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Microbial inoculants combined with biochar modulate rhizosphere microbial structure and enhance salt–alkali tolerance in alfalfa seedlings

ecology
microbiology
soil-science
plant-ecology
agronomy

Abstract

Alfalfa ( Medicago sativa L .) is widely used for the improvement of saline-alkali soils worldwide, however, the establishment of alfalfa is challenging in moderately and highly saline-alkali soils due to its poor salt tolerance at the seedling stage. The mechanisms by which microbial inoculants combined with biochar enhance alfalfa tolerance to saline–alkali stress remain unclear. In this study, a greenhouse experiment was conducted to evaluate the effects of biochar-based microbial inoculant F1 (mixture of Bacillus subtilis , Bacillus licheniformis and biochar) and microbial inoculant F2 (mixture of Bacillus subtilis , Pseudomonas synxantha and Sinorhizobium meliloti ) on alfalfa seedling growth, rhizosphere soil properties and microbial community structure under moderately and highly saline-alkali stress. The results showed that increasing saline-alkali stress significantly inhibited alfalfa seedling growth, with all growth parameters under highly saline-alkali stress being significantly lower than those under moderate stress. Treatments with F2 application (MSF2, HSF2) partially alleviated saline-alkali stress on alfalfa by significantly increasing plant height and stem diameter ( p < 0.05), but it had no significant effect on shoot or root biomass. In contrast, Treatments with F1 application (MSF1, HSF1) markedly promoted aboveground biomass accumulation and root development, enhanced root activity, increased soluble sugar content and superoxide dismutase (SOD) activity, and reduced malondialdehyde (MDA) content compared with controls (CK1, CK2). Moreover, Treatments with F1 application significantly decreased rhizosphere soil electrical conductivity and pH, while increasing ammonium nitrogen and available phosphorus contents, thereby improving nutrient availability. Microbial community analysis revealed that treatments with F1 application increased the relative abundance of Bacteroidota, Actinomycetota, and the order Aggregatilineales within Chloroflexota, and differentially regulated bacterial α-diversity under MS and HS conditions. Overall, biochar-based microbial inoculant F1 effectively mitigated ionic toxicity, strengthened physiological stress resistance, improved nutrient supply, and optimized the rhizosphere microecological environment, thereby significantly enhancing saline-alkali tolerance and seedling establishment of alfalfa.
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