To reveal how the altitude gradient regulates the effects of rhizosphere microbial dynamics on ecosystem multifunctionality in Hylodesmum podocarpum, a field experiment was conducted across four elevation transects (a.s. 896–1805 m) in the Qinling Mountains. The results showed that rhizosphere soil exhibited peak microbial diversity richness at 1805 m (HB4), with bacterial communities showing a strong interspecific cooperative relationship, while the fungal communities showed a competitive relationship. In addition, this study found the assembly process to be different. Bacterial assemblages changed from random processes (HB1, HB2, HB3) to deterministic processes (HB4), whereas fungal assemblages remained stochastic processes across all elevations. Our results also revealed that synergistic interactions among soil carbon, phosphorus, and nitrogen nutrient functions collectively enhanced nutrient-centered soil multifunctionality. Notably, carbon and phosphorus nutrient functions emerged as the primary drivers of soil multifunctionality. Further mechanistic analysis revealed that while soil pH exerted significant control over both carbon and nitrogen nutrient functions, microbial mediation exhibited functional specialization: bacterial communities predominantly regulated carbon cycling, whereas fungal communities played a more comprehensive role in modulating carbon, nitrogen, and phosphorus dynamics along with overall ecosystem multifunctionality. This finding suggested that altitude gradients indirectly affect the characteristics of the microbial community by regulating soil nutrient status, thereby driving changes in ecosystem multifunctionality. This finding provides new insights into how nutrients regulate ecosystem functions through microbial pathways.

The efficacy of microbial inoculants in enhancing plant drought tolerance is context-dependent, varying with drought severity and the dynamic interactions within the plant–soil microbe system. We conducted a two-factor pot experiment using Achnatherum splendens under three moisture regimes (well-watered (control), moderate drought, and severe drought) and four inoculation treatments (sterile inoculum (control), Bacillus subtilis (BS), arbuscular mycorrhizal fungi (AMF), and their co-inoculation (MIX)). The resulting 12 treatment combinations were each replicated four times. Drought stress significantly reduced both shoot and root biomass, disrupted redox homeostasis by altering antioxidant enzyme activities and osmolyte accumulation, and induced pronounced shifts in rhizosphere microbial community composition and structure. Microbial inoculation modified osmolyte accumulation and antioxidant enzyme activity, with effects varying in magnitude and direction across moisture regimes and physiological indicators, and was generally associated with elevated soil nutrient availability and extracellular enzyme activities, though the magnitude of these effects was modulated by moisture regime. The MIX treatment showed the strongest positive responses for a subset of plant physiological and soil biochemical indicators. Bacterial and fungal co-occurrence networks exhibited divergent structural responses to inoculation and drought, with network complexity (e.g., connectivity and modularity) differentially shaped by both factors. Mantel tests and partial least squares path modeling revealed that under well-watered and moderate drought conditions, soil physicochemical properties were the primary drivers of microbial alpha and beta diversity; in contrast, under severe drought, osmotic adjustment emerged as the dominant physiological determinant of biomass maintenance. Collectively, these results reveal that the functional outcomes of microbial inoculation are contingent upon abiotic context, particularly drought intensity. This study provides a mechanistic basis and preliminary empirical evidence for subsequent field validation and further development of moisture-matched microbial intervention strategies for the ecological restoration of drought-vulnerable A. splendens ecosystems.