Green Manure Cover Enhances Soil Carbon Sequestration and Mitigates Carbon and Nitrogen Footprints in Mountainous Apple Orchards of China’s Loess Plateau

Green manure cultivation is recognized as a viable measure for mitigating agro-environmental degradation. However, it remains unclear whether diversified cropping can further enhance the environmental benefits of green manure systems, which impedes the full exploitation of their ecological benefits for sustainable agriculture. Here, we conducted a three-year field experiment in mountainous apple orchards of China’s Loess Plateau, and employed a life cycle assessment to evaluate the effects of five green manure cropping systems (clear tillage (CK); hairy vetch monoculture; oil rapeseed monoculture; hairy vetch intercropped with rapeseed; hairy vetch mixed with rapeseed). Green manure planting significantly improved the soil quality index and comprehensive evaluation index, with the highest soil quality and comprehensive evaluation indices observed in the hairy vetch intercropped with rapeseed. The hairy vetch intercropped with rapeseed can enhance the comprehensive benefits of green manure while reducing carbon and nitrogen footprints. Compared with the control, all green manure treatments except for the hairy vetch monoculture significantly reduced the carbon footprint by 9.39%–30.37%, the nitrogen footprint by 14.23%–22.70%, and environmental damage costs by 16.36%–25.86%. The carbon footprint was primarily regulated by N2O emissions and soil organic carbon sequestration rate, with contribution rates of 31.36% and 62.46%, respectively. The nitrogen footprint was mainly associated with NO3–N and N2O emissions, with contribution rates of 46.88% and 36.79%, respectively. Therefore, the diversified cropping (i.e., the hairy vetch–rapeseed intercropping) system exceed the monoculture for sustainable apple orchard production.