Saline–Alkaline Stress-Driven Rhizobacterial Community Restructuring and Alleviation of Stress by Indigenous PGPR in Alfalfa

Background: The Songnen Plain in China contains soda saline–alkaline soil, wherein salinity and alkalinity severely constrain crop productivity. Alfalfa (Medicago sativa L.) is a forage legume that has adapted to moderate saline–alkaline conditions, but how its rhizosphere microbial community facilitated this adaptation remains unclear. Methods: Using 16S rRNA gene sequencing, we compared alfalfa rhizosphere bacteria in saline–alkaline soil (AS) and control soil. Bacteria isolated from AS were screened for plant growth-promoting traits, with the most effective strains validated in pot experiments involving 50 mM NaHCO3. Results: Compared with the control soil bacterial community, the AS bacterial community was significantly enriched with Methylomirabilota and unclassified bacteria (phylum level), with the genus RB41 identified as the most discriminative biomarker. Gene functions predicted using PICRUSt2 reflected the responsiveness of this community to environmental stressors. Inoculations with Pseudomonas laurentiana strain M73 and Stenotrophomonas maltophilia strain M81, which were isolated from AS, significantly improved alfalfa growth and health under NaHCO3 stress. Conclusions: Saline–alkaline conditions in the Songnen Plain reshape the alfalfa rhizosphere bacterial community, enriching for specific taxa and potentially enhancing microbial functions associated with stress resistance. Strains M73 and M81 can effectively promote stress tolerance, making them useful microbial resources for improving soil conditions.