Objective The aim was to clarify the effects of soybean green manure on soil quality and organic carbon fractions under different fertilizer reduction ratios, and to provide theoretical support for the effects of soybean green manure on soil fertilization and carbon accumulation.

Method The experiment was a two-year pot experiment with six treatments: no fertilizer (CK), conventional fertilizer (CF) (N and P fertilizer) and nitrogen and P fertilizer (RF6, RF12, RF18 and RF24) reduced by 6%, 12%, 18% and 24%, respectively. The contents of plant dry matter, carbon, nitrogen, phosphorus and potassium, and soil nutrients and organic carbon fractions were measured.

Result Multiple planting of soybean green manure after wheat and returning to field increased the grain yield of wheat in the second year, and CF, RF6 and RF12 treatments were significantly higher than other treatments (P < 0.05). The grain yields of CF, RF6 and RF12 treatments in 2022 were increased by 32.2%-40.6% (P < 0.01) compared with 2021. The contents of alkaline nitrogen and available phosphorus increased by 8.4%-22.0% and 49.9%-56.9%. Available potassium decreased by 9.7%-14.2%. The contents of soil organic carbon, high reactive organic carbon and microbial biomass carbon increased by 14.4%-14.5%, 42.1%-42.8% and 204.0%-252.6%, respectively (P < 0.01). Soil quality evaluation showed that soil quality index of RF6 was significantly higher than those of CK and RF24 (P < 0.05), but there was no significant difference between CF and reduced fertilizer treatment.

Conclusion In conclusion, soybean green manure returned to the field increased wheat yield, soil available nitrogen, phosphorus nutrients and organic carbon content within the range of reducing nitrogen and phosphorus fertilizer by 12%. The increase of organic carbon content was mainly due to the increase of high labile fraction and microbial biomass carbon.