Mining rhizobacteria from indigenous halophytes to enhance alfalfa (Medicago sativa L.) growth and soil reclamation in saline soils of Northwest China

Enhancing the growth of alfalfa ( Medicago sativa L.) through inoculation with rhizobacteria represents a sustainable strategy for reclaiming saline soils. However, the lack of suitable strains and practical application guidelines poses significant challenges to the utilization of Plant Growth-Promoting Rhizobacteria (PGPR) in salt-affected soils of Northwest China. In this study, we selected four PGPR strains derived from indigenous halophytes based on their growth-promoting characteristics. These strains underwent further selection via a petri dish assay. Subsequently, the effects of the selected PGPR strains on alfalfa growth and soil fertility were rigorously examined through pot trials. The results demonstrated that Bacillus filamentosus HL3, B. filamentosus HL6, Bacillus subtilis subsp. stercoris HG12, and Paenibacillus peoriae HG24 significantly produced indole-3-acetic acid (IAA), solubilized phosphorus, and fixed nitrogen (except for B. filamentosus HL6, which did not significantly fix nitrogen). Compared to non-inoculated plants, B. filamentosus HL6 and B. subtilis subsp. stercoris HG12 significantly enhanced seed germination, root elongation, and seedling biomass in a 150 mmol/L NaCl saline solution. In saline-alkaline soils, PGPR inoculation under brackish water irrigation did not restore alfalfa growth to the levels observed under freshwater irrigation. Principal Component Analysis (PCA) condensed ten indicators into two indices, explaining 86.85% of the variance. Using these two indices as weights, an evaluation model for the PGPR-alfalfa symbiosis indicated that B. subtilis subsp. stercoris HG12 had the most substantial effect under freshwater irrigation, while co-inoculation with B. subtilis subsp. stercoris HG12 and B. filamentosus HL6 had the most significant impact on alfalfa growth and soil improvement under brackish water irrigation. Available phosphorus was identified as the primary factor influencing alfalfa growth, contributing 82.3% to the growth variation. These findings provide suitable microbial strains for the utilization of saline-alkali land and underscore the potential of applying indigenous PGPR-alfalfa symbiotic techniques to improve soil fertility and crop yield in the arid regions of Northwest China.