Assessment of the methane mitigation potentials of alternative water regimes in rice fields using a process‐based biogeochemistry model

Rice production is a substantial source of atmospheric CH 4 , which is second only to CO 2 as a contributor to global warming. Since CH 4 is produced in anaerobic soil environments, water management is expected to be a practical measure to mitigate CH 4 emissions. In this study, we used a process‐based biogeochemistry model (DNDC‐Rice) to assess the CH 4 mitigation potentials of alternative water regimes (AWR) for rice fields at a regional scale. Before regional application, we tested DNDC‐Rice using site‐scale data from three rice fields in Japan with different water regimes. The observed CH 4 emissions were reduced by drainage of the fields, but were enhanced by organic amendments. DNDC‐Rice gave acceptable predictions of variation in daily CH 4 fluxes and seasonal CH 4 emissions due to changes in the water regime. For regional application, we constructed a GIS database at a 1 × 1 km mesh scale that contained data on rice field area, soil properties, daily weather, and farming management of each cell in the mesh, covering 3.2% of the rice fields in Japan's Hokkaido region. We ran DNDC‐Rice to simulate CH 4 emissions under five simulated water regimes: the conventional water regime and four AWR scenarios with gradually increasing drainage. We found that AWR can reduce CH 4 emission by up to 41% compared with the emission under conventional water regime. Including the changes in CO 2 and nitrous oxide emissions, potential mitigation of greenhouse gas (GHG) was 2.6 Mg CO 2 Eq. ha −1 yr −1 . If this estimate is expanded to Japan's total rice fields, expected GHG mitigation is 4.3 Tg CO 2 Eq. yr −1 , which accounts for 0.32% of total GHG emissions from Japan. For a reliable national‐scale assessment, however, databases on soil, weather, and farming management must be constructed at a national scale, as these factors are widely variable between regions in Japan.