Mixed cropping green manure can simultaneously improve the nutrient yield and quality of spring wheat grain under reduced chemical nitrogen supply

Ensuring the provision of adequate and nutritious food for humans through sustainable agricultural development poses a major challenge. Optimizing the nitrogen supply is one of the key factors for improving crop grain yield and quality. Green manure is often used to optimize the nitrogen supply in crop production, but it is unclear whether green manure can maintain nutrient yield and quality of spring wheat under reduced chemical nitrogen input. A split-plot field experiment of several varieties of green manure and mixed cropping with green manure was established in an arid area in 2018. This study aimed to explore the feasibility of using mixed common vetch and hairy vetch, which could simultaneously maintain high nutrient production and grain quality of spring wheat under reduced chemical nitrogen input, and to reveal the mechanism of nitrogen metabolism. The effects of green manure and reduced chemical nitrogen on nutrient yield, amino acid contents, vitamin B contents, mineral contents, and processing quality of spring wheat grain, as well as nitrogen accumulation, remobilization, and assimilation, were examined from 2020 to 2022. Our results showed that a reduced chemical nitrogen input led to lower nutrient production, but green manure could increase the protein and starch yields of wheat grain. HCVN2 (mixed hairy vetch and common vetch under 20% nitrogen reduction) displayed higher protein and starch yields, which increased by 35.9 and 16.2% compared to fallow after wheat harvest with conventional nitrogen application (FN3). Meanwhile, reduced chemical nitrogen reduced the wheat grain quality, but green manure improved wheat grain quality. HCVN2 had higher wheat grain qualities, which improved by 13.2 and 10.0% for essential amino acid and non-essential amino acid contents, by 20.0 and 22.2% for vitamin B and zinc contents, and by 14.0 and 8.6% for falling number and wet gluten compared to FN3, respectively. HCVN2 could simultaneously improve the nutrient production and quality of wheat grain. This was attributed to significantly increasing the nitrogen accumulation and distribution in grain; enhancing the contribution rates of leaf, stem-sheath, and cob-glume nitrogen to grain nitrogen; and promoting the activities of nitrate reductase and glutamine synthetase. Therefore, mixed sown green manure under chemical nitrogen reduced by 20% is promising for improving the nutrient production and quality of spring wheat grain by promoting nitrogen accumulation, remobilization, and assimilation.