Effects of Replacing Bare Fallow with Cover Crops on Soil Physicochemical Properties in Southern Tibetan Cereal Cropping Systems

Cover crops have been recognized as a sustainable practice to improve soil structure and fertility in intensive agroecosystems, yet quantitative evidence on their effects and underlying mechanisms remains limited in high-altitude cereal cropping systems. This study investigated the effects of diverse cover crops on soil physicochemical properties during fallow in southern Tibetan cereal cropping systems, quantified their effect sizes relative to bare fallow, and explored the associations among key soil properties. Results showed significant variations in 0–20 cm soil physicochemical properties among cover crop species. Specifically, the oat (Avena sativa)–common vetch (Vicia sativa) mixture significantly reduced soil pH and increased water-stable aggregate rate (WSAR), total carbon (TC), and C/N ratio compared with turnip (Brassica rapa var. L.) and common vetch monocultures, whereas bulk density, total nitrogen, and soil organic matter (SOM) did not differ among cover crop treatments. Effect size analysis indicated that cover crops significantly reduced soil pH by 3.39% and increased WSAR by 32.85%. Principal component analysis and the piecewise structural equation model revealed that soil pH, WSAR, TC, and SOM collectively explained 86.18% of the observed variation in soil physicochemical properties, with pH significantly and positively affecting TC but negatively influencing WSAR, which in turn strongly promoted TC and thereby greatly enhanced SOM. These findings demonstrate that cover crops substantially improve selected soil physicochemical properties, providing critical evidence for their integration into bare fallow to enhance soil health in high-altitude cereal cropping systems.