Rational utilization of leguminous green manure to mitigate methane emissions by influencing methanogenic and methanotrophic communities

Although rice straw return is widely recommended for maintaining or improving soil fertility and rice yields, it inevitably increases methane (CH4) emissions. Effective measures to mitigate CH4 emissions while guaranteeing grain yields are urgently needed. A 2-year mesocosm experiment was conducted. Six management systems, i.e., no residue (traditional management, CF), 100% rice straw (a popular practice among farmers, FR), 50% rice straw (F1/2R), leguminous green manure (FM), mixture of leguminous green manure and 100% rice straw (FMR), and a mixture of leguminous green manure and 50% rice straw (FM1/2R), were used. Compared with the CF, the organic residue returns could effectively improve rice productivity but increase the CH4 emissions. Notably, when compared with the FR, both F1/2R and FM could reduce the CH4 emissions, with equal crop yields, and FM1/2R not only increased the rice yields but also reduced the CH4 emissions, whereas FMR increased the rice yields and the CH4 emissions. Applications of organic residues significantly increased the soil methane production potentials (MPPs), methane oxidation potentials (MOPs), the abundances of methanogens and methanotrophs, and changed methanogenic and methanotrophic community structures in comparison with the CF. The partial least squares path model (PLS-PM) results showed that the MPPs had a strong direct effect on CH4 emissions, whereas an abundance of methanogens could serve as a proxy for predicting CH4 emissions. Moreover, relatively high MOPs associated with high Type I/Type II methanotroph ratios and relatively low dissolved organic carbon (DOC) concentrations were found to be central in lowering CH4 emission fractions under the F1/2R and FM1/2R treatments. Above all, the combined utilization of leguminous green manure and rice straw exhibited potential prospects for the enhancement of rice yields and for the mitigation of CH4 emissions, and the practice of mixing leguminous green manure with 50% rice straw performed the best of the treatments tested.