Do nitrogen fertilizers stimulate or inhibit methane emissions from rice fields?

In rice cultivation, there are controversial reports on net impacts of nitrogen (N) fertilizers on methane (CH 4) emissions. Nitrogen fertilizers increase crop growth as well as alter CH 4 producing (Methanogens) and consuming (Methanotrophs) microbes, and thereby produce complex effects on CH 4 emissions. Objectives of this study were to determine net impact of N fertilizers on CH 4 emissions and to identify their underlying mechanisms in the rice soils. Database was obtained from 33 published papers that contained CH 4 emissions observations from N fertilizer (28406kgNha-1) treatment and its control. Results have indicated that N fertilizers increased CH 4 emissions in 98 of 155 data pairs in rice soils. Response of CH 4 emissions per kg N fertilizer was significantly (P<0.05) greater at<140kgNha-1 than>140kgNha-1 indicating that substrate switch from CH 4 to ammonia by Methanotrophs may not be a dominant mechanism for increased CH 4 emissions. On the contrary, decreased CH 4 emission in intermittent drainage by N fertilizers has suggested the stimulation of Methanotrophs in rice soils. Effects of N fertilizer stimulated Methanotrophs in reducing CH 4 emissions were modified by the continuous flood irrigation due to limitation of oxygen to Methanotrophs. Greater response of CH 4 emissions per kg N fertilizer in urea than ammonia sulfate probably indicated the interference of sulfate in the CH 4 production process. Overall, response of CH 4 emissions to N fertilizers was correlated with N-induced crop yield (r=+0.39; P<0.01), probably due to increased carbon substrates for Methanogens. Using CH 4 emission observations, this meta-analysis has identified dominant microbial processes that control net effects of N fertilizers on CH 4 emissions in rice soils. Finally, we have provided a conceptual model that included microbial processes and controlling factors to predict effects of N fertilizers on CH 4 emissions in rice soils.