Multi‐level nitrogen and phosphorus addition synergistically reduces alpine grassland stability via species compositional changes

Nutrient addition generally weakens the stabilizing effects of plant diversity on grassland productivity. However, whether this destabilizing effect is associated with species compositional changes and varies with nutrient input levels remains unclear. Here, we applied the Price equation to separate community species composition into lost, gained and persistent species, and quantified their contributions to the temporal stability of community productivity, defined as the ratio of temporal mean productivity to its standard deviation over year, in a 6‐year multi‐level nitrogen (N; 0, 5, 10, 15, 20 g N m −2 year −1 ) and phosphorus (P; 0, 2.5, 5, 7.5, 10 g P m −2 year −1 ) addition experiment in the Qinghai–Tibetan alpine grasslands. We found that community stability and species richness decreased with increasing N and P addition, with more rapid declines when both nutrients were applied together. The reduction in species richness resulted from fewer gained species and a decline in persistent species richness. Remarkably, species richness contributed only 5.3% to the observed variation in community stability along nutrient addition gradients. In contrast, the combined richness of gained and persistent species accounted for 53.1% of this variation. Importantly, fewer persistent species in the fertilized communities reduced the asynchronous dynamics among themselves, while fewer colonizing species decreased asynchronous dynamics between gained and persistent species, collectively weakening community stability along the nutrient addition gradients. Our results demonstrate that the destabilizing effects of nutrient addition are primarily driven by changes in species composition, which reduce compensatory dynamics among species that are persisting within the communities, rather than by the loss of species richness in grasslands. Read the free Plain Language Summary for this article on the Journal blog.