Comprehensive Responses of Physiology and Rhizosphere Microbiome to Saline–Alkaline Stress in Soybean Seedlings with Different Tolerances

Soil salinization severely threatens global crop production. Understanding the relationship between crop saline–alkaline tolerance physiology and the rhizosphere microbiome, and leveraging beneficial microorganisms to enhance crop stress resistance, holds importance for sustainable agricultural development. This study investigated the physiological and rhizosphere microbial responses of two soybean cultivars with different saline–alkaline tolerance to stress. Under saline–alkaline conditions, the tolerant cultivar exhibited superior physiological performance, including higher chlorophyll content, photosynthetic efficiency, and elevated activities of antioxidant enzymes (SOD, POD, and CAT), alongside reduced oxidative damage (MDA) and greater biomass accumulation. Combined metagenomic and physiological analyses revealed significant correlations of Bradyrhizobium and Solirubrobacter with key physiological indicators, including dry weight, PIABS, φpo, and MDA. The tolerant cultivar selectively enriched distinct marker microbes, such as Bradyrhizobium sp. and Bradyrhizobium liaoningense, in its rhizosphere. We conclude that the tolerant cultivar exhibits strong intrinsic physiological resistance. This resistance is further enhanced by a beneficially assembled rhizosphere microbiome, while the host plant’s physiology remains the dominant factor.