Microbial Inoculants Combined with Biochar Modulate Rhizosphere Microbial Structure and Enhance Salt-Alkali Tolerance in Alfalfa Seedlings

Alfalfa (Medicago sativa L.) is widely used for reclamation of saline-alkali soils, but its establishment is limited by poor salt tolerance at the seedling stage. This greenhouse study evaluated the effects of biochar-based microbial inoculant F1 (Bacillus subtilis, Bacillus licheniformis + biochar) and inoculant F2 (Bacillus subtilis, Pseudomonas synxantha, Sinorhizobium meliloti) on alfalfa growth, rhizosphere properties, and microbial communities under moderate and high saline-alkali stress.Increasing stress significantly inhibited seedling growth. F2 application (MSF2, HSF2) partially alleviated stress by improving plant height and stem diameter (p < 0.05), without significant effects on biomass. In contrast, F1 application (MSF1, HSF1) markedly promoted aboveground biomass and root development, enhanced root activity, increased soluble sugar and SOD activity, and reduced MDA content. F1 also decreased soil electrical conductivity and pH, increased available nutrients (ammonium nitrogen, available phosphorus), and optimized rhizosphere microbial community structure (increasing Bacteroidota, Actinomycetota, and Aggregatilineales within Chloroflexota).Overall, biochar-based inoculant F1 effectively mitigated ionic toxicity, strengthened stress resistance, improved nutrient availability, and optimized rhizosphere microecology, thereby significantly enhancing saline-alkali tolerance and seedling establishment of alfalfa.