Combing nitrogen fertilizer reduction with green manure returning via livestock digestion decreased greenhouse gas emissions in wheat fields at the Qinghai Plateau

To address challenges in wheat production in high-altitude and cold regions—such as excessive nitrogen fertilizer use and substantial greenhouse gas emissions—this study investigated the effects of different green manure return methods and nitrogen application rates during the wheat growing season on greenhouse gas emissions and crop yield, providing a theoretical basis and technical support for sustainable wheat cultivation in these regions. Field experiments were conducted from 2023 to 2024 at the College of Agriculture and Forestry Sciences, Qinghai University, using a split-plot design. The main plots included three nitrogen application levels: conventional nitrogen application (225 kg hm-2, N2), a 30% reduction in nitrogen application (158 kg hm-2, N1), and no nitrogen application (0 kg hm-2, N0). Based on the previous year’s green manure, three return methods were tested: root stubble return after removal of the upper plant (RR), combined return of root stubble and processed upper plant (SDRR), and full return of above-ground parts and roots (RROS). Results showed that reducing nitrogen by 30% combined with green manure incorporation and stubble return (N1SDRR) significantly mitigated greenhouse gas emissions. Total CO2 emissions decreased by 4.2% compared to full nitrogen reduction with full manure return (N1RROS), while total N2O emissions decreased by 19.1%, and net CH4 uptake increased by 15.8%. The global warming potential (GWP) was reduced by 5.0% compared to N1RROS. The N1SDRR treatment also increased wheat grain yield by 4.1% and reduced the greenhouse gas emission intensity (GHGI) by 14.6%, effectively balancing emission reduction with yield stability. Additionally, soil organic matter and ammonium nitrogen content increased by 9.1% and 22.8%, respectively, while nitrate nitrogen decreased by 10.0% compared to N1RROS. Soil sucrase and urease activities increased by 3.2% and 7.8%, respectively, whereas nitrite reductase and nitrate reductase activities decreased by 11.9% and 5.7%. This model demonstrates the potential to reduce greenhouse gas emissions while maintaining productivity through the enhancement of soil organic matter and regulation of ammonium/nitrate nitrogen balance. A random forest model further identified soil sucrase activity, grain yield, and soil organic matter as key factors influencing greenhouse gas emission intensity per unit yield. Adjusting nitrogen fertilization rates and green manure return methods significantly reduced carbon emissions per unit yield by affecting soil carbon and nitrogen dynamics. Therefore, reducing nitrogen fertilizer by 30% in combination with above-ground green manure incorporation and stubble return can improve soil physicochemical properties and enzyme activity, effectively lower greenhouse gas emissions, and stabilize wheat grain yield. This represents a promising management strategy for achieving both yield stability and emission reduction in wheat farmland on the Qinghai Plateau and other alpine regions.