Soil Nitrogen Prevails in Controlling Alpine Meadow Productivity Despite Medicago ruthenica Reseeding and Phosphorus Application

Under intensified global climate change and anthropogenic pressures, alpine ecosystems confront unprecedented stress. The degradation of alpine meadows has caused significant declines in productivity and in the abundance of high-quality forage species. This study aims to explore the effects of phosphorus (P) application and reseeding of Medicago ruthenica (L.) Trautv. on the biomass and quality of forage in degraded alpine meadows, and to identify the key soil factors influencing forage growth. Three reseeding rates (V1: low, V2: medium, V3: high) and three P levels (P0: none, P1: low, P2: high) were established in this experiment. The factors were arranged in a completely randomized design, resulting in nine distinct treatment combinations, that is V1P0, V1P1, V1P2, V2P0, V2P1, V2P2, V3P0, V3P1, and V3P2. The results showed that the interaction between the reseeding and P addition exerts a significant effect on the biomass of M. ruthenica, forbs, and aboveground biomass (p < 0.05). Additionally, the interaction between the reseeding and P addition had a significant effect on crude protein content (p < 0.05). Phosphorus addition and the interaction between the reseeding and P addition had a significant effect on ether extract content (p < 0.05). However, it is only reseeding that can significantly influence the neutral detergent fiber content (p < 0.05). Grey correlation analysis revealed that the V3P2 treatment optimized both forage biomass and nutritional quality. Hierarchical partitioning further identified soil total nitrogen as the factor that contributed the most to forage biomass and quality following reseeding and phosphorus application.