Objectives To investigate the effects of green manure combined with synergistic materials on soil quality and rice growth in the hilly regions of Sichuan, aiming to provide technical and theoretical support for the rational utilization of green manure and the application of synergistic materials in paddy fields within this region.

Methods The long-term field experiment included two planting systems: winter fallow (WF) and winter planting milk vetch (GM), as well as four combinations of synergistic materials involving without synergistic materials application (CK), sesbania biochar (Z1), bentonite (Z2), and urease inhibitor (Z3). The effects of different synergistic materials on rice yield and nutrient uptake under two cropping systems were analyzed. Additionally, physical properties (bulk density, porosity, aggregates), chemical properties (pH and nutrients), and biological properties (microbial biomass carbon and nitrogen, enzyme activity) were analyzed. The soil quality was fully evaluated by calculating the soil quality index (SQI).

Results Compared with GM-CK, the yield of rice under the GM-Z2 treatment was significantly increased by 7.8% in 2023, while that under the GM-Z3 treatment was significantly increased by 9.7% and 13.0% in 2023 and 2024, respectively. Compared with WF-Z2, the yield of rice under the GM-Z2 treatment was significantly increased by 18.3% and 17.6% in 2023 and 2024, respectively. Compared with WF-Z3, the yield of rice under the GM-Z3 treatment was significantly increased by 14.6% and 17.7% in 2023 and 2024. The GM-Z3 treatment had the best effect on improving grain yield. Compared with GM-CK and WF-Z3 treatments, GM-Z3 significantly increased the nitrogen, phosphorus, and potassium uptake by the above-ground parts of rice plants. Compared with GM-CK and WF-Z1 treatments, the GM-Z1 treatment significantly reduced soil bulk density while increasing soil organic carbon content, total nitrogen content, microbial biomass carbon content, porosity, and sucrase activity. Compared with GM-CK and WF-Z2 treatments, the GM-Z2 treatment significantly increased soil available phosphorus content. Under both cropping systems, the Z1 treatment significantly reduced soil bulk density and significantly increased soil aggregate size distribution, soil porosity, available phosphorus content, and available potassium content. In contrast, the Z3 treatment significantly increased soil aggregate size distribution, soil total nitrogen content, and available potassium content. A comprehensive evaluation of soil quality using the minimum data set method indicated that compared with GM-CK and WF-Z1, the soil quality index under the GM-Z1 treatment was significantly increased by 14.5% and 28.9%, respectively. Compared with GM-CK and WF-Z3, the soil quality index under the GM-Z3 treatment was significantly increased by 5.9% and 11.8%, respectively. The GM-Z1 treatment had the best effect on improving soil quality, and the soil quality index was 0.581.

Conclusions In the purple soil paddy rice cropping system of the hilly region of Sichuan, winter cultivation of Astragalus sinicus (Chinese milk vetch) significantly enhances integrated soil fertility. The co-application of synergistic amendments further optimizes its effects on yield improvement and soil amelioration. Specifically, the combined use of A. sinicus with a urease inhibitor primarily elevates soil total nitrogen content by mitigating ammonia volatilization and nitrogen loss. The integration of milk vetch with sesbania biochar stimulates microbial activity, thereby facilitating organic carbon transformation and nutrient cycling. Concurrently, it reinforces soil aggregate stability and improves water and nutrient retention capacity, effectively minimizing nitrogen leaching losses during the decomposition of milk vetch.