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 view of the problems of excessive application of nitrogen fertilizer and lower water resource utilization efficiency and economic benefits in crop production in the irrigation area of Hexi Oasis, the aim of this study was to explore the effects of multiple green manures after wheat and moderate reduction of chemical nitrogen fertilizer on the water consumption characteristics and economic benefit of wheat field.

From 2019 to 2020, a split plot experiment was conducted in the oasis irrigation area of Hexi, Gansu province. Two planting patterns were set up in the main area, namely, multiple cropping of green manure after wheat harvest (W-G) and single cropping of wheat (W). In the sub-plot, there were five nitrogen application levels, namely no nitrogen application (N₀), conventional nitrogen application level 180 kg·hm^-2 (N₄), reduced 45% nitrogen application (N₁), reduced 30% nitrogen application (N₂), and reduced 15% nitrogen application (N₃).

Grain yield of wheat and systematic biothermal energy were significantly increased by multiple green manure after wheat (W-G) and the moderate reduction of chemical nitrogen fertilizer, in 2019 and 2020, which increased by 10.8% and 12.4%, respectively, and the yield of systematic biothermal energy increased by 37.8% and 40.3%, respectively. Compared with nitrogen reduction of 15% (W-G-N₃) in sole wheat and traditional nitrogen application (W-N₄) in sole wheat, the grain yield increased by 6.9%-16.7% and 7.9%-13.6%, respectively, and the biothermal energy yield increased by 52.0%-62.2% and 27.1%-58.9%, respectively. The water consumption of W-G decreased by 6.3%-16.0% compared with that of W wheat growing stage, and W-G-N₃ decreased the seasonal water consumption of W-N₃ and W-N₄ by 13.4%-20.5% and 20.8%-29.0%, respectively. Repeated planting of green manure could improve the water use efficiency of wheat, and W-G increased by 7.9% and 19.2%, respectively. In 2019, compared with W-N₃ and W-N₄, the WUE of W-G-N₃ increased by 23.5% and 5.1%, respectively. Compared with W-N₃ and W-N₄, W-G-N₃ could improve the energy yield of per unit water efficiency of the system, which was increased by 2.7%-14.5% and 9.3%-17.5%, respectively. Compared with the W and W-G increased the cost input, and the gross output also increased. In 2019, the net return of W-G-N₃ increased by 9.8% and 9.5% compared with W-N₃ and W-N₄, respectively; in 2020, the net return of W-G-N₃ decreased by 15.6% and 15.7% compared with W-N₃ and W-N₄, respectively. In 2019 and 2020, the output/input of multiple cropping green manure after wheat harvest reduced by 20.7% and 23.1% compared with sole wheat, and the output/input of W-G-N₃ was 14.8%-23.1% compared with W-N₃ and W-N₄, and W-G reduced the benefit per cubic meter water of the system due to more resources input.

In the Hexi oasis irrigation areas, multiple green manure after wheat combined with moderate reduction of chemical nitrogen fertilizer could improve crop yield and economic benefit, as well as water resource utilization efficiency, among which the comprehensive effect of multiple green manure after wheat combined with 15% nitrogen treatment is outstanding, which could be used as an ideal planting pattern and nitrogen application level to improve water resource utilization and farmers' income.