Changes in soil available nitrogen mediate divergent responses in plant CSR strategies with montane meadow degradation along an aridity gradient

Background and Aims Grassland degradation alters plant communities and ecosystem functioning, but whether it induces predictable shifts in plant CSR (competitor-stress tolerator-ruderal) strategies, how aridity modulates these shifts, and the relative roles of soil N vs. P remain unclear. Methods We investigated three montane grasslands along an aridity gradient (semi-arid alpine meadow, semi-arid marsh meadow, hyper-arid mountain steppe) using paired plots (undegraded vs. degraded). We measured community composition, above- and belowground net primary productivity (ANPP, BNPP), soil available nutrients (NH ₄ ⁺ , NO ₃ ⁻ , AP), and community-weighted CSR strategies. Random forest and structural equation modelling identified drivers and pathways. Results Degradation triggered site-specific CSR reconfigurations. In the semi-arid alpine meadow, stress-tolerator (S) increased from 80% to 91%, while ruderal (R) decreased from 14% to 2%. In the hyper-arid steppe, S increased (78%→85%) and R decreased (20%→12%). Soil available N dominated CSR changes: ΔS positively correlated with ΔNH₄⁺ (+ 30%) and ΔP (+ 265%), but negatively with ΔNO₃⁻ (− 64%); ΔR showed opposite patterns. Nutrient effects on CSR were fully mediated by changes in BNPP (e.g., + 439% at the alpine site, − 59% at the hyper-arid site), with no direct nutrient–CSR pathway. Conclusion Grassland degradation drives divergent CSR reconfigurations depending on aridity. Soil N dynamics act as a dominant biogeochemical filter, influencing CSR shifts indirectly via belowground productivity reallocation. This provides a nutrient‑explicit, mechanistic framework for predicting plant community responses to degradation across climatic gradients.