A 3-year record of N2O and CH4 emissions from a sandy loam paddy during rice seasons as affected by different nitrogen application rates

Greenhouse gas fluxes from rice paddies under nitrogen fertilization merit serious attention because nitrogen fertilizer is increasingly used for the intensification of rice cultivation. A 3-year field study was conducted to measure methane (CH4) and nitrous oxide (N2O) fluxes simultaneously in a sandy loam paddy field under three nitrogen application rates (0, 150 and 250 kg N ha−1) in the Yangtze River Delta from 2005 through 2007. The rice paddies were under a typical Chinese water regime, characterized by intermittent irrigation with midseason drainage. The results revealed a trade-off between CH4 and N2O emissions as influenced by the application of urea-based fertilizers, i.e., the nitrogen fertilization reduced CH4 emissions from rice paddies but increased N2O emissions. The seasonal CH4 emissions averaged 155.9 kg C ha−1 in the absence of nitrogen amendment. Compared to no nitrogen addition, the seasonal CH4 emissions were decreased by 27% and 53% in the fertilized plots at rates of 150 and 250 kg N ha−1, respectively. It was most likely that the sandy loam texture combined with the addition of urea-based fertilizers stimulated growth and activity of methane oxidizers. In contrast, nitrogen addition increased N2O emissions 2.5 times for an application rate of 150 kg N ha−1 and 6.0 times at 250 kg N ha−1, compared to no nitrogen addition (0.38 kg N ha−1). The direct emission factor of fertilizer N for N2O was estimated to be 0.39–1.22% for rice fields, with a mean value of 0.77%. The overall emissions of CH4 and N2O, expressed as carbon dioxide equivalents were affected by the nitrogen addition rate, with the minimum emissions occurring at 250 kg N ha−1. This result indicated that the commonly applied nitrogen rate in this region might be an effective option for mitigating the combined impacts of rice production. Our results also demonstrate the presence of large interannual variations in the CH4 and N2O fluxes. It is probable that these variations resulted from differences in the amount and distribution of precipitation.