In order to solve the problems of long-term continuous cropping of wheat and high amount of nitrogen fertilizer in the production process in the arid irrigation area of Northwest China, the effects of green manure returning combined with reduced nitrogen application on soil hydrothermal variation characteristics and yield of wheat were studied, so as to provide the theoretical basis for the optimization of nitrogen application system in this area.

The field experiment was carried out in Wuwei Oasis Agricultural Experimental Station from 2021 to 2022. The treatments included no green manure (G0) and conventional nitrogen application (N1), as well as three green manure returning treatments (G1, G2, G3, applying green manure 15 000, 22 500, 30 000 kg·hm^-2, respectively) and two nitrogen fertilizer reduction treatments (N2, N3, reducing 15% and 30% compared with conventional nitrogen application, respectively). The effects of green manure returning combined with nitrogen reduction on soil water and heat variation characteristics, leaf area index and yield of wheat field were analyzed.

The green manure returning combined with nitrogen reduction could increase soil water storage in 0-120 cm soil layer of wheat field. Compared with G0N1, G2N2, G3N2 and G3N3 increased soil water storage by 4.0%-7.8%. Among them, G3N2 maintained higher soil water content in all soil layers during sowing, vegetative, reproductive and harvesting stages. From sowing to jointing stage, the soil temperature under G2N2, G3N2 and G3N3 increased by 0.6-1.3 ℃ and the soil accumulated temperature increased by 24.8-55.3 ℃ compared with G0N1. From the filling stage to the mature stage, the soil temperature of each green manure returning combined with nitrogen reduction treatment was 0.4-1.0 ℃ lower than that under G0N1, and the soil accumulated temperature decreased by 7.9-20.0 ℃. At the same time, the temperature change range under G3N2 in soil warming and cooling stage was smaller than that under other treatments. Green manure returning combined with nitrogen reduction significantly increased the leaf area index of wheat from booting stage to maturity stage, providing sufficient photosynthetic source for dry matter accumulation at late growth stage. Under this condition, compared with G0N1, the biomass and grain yield of wheat increased by 13.7%-28.0% and 11.7%-31.3%, respectively, and the increase under G3N2 was the largest. Correlation analysis showed that grain yield and its components were significantly positively correlated with leaf area index, soil water content and soil temperature in 0-60 cm soil layer. Structural equation model analysis found that soil hydrothermal conditions indirectly affected yield changes by directly affecting leaf area index.

Green manure returning combined with nitrogen reduction could improve the soil hydrothermal environment of wheat field and increase the leaf area index of wheat, so as to obtain high yield. Therefore, 30 000 kg·hm^-2 green manure+15% nitrogen reduction was the best green manure nitrogen fertilizer application mode to optimize the field hydrothermal environment and obtain high yield in oasis irrigation area.

The aim of this study was to explore the effects of returning green manure to soil on wheat yield under different nitrogen application levels in oasis irrigation areas, which had important guiding significance for establishing a green wheat production pattern based on the combined application of green manure and chemical nitrogen fertilizer.

From 2018 to 2020, a split plot experiment was conducted in the Hexi oasis irrigation areas of Gansu province. The main plot was set up with two planting patterns of sole wheat (W) and multiple cropping hairy vetch after wheat harvest (W-G), and the sub-plot had 5 nitrogen levels, including no nitrogen application (N₀), 55% N (N₁), 70% N (N₂), 85% N (N₃), 100% N (N₄), among which 100% N was the conventional nitrogen application rate of 180 kg·hm^-2. The leaf area index (LAI), leaf area duration (LAD), net assimilation rate (NAR) and grain yield at maturity stage and its constituent factors were measured during the whole growth period of wheat, in order to provide the basis for optimizing planting pattern and nitrogen application level in this area.

