Objectives This study aimed to investigate the effects of four years of Chinese milk vetch (Astragalus sinicus L.) incorporation combined with different rates of nitrogen (N) reduction on paddy soil fertility, including physicochemical properties, enzyme activities, and microbial abundance, and rice growth. The study further aimed to clarify the synergistic effects of Chinese milk vetch incorporation and N reduction in maintaining high yield, improving soil quality, and reducing chemical fertilizer inputs, thereby providing a theoretical basis for green and efficient fertilization strategies in paddy fields.

Methods A four-year field experiment was conducted from 2020 to 2024 in Jiande City, Zhejiang Province, China. Five treatments were established: winter fallow with conventional fertilization (CK), Chinese milk vetch incorporation with conventional fertilization (CT0), Chinese milk vetch incorporation with 10% N reduction (CT1), Chinese milk vetch incorporation with 20% N reduction (CT2), and Chinese milk vetch incorporation with 30% N reduction (CT3). Each treatment was replicated three times. Soil samples from the 0-20 cm soil layer and plant samples were collected at the rice maturity stage each year to determine soil nutrient contents, enzyme activities, microbial abundance, and rice yield. Rice yield components were also analyzed, and the relationships between soil properties and rice yield were evaluated using correlation analysis.

Results The results of the four consecutive years experiment demonstrated that all Chinese milk vetch incorporation treatments significantly increased rice yield, with the CT1 treatment (10% N reduction) showing the greatest improvement. In 2024, the rice yield under CT1 reached 9.64 t/hm², representing a 49.5% increase compared with conventional fertilization (CK). The four-year average yield under CT1 reached 8.28 t/hm², which was 11% higher that under CK, and the lower coefficient of variation indicated improved yield stability. Regarding soil nutrients, the CT1 treatment significantly increased soil available phosphorus content for three consecutive years, while CT0 and CT1 significantly enhanced available potassium content. Furthermore, Chinese milk vetch incorporation significantly improved soil enzyme activities and microbial abundance. Bacterial, fungal, and archaeal abundances under CT0 and CT1 treatments increased by 84.5%–101.8% compared to CK. Correlation analysis indicated that thousand-grain weight was extremely significantly positively correlated with rice yield (P < 0.01), while soil pH was significantly negatively correlated with yield. Random Forest analysis showed that organic matter content and enzyme activities were the primary factors affecting rice yield. Chinese milk vetch incorporation enhanced thousand-grain weight by increasing soil organic matter and microbial abundance, thereby achieving rice yield improvement.

Conclusions Winter planting of Chinese milk vetch combined with moderate nitrogen reduction effectively improved soil nutrient content, enzyme activities, and microbial abundance, optimized rice yield components, and enhanced yield stability. Among the tested fertilization regimes, 10%–20% nitrogen reduction combined with Chinese milk vetch incorporation exhibited the best overall performance and represents a promising strategy for sustainable rice production.