Identification of PSB in soybean rhizosphere and secretion of organic acids in the process of phosphate solubilizing

Objective In order to determine the species and characteristics of phosphate solubilizing bacteria (PSB) in soybean, Glycine max, rhizosphere soil in Jilin Province.

Method PSB were screened based on the formation of a halo zone of dissolving phosphate, and were identified by 16S rDNA gene sequencing and Vitek 2 system. Bacterial growth rates, soluble phosphorus concentrations, pH changes of the media and the types and amounts of secreted organic acids were determined.

Result WJ1, WJ3, WJ5 and WJ6 were isolated from soybean rhizosphere soil, and were identified as Pseudomonas sp., Enterobacter sp., Ochrobacterum sp. and Klebsiella sp. respectively. The maximum soluble phosphorus concentrations within 96 h of inoculating four PSB were 558, 478, 596 and 586 μg·mL^-1 respectively. The methyl red tests and pH measurements indicated that media pH all decreased during the phosphate solubilizing process of four PSB. Bacterial growth was limited when pH was below 4. Growth of WJ1 and WJ3 were slightly affected at pH 5, whereas WJ5 and WJ6 grew normally. Various organic compounds secreted from four PSB were analyzed by GC-MS, and among them α-ketoglutaric acid was identified as an abundant compound secreted by WJ1, WJ3 and WJ6.

Conclusion All four PSB can effectively solubilize phosphate. The types of organic acids produced by the four PSB are not exactly the same, and the secretion of organic acids causes medium pH to decline, which inhibit the bacterial growth. The number of the bacteria determinines the amount of dissolved phosphorus.