wheat rotation systems in southeast China

A field experiment was conducted in a rice–winter wheat rotation agroecosystem to quantify the direct emission of N2O for synthetic N fertilizer and crop residue application in the 2002–2003 annual cycle. There was an increase in N2O emission accompanying synthetic N fertilizer application. Fertilizer-induced emission factor for N2O (FIE) averaged 1.08% for the rice season, 1.49% for the winter wheat season and 1.26% for the whole annual rotation cycle. The annual background emission of N2O totaled 4.81 kg N2O–N ha−1, consisting of 1.24 kg N2O–N ha−1 for rice, 3.11 kg N2O–N ha−1 for wheat seasons. When crop residue and synthetic N fertilizer were both applied in the fields, crop residue-induced emission factor for N2O (RIE) was estimated as well. When crop residue was retained at the rate of 2.25 and 4.50 t ha−1 for each season, the RIE averaged 0.64% and 0.27% for the whole annual rotation cycle, respectively. Based on available multi-year data of N2O emissions over the whole rice–wheat rotation cycle at 3 sites in southeast China, the FIE averaged 1.02% for the rice season, 1.65% for the wheat season. On the whole annual cycle, the FIE for N2O ranged from 1.05% to 1.45%, with an average of 1.25%. Annual background emission of N2O averaged 4.25 kg ha−1, ranging from 3.62 to 4.87 kg ha−1. It is estimated that annual N2O emission in paddy rice-based agroecosystem amounts to 169 Gg N2O–N in China, accounting for 26–60% of the reported estimates of total emission from croplands in China.