Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N2O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha(-1) (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N2O emission was measured periodically during the winter season and double rice growing seasons. The average N2O flux was 11.0 and 18.1 mug N m(-2) h(-1) for plot V and plot F, respectively, during winter season. During the early rice growing period, N2O emission from plot VN averaged 167 mug N m(-2) h(-1), which was eight-to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 mug N m(-2) h(-1) for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N2O flux was 14.5, 11.1, 12.1 and 9.9 mug N m(-2) h(-1) for plots VN, VU, FN and FU, respectively. The annual N2O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha(-1) and 1.3 kg N ha(-1), respectively, with synthetic N fertilizer, and were 0.99 kg N ha(-1) and 1.12 kg N ha(-1), respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N2O emission during double rice season.