Screening, mechanisms of action, and application effect evaluation of salt-alkali reducing functional strains

Microbial remediation has been recognized as an environmentally friendly and cost-effective approach for improving saline-alkali soils. This study aimed to isolate bacterial strains capable of reducing salinity and alkalinity from saline-alkali soils, elucidate their functional mechanisms, and evaluate their remediation potential. Salt-alkali reduction efficiency was assessed by titration and pH measurement, and tolerance was tested across different NaCl concentrations and pH gradients. The effects of exogenous additives and metal ions on salt-alkali reduction efficiency were analyzed, whole-genome sequencing was performed to identify functional genes. Soil improvement experiments were conducted to evaluate remediation potential by measuring pH, electrical conductivity, exchangeable sodium percentage(ESP), and total salt content. Fermentation conditions were optimized using response surface methodology. Two salt-alkali-reducing bacterial strains, designated as 11 and 14, were identified as Microbacterium paludicola and Pseudomonas veronii, respectively. Under an initial salinity of 10, salt reduction rates reached 3.95%±0.24% and 3.74%±0.16%; under an initial pH of 9.0, alkali reduction rates were 17.19%±0.45% and 17.12%±0.40%, respectively. Both strains grew stably under 1%-3% NaCl and pH 7-9. Citric acid and betaine significantly enhanced the salt reduction rate of strain 11, while glutamic acid and citric acid significantly improved its alkali reduction rate. Metal ions had no significant effect on the alkali reduction of either strain. Genomic analysis revealed 59 and 75 salt-alkali-related genes in strains 11 and 14, respectively, spanning ion transport, osmotic regulation, and organic acid metabolism. In soil improvement experiments, strain 11 significantly reduced ESP and pH, while strain 14 significantly decreased electrical conductivity and total salt content; mixed inoculation showed no synergistic effect. Optimal fermentation conditions for strain 11 were an inoculation volume of 3.46%, salinity of 9.83, and pH of 7.97, with a verified OD₆₀₀ of 0.668; for strain 14, inoculation volume of 2.09%, salinity of 3.19, and pH of 8.00, with a verified OD₆₀₀ of 0.963. Strains 11 and 14 reduce soil salinity and alkalinity through distinct mechanisms of organic acid secretion and membrane transport-mediated ion uptake, respectively, and both demonstrate promising potential for the biological remediation of saline-alkali soils.