Optimizing the Incorporated Amount of Chinese Milk Vetch (Astragalus sinicus L.) to Improve Rice Productivity without Increasing CH4 and N2O Emissions

Chinese milk vetch (CMV) is a leguminous green manure that is commonly cultivated in paddy fields and can partially substitute synthetic nitrogen fertilizer. However, the impacts of incorporating CMV on CH4 and N2O emissions are still a subject of controversy. Therefore, we conducted a field experiment over three years to investigate emissions under different substitution ratios: urea only (CF); incorporating a traditional amount of CMV (MV); and with incorporation ratios of 1/3 (MV1/3), 2/3 (MV2/3), and 4/3 (MV4/3) of MV for partial urea substitution. Compared with CF, MV2/3, MV, and MV 4/3 resulted in increased yields. MV and MV4/3 reduced N2O emissions but increased CH4 emissions by 28.61% and 85.60% (2019), 32.38% and 103.19% (2020), and 28.86% and 102.98% (2021), respectively, resulting in an overall increase in total global warming potential (except for MV in 2021). MV2/3 exhibited a low greenhouse gas intensity value ranging from 0.46 to 0.47. Partial least-squares-path model results showed that CH4 and N2O emissions were influenced by substitution ratios, which indirectly regulated the gene abundances of mcrA and nosZ. Overall, the impact of CMV on CH4 and N2O emissions was determined by substitution ratios. MV2/3, which involved partial substitution of synthetic N fertilizer with 15.0 t ha−1 of CMV, resulted in improved rice productivity without increasing CH4 and N2O emissions, making it a recommended approach in the study area.