Legume green manure can intensify the function of chemical nitrogen fertilizer substitution via increasing nitrogen supply and uptake of wheat

Achieving the green development of agriculture requires the reduction of chemical nitrogen (N) fertilizer input. Previous studies have confirmed that returning green manure to the field is an effective measure to improve crop yields while substituting partial chemical N fertilizer. However, it remains unclear how to further intensify the substituting function of green manure and elucidate its underlying agronomic mechanism. In a split-plot field experiment in spring wheat, different green manures returned to the field under reduced chemical N supply was established in an oasis area since 2018, in order to investigate the effect of green manure and reduced N on grain yield, N uptake, N use efficiency (NUE), N nutrition index, soil organic matter, and soil N of wheat in 2020–2022. Our results showed that mixed sown common vetch and hairy vetch can substitute 40% of chemical N fertilizer without reducing grain yield or N accumulation. Noteworthily, mixed sown common vetch and hairy vetch under reduced N by 20% showed the highest N agronomy efficiency and recovery efficiency, which were 92.0% and 46.0% higher than fallow after wheat harvest and conventional N application rate, respectively. The increase in NUE of wheat was mainly attributed to mixed sown common vetch and hairy vetch, which increased N transportation quantity and transportation rate at pre-anthesis, enhanced N harvest index, optimized N nutrition index, and increased activities of nitrate reductase and glutamine synthetase of leaf, respectively. Meanwhile, mixed sown common vetch and hairy vetch under reduced N by 20% improved soil organic matter and N contents. Therefore, mixed sown common vetch and hairy vetch can substitute 40% of chemical N fertilizer while maintaining grain yield and N accumulation, and it combined with reduced chemical N by 20% or 40% improved NUE of wheat via enhancing N supply and uptake.