Aiming at the issues of insufficient input of organic matters and high reliance on nitrogen fertilizers of wheat production in the Qinghai Plateau, the study investigated the influences of different incorporation methods of green manure after wheat combing nitrogen fertilizer reduction on wheat yield and soil quality, so as to provide the theoretical basis and practical guidance of efficiently and sustainably producing wheat in this area.

The experiment was conducted in the experimental field of the Academy of Agriculture and Forestry Sciences of Qinghai University from 2022 to 2023. A split plot design was adopted, and three nitrogen application levels were set up in the main plot of wheat season: conventional nitrogen application (225 kg·hm^-2, N2), nitrogen reduction by 30% (158 kg·hm^-2, N1), and no nitrogen application (N0). The subplot was setup as three green manure incorporation methods in the previous season: only green manure root returning (RR), overground straw returning via sheep digest combined with root returning (SDRR), and overground straw and root returning (OSRR).

The content of soil organic matter, total nitrogen, ammonium nitrogen, available phosphorus, and available potassium of treatment nitrogen reduction by 30% and overground straw returning via sheep digest combined with root returning (N1SDRR) were increased by 10.3%, 8.6%, 23.8%, 9.1%, and 8.4%, respectively, than those under nitrogen reduction by 30% and overground straw and root returning (N1OSRR). Meanwhile, the content of soil nitrate nitrogen under N1SDRR was 4.4% and 11.3% lower than that of treatment conventional nitrogen application and overground straw returning via sheep digest combined with root returning (N2SDRR) and N1OSRR, respectively. Additionally, the activities of soil alkaline phosphatase, sucrase, and urease under N1SDRR treatment were increased by 8.5%, 10.2%, and 3.7%, respectively, compared with the N1OSRR treatment; however, the activity of soil catalase was 7.1% and 10.0% lower than that under N2SDRR and N1OSRR, respectively. The soil quality index of the N1SDRR treatment was the highest (0.79), showing no significant difference from the N2SDRR treatment, but significantly increasing by 72.0% compared with the N1OSRR treatment. Furthermore, SDRR significantly enhanced the grain yield of wheat by 28.1% and 7.3% compared with RR and OSRR, respectively, and no yield reduction occurred even 30% nitrogen fertilizer was reduced. Linear regression analysis demonstrated a significant positive correlation between the soil quality index and yield. The random forest model indicated that the main factors influencing soil quality index were soil organic matter, available phosphorus, ammonium nitrogen, and total nitrogen, while the main factors influencing grain yield of wheat were soil organic matter, total nitrogen, ammonium nitrogen, available phosphorus, and urease activity.

Under the condition of 30% reduction of nitrogen fertilizer, the overground straw returning via sheep digest combined with root returning could effectively improve soil quality and wheat grain yield by reducing soil bulk density, increasing soil organic matter, total nitrogen, ammonium nitrogen, available phosphorus, available potassium content and alkaline phosphatase, urease and sucrase activity, which was a suitable management measure for sustainable wheat production in this area.

This study aimed to screen a suitable green manure configuration mode to reduce the amount of nitrogen applied to wheat, so as to provide the theoretical basis and technical support for improving soil quality and increasing wheat yield and stable yield in Qinghai Plateau.

The split plot experiment was carried out in the experimental base of the Academy of Agricultural and Forestry Sciences of Qinghai University from 2022 to 2023. The main plot was set as nitrogen application level, namely 225 kgN·hm^-2 (N2, customary nitrogen application), and 158 kgN·hm^-2 (N1, 30% reduction of customary nitrogen application); the split plot was divided into four green manure configuration modes, namely common vetch monoculture (CV), common vetch and highland barley mixed cropping (CV×CH), common vetch and hairy vetch mixed cropping (CV×HV), and common vetch and rapeseed mixed cropping (CV×RS).

