Methane and nitrous oxide emissions from rice paddy fields as affected by nitrogen fertilisers and water management

Methane and N2O emissions affected by nitrogen fertilisers were measured simultaneously in rice paddy fields under intermittent irrigation in 1994. Ammonium sulphate and urea were applied at rates of 0 (control), 100 and 300 kg N ha(-1). The results showed that CH4 emission, on the average, decreased by 42 and 60% in the ammonium sulphate treatments and 7 and 14% in the urea treatments at rates of 100 and 300 kg N ha(-1), respectively, compared to the control. N2O emission increased significantly with the increase in the nitrogen application rate. N2O emission was higher from ammonium sulphate treatments than from the urea treatments at the same application rate. A trade-off effect between CH4 and N2O emission was clearly observed. The N2O flux was very small when the rice paddy plots were flooded, but peaked at the beginning of the disappearance of floodwater. In contrast, the CH4 flux peaked during flooding and was significantly depressed by mid-season aeration (MSA). The results suggest that it is important to evaluate the integrative effects of water management and fertiliser application for mitigating greenhouse gas emissions in order to attenuate the greenhouse effect contributed by rice paddy fields.