Faba Bean–Oat Mixtures Modify Rhizosphere Microbiota and Nutrient–Biomass Regulation on the Qinghai–Tibetan Plateau

Grass–legume mixtures are increasingly recognized for their potential to enhance soil health and forage productivity through belowground biotic interactions. In this study, we evaluated the effects of Vicia faba L. (faba bean 4060)–Avena sativa L. (oat ‘Baylor II’) mixtures on biomass, soil properties, and bacterial community dynamics. Results showed that mixtures significantly reduced the fresh weight of faba bean (6.2 kg/m2) compared to monoculture (8.8 kg/m2, p < 0.001), while oat biomass increased under mixtures (3.2 kg m−2 vs. 2.8 kg m−2, p < 0.01). Available phosphorus (AP) and available potassium (AK) significantly decreased in the rhizosphere of both mixtures, whereas alkali-hydrolyzable nitrogen (AN) significantly increased, particularly in oat. Mixtures significantly enhanced bacterial richness, evenness, and Shannon diversity in faba bean (p < 0.01) but had no significant effect on oat diversity metrics. NMDS indicated distinct shifts in bacterial community structures under mixtures. Acidobacteriota and Vicinamibacteraceae were enriched in faba bean mixtures, whereas Actinobacteriota decreased in both forages under mixtures. Source Tracker analysis suggested substantial microbial exchange between species, with over 40% of the bacterial community in mixed roots originating from the partner monoculture. Although microbial community stability tended to decline under mixtures, differences were not significant. Niche breadth was significantly expanded in faba bean mixtures. Community assembly processes remained predominantly stochastic; however, mixtures slightly shifted the balance toward deterministic processes. Structural equation model revealed that soil physicochemical properties had a significant negative effect on diversity (β = −0.371, p = 0.007), and diversity had a significant negative effect on freshweight (β = −0.770, p < 0.001), suggesting that bacterial diversity may play a mediating role in the relationship between soil properties and plant fresh weight (β = 0.285, p = 0.011). These findings demonstrate that mixture-induced changes in soil nutrient status and microbial community characteristics collaboratively mediate plant performance through altered community assembly and diversity–function relationships.