Compared with W, W-G significantly increased the mean LAI and LAD during the whole growth period of wheat by 9.5%-19.7% and 9.7%-21.0%, respectively; The moderate reduction of nitrogen fertilizer was beneficial to increase the mean LAI and LAD of wheat, with N₃ being the most prominent. Compared with W-N₃ and W-N₄, W-G-N₃ increased mean LAI by 4.1%-15.4% and 8.8%-17.5%, respectively, and the total LAD increased by 4.6%-9.2% and 16.8%-18.8%, respectively. W-G reduced the mean NAR of the whole growth period of wheat, which was 17.7% and 17.8% lower than that of sole wheat, respectively. Compared with W-N₃ and W-N₄, W-G-N₃ reduced mean NAR by 16.4%-17.5% and 26.5%-40.1%, respectively. Planting over-pressing hairy vetch after wheat harvest and moderately reducing nitrogen fertilizer increased the grain yield of wheat. Compared with W-N₃ and W-N₄, W-G-N₃increased the yield by 6.9%-16.7% and 7.9%-13.6%, respectively. Grey correlation analysis and correlation analysis showed that multiple planting hairy vetch after wheat harvest and appropriate nitrogen fertilizer yield high crop yields were mainly due to the synergistic increase of mean LAI, total LAD and yield components.

The treatment of multiple cropping hairy vetch after wheat harvest combined with 85% N fertilizer treatment promoted the growth of vegetative organs, which was conducive to the establishment, expansion and enrichment of the grain bank, thereby obtaining high yields. Therefore, W-G-N₃ was an ideal planting pattern and nitrogen application level for optimizing wheat yield performance indicators in oasis irrigation areas to obtain high yields.

In order to provide the theoretical basis for the development of high-yield and high-efficiency maize production technology in this area, the characteristics of dry matter accumulation and distribution and the response of grain yield to the return of green manure to the field combined with nitrogen reduction were analyzed in the oasis irrigation area.

The field experiment was carried out in Hexi Oasis Irrigation Area, Gansu Province from 2020 to 2021. The combination of green manure returning to the field and different nitrogen reduction ratios (green manure combined with nitrogen reduction 0%, N100; green manure combined with nitrogen reduction 10%, N90; green manure combined with nitrogen reduction 20%, N80; green manure combined with nitrogen reduction 30%, N70; green manure combined with nitrogen reduction 40%, N60) on the distribution of dry matter accumulation in maize and impact on production.

After the jointing stage, the aboveground dry matter accumulation under N80 and N90 treatments was significantly higher than that of N70 and N60 treatments. At the mature stage, the aboveground dry matter accumulation under N80 increased by 13.3%-23.2% compared with N70 and N60 treatments, and N90 was higher than N70 and N60 treatments. Compared with the N70 and N60 treatments, the maximum growth rate and average growth rate of the aboveground dry matter under N80 were significantly increased by 9.5%-21.2% and 13.0%-23.2%, respectively; N90 significantly increased by 10.2%-21.8% and 13.9%-23.7% compared with N70 and N60 treatments, both of which effectively delayed the decrease of aboveground dry matter accumulation rate of maize from silking stage to grain filling stage. Compared with N70 and N60, the maximum growth rate of dry matter under N80 treatment was 2.44 d and 2.77 d earlier than that under N70 and N60, respectively, and N90 was 1.92 d and 2.3 d earlier than the N70 and N60 treatments, respectively. N80 and N90 promoted the distribution of dry matter in the ears of maize, and effectively increased the contribution rate of dry matter transport before flowering to grain dry matter accumulation. At the same time, the post-flowering dry matter accumulation under N80 increased by 12.2% and 20.4% compared with N70 and N60 treatments, respectively. Compared with the N70 and N60 treatments, the post-flowering dry matter accumulation under N90 was increased by 12.4% and 20.5%, respectively, and the difference was significant. There was no significant difference in maize grain yield among N100, N90, and N80 treatments, but the maize grain yield under N80 increased by 16.8% and 27.4%, respectively. Compared with N70 and N60 treatments, the yield under N90 treatment increased by 17.4% and 27.9%, respectively, with significant differences.

The return of leguminous green manure to the field combined with 10% and 20% nitrogen reduction treatments increased the aboveground dry matter accumulation and accumulation rate of maize, promoted the distribution of dry matter in the ears at maturity, and improved the pre-flowering dry matter transport on grain dry matter. The cumulative contribution rate could be used as the recommended nitrogen application method for high maize yield in oasis irrigated areas.