Methane emission from rice fields as affected by land use change

The purpose of this study was to evaluate how former upland cultivation history affects CH4 emission from rice paddies. We measured CH4 flux, methanogen population and in situ Fe(III) reduction in the rice paddies following three different lengths of time since upland crop (Soybean) cultivation. Results showed that CH4 emissions from long-term rice paddy (19 yearʹs continuous cultivation) were significantly higher than recently converted ones. Temporal dynamics of methanogens on rice roots also varied among the plots, and showed a good correlation with CH4 emission rates. Cumulative Fe(III) reduction acted as the dominant electron acceptor in all plots, accounting for 68–94% of the total electron consumption. Fe(II) concentration was highest in the 19-year plot and lowest in the 1-year plots, indicating lower electron availability in recently converted paddies necessary for Fe reduction and CH4 production. Anoxic laboratory soil incubation also suggested poor availability of electron donors in the recent paddies. Collectively, our results demonstrate that the conversion of upland to paddy rice cultivation significantly affected CH4 emission through changing availability of electron donors, redox status of soil Fe and activity of methanogens, which ultimately caused low CH4 emissions.