Screening and identification of the phosphate-solubilizing bacterium <i>Sinorhizobium meliloti</i>, and its fermentation process optimization.

Phosphate-solubilizing bacteria (PSB), as a green alternative to chemical phosphorus fertilizers, can enhance phosphorus use efficiency in crops through an environmentally friendly manner, thereby increasing crop yields. To address the limitations in the application and promotion of phosphate-solubilizing bacterial inoculants caused by the constraints in strain resources and viable counts, a potential phosphate-solubilizing bacterium, designated H9, was isolated from farmland in the suburban area of Luoyang. After six days of cultivation, the maximum soluble phosphorus content in the fermentation broth reached 71.17 mg·L^-1. Based on morphological characteristics and 16S rDNA sequencing, the strain H9 was identified as <i>Sinorhizobium meliloti</i>. We further conducted high-density fermentation optimization to determine the optimal culture conditions. The results showed that at the condition of lactose 48.32 g·L^-1, yeast extract 11.32 g·L^-1, NaCl 3.29 g·L^-1, an initial pH of 7.0, a working volume of 20 mL, inoculum size of 3%, and incubation at 34 ℃ with shaking at 180 r·min^-1, the viable density of H9 reached (1.491±0.05)×10¹⁰ CFU·mL^-1. Collectively, these findings suggested that <i>S. meliloti</i> H9 is a promising microbial resource for the development of high-quality phosphate-solubilizing biofertilizers and offers considerable potential for sustainable agricultural applications.