Regulatory effects of row spacing on plant-soil feedbacks in an oat–forage pea intercropping system: a two-year, dual-location study

To clarify the effects of row spacing on forage yield, quality, and soil function in an oat–forage pea intercropping system in the high-altitude Qinghai–Tibet region, this study systematically evaluated the combined effects of row spacing on dry matter production, nutritional attributes, and soil physicochemical characteristics based on a two-year, multi-site field experiment conducted at two locations (Hainan and Haixi Prefectures). A DTOPSIS model was employed to comprehensively assess system performance by integrating forage yield, quality, and soil nutrient indicators, thereby facilitating optimized spatial structure evaluation. Structural equation modeling was further applied to quantify the direct and indirect effects of row spacing on forage yield, quality, and soil quality indicators, as well as to elucidate the complex trade-offs among these factors. Key findings revealed that optimal trade-offs between forage yield and quality were achieved at a 30 cm row spacing in Hainan Prefecture, whereas a 25 cm spacing was more effective in Haixi Prefecture, highlighting site-specific differences in spatial resource-use efficiency and nutrient-balancing mechanisms under varying ecological conditions. In Hainan Prefecture, a wider row spacing (30 cm) promoted overyielding effects through improved spatial resource partitioning; however, this was accompanied by a potential trade-off between enhanced forage quality and accelerated soil nutrient depletion, indicating a negative feedback mechanism linking high productivity to reduced soil ecological sustainability. In contrast, the narrower optimal row spacing (25 cm) in Haixi Prefecture enhanced soil nutrient use efficiency and forage quality, reflecting the constraints imposed by nutrient limitations in arid alpine ecosystems. However, a trade-off in resource allocation between forage yield and soil quality remained evident, underscoring the inherent challenge of balancing ecosystem multifunctionality. This study contributes to advancing the ecological mechanistic understanding of spatial structure regulation in intercropping systems and offers precision management strategies for multi-objective optimization in alpine pastoral systems and other similarly complex environments.