Altered soil dominant N and P supply fractions mediate ANPP via plant nutrient uptake efficiency under alpine meadow degradation.

Soil nitrogen (N) and phosphorus (P) contents impact grassland net primary productivity. Nevertheless, the regulatory mechanisms of soil N and P fractions through N/P ratio and nutrient uptake efficiency of aboveground plants on aboveground net primary productivity (ANPP) across degraded meadows remain unclear. We determined the dynamics of soil N and P fractions, N uptake efficiency (NupE) and P uptake efficiency (PupE) of aboveground plants, and ANPP across differently degraded meadows in the Zoige Basin, China. Compared with non-degraded meadow (NDM), soil N fraction contents decreased by 11.6%-96.4% integrally in degraded meadows, suggesting a decline in both soil N accumulation and availability. Soil labile and potentially labile P contents significantly decreased by 11.0%-75.2% in moderately degraded meadow (MDM) and 10.4%-74.5% in heavily degraded meadow (HDM), demonstrating a reduction in P availability in both MDM and HDM. Highly labile N, as well as labile and potentially labile P, were the preferential supply fractions in the meadows. Meadow degradation reduced NupE and PupE of aboveground plants by 12.0-65.9% and 11.3-77.7%, respectively, inducing the risk of N and P co-limitation. Partial Least Squares Structural Equation Modeling (PLS-SEM) demonstrated that soil N and P fractions affected ANPP via the N/P ratio, NupE, and PupE of aboveground plants under meadow degradation. Our findings highlight that soil labile N and P fractions regulated ANPP through N/P ratio and nutrient uptake efficiency of aboveground plants, providing new insights for optimizing fertilization measures and improving net primary productivity to restore degraded meadows.