Effects of Nitrogen Fertilizer Reduction and Green Manure Mixed Cropping on Soil Quality and Wheat Yield

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.