Methane and nitrous oxide emissions from agricultural fields in Japan and mitigation options: a review

Agriculture is a significant anthropogenic source of methane (CH4) and nitrous oxide (N2O). This review provides an overview of the emissions of the two greenhouse gases (GHGs) from agricultural fields and mitigation options based on previously published field studies mainly conducted in Japan. Rice paddy fields in Japan emitted 11,982 kt CO(2)eq of CH4, accounting for 43.6% of the total CH4 anthropogenic emissions in 2021. Global meta-analysis shows that mid-season drainage (MD) is more effective in reducing CH4 emissions from rice paddy fields than continuous flooding. In the regions where MD is already practiced, prolonged MD (a week longer than usual MD) has been proposed to further reduce CH4 emissions compared to conventional MD. A field survey conducted at nine sites across Japan showed that prolonging MD reduced CH4 emissions and global warming potential (CH4 + N2O) by 30% and 28%, respectively. Moreover, in Japan, the autumn incorporation of rice straw was shown to reduce annual CH4 emissions by 54% on average compared with spring incorporation. Subsurface drainage and the application of iron-containing materials have been shown to reduce CH4 emissions from paddy fields, although further field studies testing various conditions are needed. In addition to CH4 emissions from rice paddy fields, agricultural soil in Japan in 2021 emitted 5962 kt CO(2)eq of N2O, accounting for 28.9% of the total anthropogenic N2O emissions. Meta-analyses indicated that nitrification inhibitors and polymer-coated fertilizers were effective in reducing N2O emissions after the application of synthetic N fertilizers. Moreover, biochar application was shown to be effective in sequestering soil carbon and has also been investigated as a mitigation option to reduce CH4 and N2O emissions; however, the results were inconclusive. The Ministry of Agriculture, Forestry, and Fisheries is now promoting prolonged MD and autumn incorporation of rice straw through a direct payment program to reduce CH4 emissions from paddy fields in Japan. In addition, prolonged MD and biochar application have been approved as J-Credit methodologies to reduce CH4 emissions from paddy fields and sequester soil carbon, respectively. Further studies are warranted to develop practical mitigation strategies to reduce GHG emissions while maintaining productivity.