Objectives This study aimed to investigate the effects of winter Chinese milk vetch combined with different carbon-amending materials on rice yield, nutrient uptake, soil nutrients and carbon fractions, so as to provide theoretical support and technical references for further improving grain productivity and carbon sequestration capacity of green manure paddy fields.

Methods A long-term field experiment was initiated in Quzhou City, Zhejiang Province in 2021. Two main treatments were set up, including winter fallow (CK) and winter Chinese milk vetch (Astragalus sinicus L.) return (GM). Under each main treatment, three carbon-amending materials, namely Sesbania biochar (T), goethite (Z) and ethephon (Y), were applied, forming a total of eight treatments. Rice yield, nitrogen, phosphorus and potassium concentrations and accumulations in grain and straw, plant nutrient utilization efficiency, harvest index, as well as basic soil nutrient contents and carbon fractions were determined from 2022 to 2024.

Results Green manure, carbon-amending materials and experimental year all exerted extremely significant effects on rice yield (P<0.001). Except for the non-significant interaction between green manure and experimental year, the two-way and three-way interactive effects were significant or extremely significant. Compared with CK, only treatment T significantly increased the 3-year average rice yield by 8.4% under winter fallow; all four green manure treatments markedly elevated average rice yield, with GM+T and GM+Y increasing yield by 13.6% and 20.4% relative to CK, respectively. Under winter fallow, treatment T significantly raised grain nitrogen concentration, straw phosphorus concentration, grain nitrogen and potassium accumulation, as well as nitrogen and potassium harvest index by 8.4%, 17.3%, 25.4%, 20.1%, 7.4% and 21.3%, respectively; treatment Y significantly increased grain phosphorus concentration by 15.7%. Treatment GM+Y substantially improved rice nutrient uptake and harvest index, relative to CK, grain potassium and straw nitrogen concentrations rose by 17.3% and 33.0%, grain nitrogen, phosphorus and potassium accumulations increased by 40.1%, 36.7% and 47.0%, and potassium harvest index was elevated by 44.7%. Under winter fallow, soil ammonium nitrogen and available potassium contents in treatment Z increased by 21.4% and 25.2% compared with CK, while soil available phosphorus and available potassium contents in treatment Y increased by 102.3% and 28.7%. Green manure return significantly increased soil total nitrogen and ammonium nitrogen contents relative to CK, and combined application with carbon-amending materials further improved soil nutrient status. Among all treatments, compared with CK, GM+Y increased soil total nitrogen, available phosphorus and available potassium contents by 78.6%, 132.7% and 29.3%, respectively, GM+Z raised soil ammonium nitrogen by 96.1%. The application of carbon-amending materials had no significant influence on soil organic carbon content and carbon stock, but altered physical fractions of soil organic carbon. Mineral-associated organic carbon (MAOC) contents in treatments Y, GM+Z and GM+Y were significantly increased by 45.3%, 38.9% and 67.9% relative to CK, which contributed to the stability of soil carbon pool.

Conclusions Under winter fallow conditions, the application of Sesbania biochar achieved the most pronounced increase in rice yield, performing on par with the practice of planting and incorporating green manure. While goethite enhanced soil ammonium nitrogen and available potassium contents, it did not translate into a significant yield boost; ethephon likewise showed no notable effect on grain yield. Under the condition of planting and incorporating green manure, combined application of Sesbania biochar and goethite exhibited no significant synergistic effect. In contrast, the combination of green manure and ethephon significantly elevated the content of mineral-associated organic carbon (MAOC) in soil. Owing to its high stability, this fraction facilitates the accumulation of the soil carbon pool and maintains elevated levels of total nitrogen, available phosphorus, and available potassium, thereby laying a solid nutrient foundation for both high and stable yields.