The soil bulk density and pH of common vetch mixed with highland barley were 8.6%-10.5% and 3.3%-3.4% lower than those of common vetch monoculture, 3.3%-5.8% and 1.6%-1.9% lower than those of common vetch mixed with hairy vetch, and 4.9%-7.5% and 2.3%-2.4% lower than those of common vetch mixed with rape, respectively. The soil organic matter, total nitrogen, total phosphorus, total potassium, nitrate nitrogen, ammonium nitrogen, available phosphorus and available potassium in the mixed cropping of common vetch were 20.0%-22.3%, 8.3%-9.7%, 11.2%-15.6%, 14.6%-16.4%, 14.8%-20.1%, 16.8%-18.9%, 16.4%-19.4%, 23.8%-24.6% higher than those in the single cropping of common vetch, 12.6%-13.8%, 3.9%-4.2%, 3.1%-4.9%, 4.8%-5.5%, 7.5%-8.2%, 8.2%-8.7%, 6.1%-7.7%, 6.9%-10.7% higher than those of common vetch mixed with hairy vetch, and 26.7%-35.4%, 14.3%-14.4%, 7.0%-8.2%, 7.9%-9.8%, 23.5%-28.0%, 23.5%-25.3%, 11.5%-12.3%, 16.5%-19.6% higher than those of common vetch mixed with rape, respectively. The activities of soil sucrase and catalase were increased by 3.2%-4.1% and 4.5%-4.9%, respectively, in the 30% nitrogen reduction treatment compared with the conventional nitrogen application. In addition, the activities of soil sucrase, urease, catalase, and alkaline phosphatase were increased by 20.1%-24.7%, 10.5%-14.4%, 12.2%-16.9%, and 12.2%-12.6%, respectively, in the common vetch mixed cropping of highland barley compared with the common vetch monoculture, 12.0%-12.5%, 4.2%-5.2%, 6.4%-7.6%, 4.7%-6.2% compared with the common vetch mixed with hairy vetch, and 28.1%-33.6%, 15.2%-19.1%, 18.7%-24.2%, 5.7%-5.8% compared with the common vetch mixed with rape, respectively. The soil quality index of 30% nitrogen reduction treatment was 17.1% higher than that of the traditional nitrogen application. Furthermore, the soil quality index for the intercropping of common vetch with hulless barley and the intercropping of common vetch with hairy vetch were 142.3% and 76.6% higher, respectively, compared with the single cropping of common vetch. Under the condition of a 30% reduction in nitrogen fertilizer application, the wheat grain yield of the treatment with common vetch intercropped with hulless barley did not show significant difference compared to traditional nitrogen application. However, it was 10.4%-12.5% and 4.8%-5.9% higher, respectively, than that of single cropping of common vetch and intercropping of common vetch with hairy vetch. The Mantel test indicates that soil quality index and wheat yield were significantly positively correlated with soil organic matter, total nutrients, available nutrients, and enzyme activity, while they were significantly negatively correlated with soil bulk density and pH. Additionally, the random forest model identified soil organic matter, total nitrogen, ammonium nitrogen, nitrate nitrogen, sucrase activity and urease activity were the key factors influencing wheat yield.

The 30% nitrogen reduction treatment combined with common vetch mixed with highland barley could improve soil quality and wheat yield by improving soil physical and chemical properties and soil enzyme activity, which could be used as a suitable production mode in Qinghai Plateau.

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.

Aiming at the problem of insufficient excavation of the potential of efficient water utilization of wheat-maize intercropping in the oasis irrigation area, which restricted the stable development of multi-maturing cultivation, this study was intended to provide the theoretical basis for the creation of a model of efficient water utilization of wheat-maize intercropping in the oasis irrigation area by investigating the effects of different nitrogen fertilizer postponing application on water consumption characteristics and water utilization of wheat-maize intercropping.

The experiment was carried out in the oasis agricultural comprehensive experimental station of Gansu Agricultural University from 2020 to 2021. Three planting patterns of wheat-maize intercropping, monocropping wheat and monocropping maize were set up. Four treatments application systems were set up for maize: no nitrogen application (N0), 20% nitrogen fertilizer postponing (N1), 10% nitrogen fertilizer postponing (N2), and traditional nitrogen fertilizer without postponing (N3). The total nitrogen application rates of intercropping maize and monocropping maize were 210 and 360 kg·hm^-2, respectively. The effects of different planting systems and nitrogen fertilizer postponing ratios on soil evaporation, water consumption characteristics and water use of wheat and maize were studied.

During wheat and maize independent growth stage, the intercropping tree evaporation was greater than that of monocropping, the intercropping wheat tree evaporation increased 15.9%-16.7% than that of monocropping wheat, and the intercropping maize tree evaporation increased 5.4%-14.7% than that of monocropping maize, while wheat and maize symbiosis of intercropping tree evaporation compared with the monocropping weighted reduction of 4.6%-6.1%; the total amount of evaporation during the whole life cycle tree performance: wheat maize in the intercropping mode, intertree evaporation was reduced by 6.5% in the 20% N fertilization setback treatment compared with N3, and intertree evaporation in the wheat belt increased by 12.6%-17.3% compared with that in the maize belt, which was the main source of intertree evaporation in the intercropping system. In the intercropping system, water consumption was 34.3 and 18.9 mm lower than that of traditional N application under the 20% and 10% N fertilizer setback treatments, respectively, but the difference between E/ET and traditional N application was not significant. The seed yield of intercropping system was increased by 21.1%-39.0% compared with the weighted average of monocrop, and the seed yield of intercropping system with 20% N fertilizer setback treatment was increased by 28.8% compared with the traditional N application, among which the intercropped wheat and intercropped maize with 20% N fertilizer setback treatments were increased by 24.3% and 30.8%, respectively, compared with the traditional application of N. The water consumption during the whole growth period under intercropping system with 20% and 10% N fertilizer setback treatment was decreased by 34.3 and 18.9 mm that under traditional application of N, respectively. The E/ET of intercropping system with 20% N fertilization was increased by 20% than that under the traditional N application. The water use efficiency of intercropping planting pattern nitrogen fertilizer setback treatment was significantly increased by 15.0% and 12.3% than that under the weighted average of monocrops; among which the nitrogen fertilizer setback 20% treatment was increased by 35.9% compared with the traditional nitrogen application, and the nitrogen fertilizer setback 10% treatment was increased by 19.3% compared with the traditional nitrogen application.

The wheat-maize intercropping pattern combined with 20% nitrogen fertilizer postponing could reduce soil evaporation and water consumption during the whole growth period, and increase yield and water productivity, which was a nitrogen application system that could be used for high-yield and high-efficiency production of wheat-maize intercropping in oasis irrigation areas.