Effects of Chinese Milk Vetch Returning on Soil Properties, Microbial Community, and Rice Yield in Paddy Soil

The application of green manure is considered a sustainable management approach to improve soil quality and crop yield in agricultural production. However, few studies have focused on the effects on soil properties and the microbial community when varied green manure rates are applied in rice cropping systems. We examined the soil and microbial properties in paddy soil and evaluated rice productivity and sustainable yield index in response to Chinese milk vetch (MV) application at various rates (0, 22.5, 30, 45, and 60 Mg ha−1, labeled MV0, MV22.5, MV30, MV45, and MV60, respectively) in a seven-year (2014–2020) field experiment. For all treatments, MV application significantly improved rice yield by 22.5–29.7% and sustainable yield index by 10.3–13.0% compared with no MV application, and the highest average yield across the 7 years of the study was found in MV60, followed by MV45, MV30, and MV22.5. Compared with MV0, MV45 and MV60 treatments significantly reduced soil bulk density (BD), increased soil organic carbon (SOC), total nitrogen (TN), total potassium (TK), and alkali solution nitrogen (AN), particularly in MV45. Moreover, compared with MV0, a marked decrease in the alpha diversity (Shannon index) of the bacterial community and a significant increase in the relative abundance of Firmicutes were observed under MV45. The alpha diversity of the fungal community did not vary across treatments, but MV45 significantly reduced the relative abundance of Ascomycota and Chytridiomycota compared with MV0. In addition, redundancy analysis indicated that the fungal community structure was significantly related to soil BD, and random forest classification analysis demonstrated that SOC and AN were the most important factors for rice yield in this study. Overall, these results indicated that MV application at 45 Mg ha−1 should be recommended to simultaneously achieve the improvement of rice yield and soil quality in the South China.