How does the long-term return of mix-sowing green manures increase nitrogen utilization and decrease ecological costs of wheatland under reduced chemical nitrogen input?

Increasing nitrogen (N) utilization and reducing ecological costs of farmland are key objectives for mitigating environmental pollution and advancing sustainable agricultural development. Green manure is widely used to increase crop N efficiency while replacing partial chemical N input. However, it remains uncertain whether reducing chemical N supply, combined with mix-sowing green manures, could increase N utilization and reduce ecological costs of wheatland, revealing its mechanisms of soil N regulation and bacteria diversity. A 6-year field experiment was conducted in an arid irrigation area of northwestern China, implementing wheat multi-cropping different green manures and mix-sowing green manures (F, fallow; CV, common vetch; R, rapeseed; HCV, hairy vetch and common vetch) under reduced chemical N (N3, conventional N application rate; N2, reduced N by 20%; N1, reduced N by 40%). Our results showed that reducing chemical N decreased wheat yield and N efficiency, while green manure return increased wheat yield and N efficiency. Under N2, HCV had a higher wheat yield and N efficiency than CV, R, and F. Wheat N uptake and active N loss were decreased with chemical N reduction but enhanced with green manure return. HCVN2 improved wheat N uptake by 8.3% while reducing ammonia volatilization, nitrous oxide emissions, and nitrate leaching by 12.5%, 17.2%, and 22.1%, respectively, compared to FN3. The mechanisms of HCVN2 enhanced N utilization and reduced ecological costs of wheatland, mainly including increased N contents in the 0–40 cm soil layer, improved soil enzyme activities of N metabolism, and enriched soil bacterial diversity. Therefore, mix-sowing green manures return enhanced N utilization and decreased ecological costs of wheatland under a 20% reduction in chemical N input. • Mix-sowing green manure (HCV) enhanced gain yield and N efficiency of wheat. • HCV and reduced N by 20% can increase wheat N uptake and decrease active N loss. • HCV and reduced N by 20% improved soil N content, enzyme activity, and bacteria. • HCV and reduced N by 20% (N2) increased ecosystem socio-economic benefits. • HCVN2 can enhance N utilization and reduce ecological costs of wheatland.