Objectives We studied the effects of different amounts of green manure (Vicia sativa) incorporation under different nitrogen (N) fertilization rates on wheat production and soil fertility in the system of planting Vicia sativa after wheat in Qinghai Plateau, aiming to propose an efficient combination of N fertilizer and green manure incorporation for wheat production.

Methods Field experiments were carried out in 2023 and 2024 at Qinghai University. Three N treatments were designed: no N application (N0), conventional N application (N100%), and 30% nitrogen reduction (N70%). Under each N treatment, the half amount (G1) and the full dose (G2) of the harvested fresh green manure were returned to the soil. Wheat yield and nitrogen uptake were measured. Soil chemical and microbial properties were also measured to calculate the integrated fertility index (IFI). The overall benefits of different treatments were quantified using the entropy weight-TOPSIS model.

Results There was no significant difference in wheat yield, number of grains per ear, thousand-grain weight, plant height, and ear length among the four treatments combining two nitrogen application rates and two green manure return rates (N70%G1, N70%G2, N100%G1, N100%G2). No significant differences were observed in nitrogen uptake, nitrogen harvest index, and nitrogen use efficiency between N70% and N100%. Similarly, there were no significant differences in nitrogen uptake, nitrogen harvest index, and nitrogen use efficiency between N100%G1 and N100%G2, but the nitrogen harvest index of N100%G2 was significantly higher than that of N100%G1 in 2024. The average soil bulk density of the two N70% treatments was significantly lower than that of N0 by 5.20%, while the decrease for N100% was not significant. There were no significant differences in soil water content between the two nitrogen application rates and their corresponding green manure return treatments. The microbial carbon-nitrogen ratio of each treatment ranged from 10.23 to 11.09, which was higher than 10∶1, indicating that the microorganisms were mainly fungi. There were no significant differences in the effects of the two nitrogen application rates and green manure return on soil microbial carbon and nitrogen. The soil comprehensive fertility index (IFI) of the N70%G2 and N70%G1 treatments was 10.64% and 12.41% higher than that of N0G2 and N0G1, respectively. There were no significant differences in total income and net income between the two nitrogen application rates and the amounts of green manure returned. The comprehensive evaluation using the entropy weight-TOPSIS model showed that the treatment combining half-amount green manure return with a 30% nitrogen reduction has a comprehensive score of 0.55, which is higher than that of other treatments.

Conclusions Under the condition of multiple cropping of green manure after wheat harvest and returning half of the harvested green manure to the field, when the conventional nitrogen application rate was reduced by 30%, there were no significant differences in the agronomic traits of the harvested wheat, such as yield, 1000-grain weight, plant height, and panicle length, as well as in soil fertility and net income, compared with the treatment of conventional fertilization combined with returning the full dose of green manure to the field. Consequently, the highest comprehensive score was achieved. Therefore, returning half of the green manure to the field with a 30% nitrogen reduction is an optimized model for simultaneously achieving stable wheat yield and maintaining soil fertility in the eastern agricultural area of Qinghai, considering both ecological and economic benefits.