Rhizosphere microbial functional potential is linked to forage quality through soil nutrient processes and leaf stoichiometric balance in wild Elymus nutans from Xizang

Rhizosphere microbial functional potential may contribute to variation in plant nutrient allocation and forage quality, yet the pathways linking microbial functions, soil nutrient processes, and plant stoichiometry remain poorly resolved in alpine grasslands. We surveyed 50 natural sites of wild Elymus nutans across Tibetan alpine grasslands and collected 150 composite rhizosphere samples during the flowering stage. Rhizosphere metagenomes were used to characterize KEGG level-3 functions, CAZy functions, and curated carbon-, nitrogen-, and phosphorus-related functional modules. Plant stoichiometric traits, forage quality, rhizosphere soil physicochemical properties, microbial biomass, and enzyme activities were measured concurrently. Functional diversity and composition were evaluated using Shannon diversity, PCoA, and PERMANOVA, while Procrustes and Mantel analyses were used to assess microbial–plant–soil coupling. RDA and piecewise structural equation modelling were further used to identify dominant functional domains and pathways associated with crude protein (CP). Rhizosphere microbial functional structure varied significantly along the elevational gradient. KEGG functional diversity and composition differed among elevation groups, whereas CAZy diversity increased with elevation but CAZy composition remained comparatively stable. Among the major biogeochemical domains, nitrogen-related functions showed the strongest elevational response, whereas carbon- and phosphorus-related functions were comparatively stable. Rhizosphere microbial functional structure was more strongly associated with leaf stoichiometry and forage quality than with root stoichiometry. In the leaf RDA, C_total was the only significant predictor, although the overall explanatory power was low, whereas forage quality was jointly associated with C_total, N_total, and soil chemical background. The final pSEM identified C_GH as the strongest positive functional contributor to CP and N_urea as the strongest negative contributor, with soil nutrient processes and Leaf N:P forming important intermediate components of the pathway. These results indicate that forage quality in wild E. nutans is associated with coordinated variation in rhizosphere microbial functional potential, soil nutrient processes, and plant stoichiometric balance, providing a process-based framework for understanding plant–soil–microbiome coupling in Tibetan alpine grasslands.