Abstract: Abstract Aims Understanding how nutrient addition influences community assembly is essential for predicting the responses of alpine grassland ecosystems to eutrophication. However, the relative importance of deterministic versus stochastic processes following nutrient addition remains debated. Methods We conducted a nutrient-addition experiment in the Bayanbulak alpine grassland on the southern slope of the Tianshan Mountains, with four treatments: control, nitrogen addition, phosphorus addition, and combined nitrogen and phosphorus addition. We measured species relative abundance (SRA), aboveground biomass, and plant functional traits. Using trait-abundance relationships, the β Raup–Crick index, functional diversity metrics, niche overlap, and the responses of soil physicochemical properties to fertilization, we evaluated how nutrient addition alters the balance between deterministic and stochastic assembly, and further assessed the relative importance of environmental filtering versus limiting similarity. Important findings Following N addition, SRA exhibited a significant positive linear relationship with plant height, while under N and NP additions SRA showed a significant negative linear relationship with seed size, indicating that taller species and species with smaller seeds became dominant via advantages in light competition and rapid colonization, respectively—consistent with trait-based niche assembly. Null-model analyses (β Raup–Crick) further revealed that between-community dissimilarity under nutrient addition was significantly lower than expected by randomization, indicating a shift from stochastic to deterministic assembly. Nitrogen-containing treatments significantly increased aboveground biomass, while significantly decreasing functional richness, functional dispersion, and Rao’s Q, and increasing niche overlap. These results suggest that nutrient addition enhanced environmental filtering, promoting functional trait convergence and compositional convergence. However, short-term nutrient addition did not significantly reduce species richness, likely because the regional species pool contains a relatively low proportion of rare species. Overall, nitrogen is the primary limiting nutrient for aboveground biomass in our study area, and nutrient enrichment promotes deterministic assembly by strengthening environmental filtering. These results reveal a trend toward functional convergence under nutrient enrichment and provide theoretical support for biodiversity conservation in alpine grasslands.

Key words: nutrient addition, community assembly, species relative abundance, functional traits, niche overlap