Responses of soybean growth and rhizosphere microbiota to combined microbial fertilizer in saline‐alkali soil

BACKGROUND: Severe soil salinization in the Yellow River Delta region substantially limits the growth and yield of crops such as soybeans. This study explored the potential of microbial fertilizer to improve saline-alkali soil and promote soybean growth. RESULTS: A pot experiment was conducted with four treatments: control (CK), salt-tolerant phosphate-solubilizing bacteria (P), microbial agent + salt-tolerant phosphate-solubilizing bacteria (KP), and spent mushroom substrate (SMS) + salt-tolerant phosphate-solubilizing bacteria (MP). Compared with CK, microbial fertilizer treatments significantly reduced soil pH by 4.4% to 8.5% but did not affect total soil salt content significantly. The MP treatment exhibited strong synergistic effects, significantly increasing dissolved organic carbon, microbial biomass carbon, and cellobiohydrolase activity (+73.4%), thereby promoting soil carbon cycling. This treatment demonstrated the strongest growth-promoting effects, increasing soybean plant height by 28.8%, fresh root weight by 171.0%, and dry root weight by 167.0%. High-throughput sequencing showed that treatments reshaped root microbiota. Treatments P and MP increased bacterial diversity, whereas MP uniquely increased fungal operational taxonomic units (OTUs) and enriched carbon-metabolism-related genera (e.g., Thermomyces). Treatments KP and MP also increased the abundance of beneficial taxa like Bacteroidota. Redundancy analysis identified soil pH, organic carbon, and enzyme activities as key drivers of microbial community evolution. CONCLUSION: The combination of SMS and salt-tolerant phosphate-solubilizing bacteria (MP) effectively optimized the saline-alkali microenvironment, enhanced soil carbon metabolic function, and significantly promoted growth of soybean roots and aboveground parts. This approach provides a promising strategy for the biological improvement of saline-alkali land in the Yellow River Delta. © 2026 Society of Chemical Industry.