Effects of organic matter incorporation on nitrous oxide emissions from rice-wheat rotation ecosystems in China

Organic matter addition is thought to be an important regulator of nitrous oxide (N2O) emissions from croplands. Contradictory effects, however, were reported in previous studies. To investigate the effects of crop residue management on N2O emissions from rice-wheat rotation ecosystems, we conducted field experiments at three sites (Suzhou, Wuxi and Jiangdu) in the Yangtze River Delta, using static chamber and gas chromatography methods. Our data show that N2O emissions throughout the rice season from plots treated with wheat straw application at a high rate (WS) prior to rice transplanting (1.1-2.0 kg N ha(-1)) were significantly lower (P < 0.05) than those from the control plots without organic matter addition or added with wheat straw at a moderate rate (1.6-2.9 kg N ha(-1)). Furthermore, the WS treatments had a residual inhibitory effect on N2O emissions in the following non-rice season, which consistently resulted in significantly lower emissions (P < 0.05) compared to the control treatments (2.2-3.1 vs. 3.9-5.6 kg N ha(-1)). In comparison to the control treatments, the WS treatments reduced both the seasonal and annual direct emission factors of the applied nitrogen (EFd) by 50-68% (mean: 57%). The addition of compost (aerobically composted rice or wheat straw harvested in the last rotation) reduced the seasonal and annual EF(d)s by 29-32%. Over the entire rice-wheat rotation cycle, annual N2O emissions from the fertilized fields at the three sites ranged from 3.3 +/- 0.3 to 16.8 +/- 0.6 kg N ha(-1), with a coefficient of variation (CV) of 61%. Similarly, the EF(d)s during the rice-wheat rotation cycle ranged from 0.4% to 2.5%, with a CV of 67%. These high spatial variations might have been related to: variations in soil properties, such as texture and soil organic carbon; management practices, such as straw treatments (i.e., compost versus fresh straw) and weather conditions, such as precipitation and rainfall distribution. Our results indicate that the incorporation of fresh wheat straw at a high rate during the rice season is an effective management practice for the mitigation of N2O emissions in rice-wheat rotation systems. Whether this practice is also effective in reducing the overall global warming potential of net N2O, CH4 and CO2 emissions needs to be seen through further studies.