This study aimed to explore the effects of multiple cropping green manure combined with different levels of nitrogen fertilizer on the yield and grain quality of spring wheat, so as to provide the theoretical guidance for the construction of chemical fertilizer reduction and high-quality production mode based on green manure in Qinghai province.

The split plot experiment was carried out in the experimental site of the Academy of Agricultural and Forestry Sciences of Qinghai University from 2023 to 2024. Two planting patterns were set up in the main area: multiple cropping green manure after wheat (W-G) and leisure after wheat (W), and three nitrogen fertilizer levels in the sub-area included: no nitrogen application (N0), nitrogen fertilizer reduction by 30% (N1, 157.5 kg N·hm^-2), and the local custom of nitrogen application (N2, 225 kg N·hm^-2). Wheat yield and grain quality (grain protein content, sedimentation value, formation time, stability time, wet gluten, etc.) were determined.

The grain yield of spring wheat under W-GN1 treatment was 5.5% and 13.4% higher than that under WN1 in 2023 and 2024, and 2.0% and 5.3% higher than that under WN2 treatment, respectively; the biological yield under W-GN1 in 2023 and 2024 was 5.1% and 10.6% higher than that under WN1, and 1.5% and 4.6% higher than that under WN2, respectively. W-G could obtain higher harvest index than W. The compensation effect of multiple cropping green manure was negative under no nitrogen application level, and it was between 2.0%-14.0% under nitrogen application level. The contribution of multiple cropping green manure combined with 30% reduction of nitrogen fertilizer to increasing crop yield was the best. At the same time, W-GN1 could improve grain quality by increasing grain protein content, sedimentation value, formation time and stabilization time. The grain protein content under W-GN1 treatment was 10.62% and 9.48% higher than that under WN1, respectively. The grain sedimentation value increased by 25.05% and 18.13%, respectively. The grain formation time increased by 34.70% and 8.66%, respectively. The grain stability time increased by 41.30% and 13.68%, respectively. Through principal component analysis, it demonstrated that the multiple cropping of green manure after wheat had a more significant promoting effect in grain protein content, sedimentation value, formation time and stability time.

The grain yield and quality of spring wheat were significantly improved by 30% reduction of nitrogen fertilizer (N1, 157.5 kg N·hm^-2) combined with green manure after wheat harvest, which could be used as a suitable planting mode and nitrogen application level for improving quality and stable yield of wheat under the condition of reducing chemical fertilizer in Qinghai Province.

The problem for film-mulched maize in the oasis irrigation region was an advanced nitrogen (N) requirement and led to the insufficient supply of N at the late growth stages. In this study, the effects of N-fertilizer postponing application on intercropped maize photosynthetic physiological characteristics and grain yield were studied, so as to reveal the photosynthetic mechanism of intercropped maize grain yield advantage in the experimental area.

From 2019 to 2021, the maize was used as experimental materials in Hexi oasis irrigation region. The split-plot experiment design was adopted, among which pea/maize intercropping and maize monoculture were the main factors, and three N fertilizer postponing application (postponing ration 20%, 10%, and traditional practice) were the secondary factors. Then, this field experiment was used to investigate the photosynthetic physiological characteristics and yield performance of maize under N-fertilizer postponing application and intercropping pattern.

The results demonstrated that the grain yield of intercropped maize under the postponing application of 20% N-fertilizer and 10% was increased by 28.5% and 13.8%, and biomass yield by 23.8% and 12.5%, respectively, compared with traditional N management practices. Similarly, compared with traditional N management practice, the grain yield of sole maize under the postponing application of 20% N-fertilizer and 10% was also increased by 29.7% and 13.3%, and biomass yield by 19.6% and 10.3%, respectively. Compared with the monoculture maize, intercropping could increase the grain yield by 33.2%-35.1% and biomass yield by 26.8%-31.5% under the same area. Furthermore, the postponing application of 20% N-fertilizer and 10% increased the population grain yield of intercropping pattern by 27.2% and 12.9%, respectively, compared with the traditional N management practice. The results showed that intercropping pattern could increase the grain yield of maize compared with the sole pattern, and the N fertilizer postponing application also boosted the improvement of grain yield in the intercropping system compared with the traditional N management practice. During the whole growth periods, the intercropping increased the net photosynthetic rate, stomatal conductance, transpiration rate, and decreased intercellular CO₂ concentration. Compared with traditional N management, the net photosynthetic rate under the postponing application of 20% N-fertilizer and 10% was increased by 12.8% and 6.0%, the stomatal conductance by 14.0% and 6.9%, and the transpiration rate by 20.5% and 9.5%, respectively, while the intercellular CO₂ concentration was decreased by 29.8% and 13.1%, respectively. The SPAD value under the postponing application of 20% N-fertilizer and 10% was increased by 7.5% and 3.7%, respectively. The principal component analysis results showed that the N-fertilizer postponing application and intercropping pattern could increase the grain yield via boosting the net photosynthetic rate, the stomatal conductance, the transpiration rate, and leaf SPAD value, and decreasing intercellular CO₂ concentration.

N-fertilizer postponing application 20% treatment (36 kg·hm^-2 N fertilizer was topdressing at maize jointing stage and 108 kg·hm^-2 at 15 d post-silking stage) had the advantage of improving the photosynthetic characteristics of intercropped maize, thereby boosting the grain yield improvement.