Regulatory Effects of Green Manure Combined with Nitrogen Reduction on Carbon-Cycling Functional Genes and Microbial Communities in Paddy Soils

Excessive nitrogen (N) fertilization in rice systems has caused soil degradation and reduced N use efficiency. Green manure, especially Astragalus sinicus (Chinese milk vetch), provides a sustainable alternative, but the microbial and functional gene mechanisms underlying its interaction with reduced N input remain unclear. In this study, a field experiment was conducted at Dingdian Village, Natong Town, Long’an County, Nanning City, Guangxi Province, China (107°51′21″ E, 23°00′41″ N) during the 2018–2019 rice growing seasons. Four treatments were established: conventional N fertilization (N100), 20% N reduction (N80), green manure plus full N (GMN100), and green manure plus 20% N reduction (GMN80). Soil physicochemical traits, microbial community composition, and carbon-cycling functional genes were analyzed using high-throughput sequencing and metagenomic profiling. Compared with N100, GMN80 significantly increased soil organic matter (by 21.3%), microbial biomass carbon (by 32.6%), and available phosphorus (by 17.8%). The Shannon index rose from 4.18 to 4.63, while Proteobacteria and Actinobacteria increased by 9.5% and 7.2%, respectively. Functional genes encoding glycoside hydrolases (GH5, GH9) and carbohydrate esterases (CE1, CE10) were enriched by 25–40%, with upregulation of carbon fixation (rbcL) and methane metabolism (mcrA) genes. Integrating A. sinicus with moderate N reduction improves soil fertility, stimulates microbial diversity, and enhances carbon turnover efficiency, offering a practical pathway toward sustainable low-carbon rice production.