Isolation, Identification, and Fermentation Optimization of Phytase-Producing Bacteria and Their Effects on Soybean Seedlings

Phosphorus in soil mostly exists in complex compounds such as phytic acid, which reduces the effectiveness of phosphorus and limits agricultural production. Phytase has the activity of hydrolyzing phytate into phosphate. The mineralization of phytate in soil by phytase secreted by microorganisms is an effective way to improve the utilization rate of phytate. This study isolated a high-yield phytase strain, identified as Pseudomonas by 16S rDNA and named Pseudomonas sp. S3-10. The fermentation medium composition and conditions were optimized using the single-factor method, Plackett-Burman design (PBD), and response surface methodology (RSM). The results showed that cane molasses, MgCl₂, and temperature significantly affected the fermentation biomass of the bacterium. The optimal fermentation conditions were cane molasses and MgCl₂ concentrations of 61.80 g/L and 5.94 g/L, respectively, at 34.4 °C. Compared with the unoptimized fermentation conditions, the maximum biomass increased by 160.17 ± 6.26% under the optimized fermentation conditions, reaching 9.13 ± 0.09 × 10⁹ CFU/mL. The pot experiment results showed that Pseudomonas sp. S3-10 has a significant promoting effect on soybean growth. The strain increased the fresh weight and length of soybean seedlings by 112.92 ± 28.41% and 74.02 ± 3.24%, respectively, and increased the phytase activity in the soil and available phosphorus concentration in the plant rhizosphere by 388.15 ± 24.24% and 365.05 ± 91.96%, respectively. This study provided a high-yield phytase strain and its optimal fermentation conditions. The bacterium has significant plant growth-promoting effects and can be used as a new type of biological fertilizer, which is of great significance for reducing phosphorus fertilizer usage, improving phosphorus utilization efficiency, and protecting the ecological environment in agricultural production.