Carrier formulation and application concentration synergistically affect soil functions and microbial communities to enhance phosphate-solubilizing bacterial biofertilizer-mediated restoration of degraded alpine meadows

Background and aims Degraded alpine meadows on the Tibetan Plateau threaten ecology. Chemical fertilizers risk secondary pollution, yet native P-solubilizing biofertilizer carriers have been scarcely researched so far. This study explores how carrier formulation and concentration synergistically restore meadows for green remediation. Methods Four indigenous phosphate-solubilizing bacterial strains were formulated with four carriers (nutrient soil, charcoal, peat-based substrate, and spent mushroom substrate) and applied at three rates (25%, 50%, and 100%) in a one-growing-season field experiment in Qinghai, China. Plant biomass, soil properties, extracellular enzyme activities, and microbial communities were analyzed to evaluate treatment effects and identify drivers. Results Biofertilizer concentration and its interaction with carrier formulation significantly affected plant productivity ( P  < 0.05). The 50% concentration level promoted growth the strongest, with F3-50 achieving the highest aboveground biomass (357.0 g·m ^-2 ), 119.4% higher than the control. Biofertilizers modified soil nutrient availability and enzyme activities, with distinct carrier-dependent responses. The F3-50 treatment showed the highest β-glucosidase activity, whereas phosphorus acquisition enzymes exhibited carrier-specific patterns. Microbial analysis indicated that it had limited effects on bacterial α-diversity, while fungal communities were more responsive. RDA showed that soil organic matter, nitrogen and phosphorus availability, and nutrient-acquisition enzymes explained 37.22% and 60.76% of bacterial and fungal community variation, respectively. Conclusion Optimized carrier-concentration combinations improved nutrient cycling, affected microbial communities, and enhanced vegetation productivity. The performance of F3-50 highlights its potential for restoring degraded alpine meadows. These findings emphasize that optimizing carrier composition and concentration is critical for maximizing biofertilizer efficacy in nutrient-deficient alpine ecosystems.