Effects of Legume–Grass Mixture Composition and Seeding Ratio on Plant Community Traits, Soil Physicochemical Properties, and Soil Fungal Diversity

Legume–grass mixtures are widely used to improve productivity and soil quality in alpine grasslands; however, their effects on soil fungal communities and the underlying mechanisms remain unclear. In this study, a two-factor field experiment (species richness (three, four, and five species) × legume–grass ratio (4:6, 3:7 and 2:8)) was conducted in an alpine artificial grassland on the Qinghai–Tibetan Plateau. Soil fungal communities were assessed using high-throughput sequencing combined with multivariate analyses. The results showed that Ascomycota, Mortierellomycota, and Basidiomycota were the dominant fungal phyla. Both species composition and seeding ratio significantly influenced fungal community structure and α-diversity primarily through indirect pathways mediated by plant community characteristics and soil properties. Plant height and soil total phosphorus (TP) were identified as key drivers of fungal α-diversity. Specifically, the four-species mixture (Z2) at a 3:7 legume–grass ratio resulted in relatively higher and more stable aboveground biomass and improved soil nutrient status, whereas increasing species richness to five species did not further enhance these benefits. Overall, our findings indicate that optimizing species composition and legume–grass ratio, rather than simply increasing species richness, is more effective for regulating soil fungal diversity and ecosystem function.