Greenhouse gas fluxes and NO release from a Chinese subtropical rice-winter wheat rotation system under nitrogen fertilizer management

Abstract

Although synthetic nitrogen fertilizers play an important role in increasing cereal grain yields, there have been increased concerns about their intensive utilization and environmental consequences. The overall goal of this study is to gain an insight into the integrated evaluation of greenhouse gas emission and nitric oxide (NO) release and grain yield as affected by nitrogen fertilization in a subtropical rice-wheat rotation system. The assessment was based on four consecutive yearly measurements of the fluxes of methane (CH 4), nitrous oxide (N 2 O) and ecosystem respiration (CO 2), and the simultaneous observation of NO emissions in nonrice seasons under three fertilization practices (i.e., the conventional farmers' practice with common nitrogen application rate, an alternative practice with reduced nitrogen input, and no nitrogen application as a control). Clearly, these trace gas fluxes showed largely intra-annual and interannual variations, highlighting the importance of entire year measurement for multiple years to achieve representative annual estimates. The annual mean CH 4 fluxes varied from 95 kg C ha -1 (7.8 kg C t -1 grain) for the farmers' practice to 205 kg C ha -1 (25.7 kg C t -1 grain) for the control, indicating that nitrogen fertilization inhibited CH 4 emissions. Across all the years, the annual N 2 O emissions increased exponentially with an increasing nitrogen rate and harvested aboveground biomass. The annual N 2 O emission averaged 1.3-5.3 kg N ha -1 (159-444 g N t -1 grain) for all treatments. The annual direct emission factors of N 2 O-N tended to increase with increasing nitrogen rate and averaged 0.61% and 0.85% for the alternative and farmers' practices, respectively. Over all nonrice seasons, the seasonal mean NO emissions ranged from 0.15 to 1.4 kg N ha -1 (58-253 g N t -1 grain), and were equivalent to 0.43% to 0.54% of the applied nitrogen. Averaging across the 4 years, the annual aggregate emissions of CH 4 and N 2 O were 7.4 t CO 2 -eq ha -1 (928 kg CO 2 -eq t -1 grain) for the control and 5.6-5.7 t CO 2 -eq ha -1 (468-494 kg CO 2 -eq t -1 grain) for the fertilized treatments. Despite the comparable greenhouse effect between the alternative and farmers' practices, reducing the common N rate by 37% resulted in decreased NO emission and increased nitrogen use efficiency, and negligible effects on economic return from grain yields. © 2013. American Geophysical Union. All Rights Reserved.