Isolation and screening of soil phosphate-solubilizing bacteria and their phosphate solubilization mechanisms in the Mu Us Desert

Phosphate-solubilizing bacteria often transform the insoluble soil phosphorus into the plant-assimilable forms across diverse ecosystems. However, it is still scarce research in desert regions. In this investigation, the soil sample was taken from the predominant sand-fixing shrub community (Artemisia ordosica) in the Mu Us desert. Tricalcium phosphate was employed as the phosphorus medium to selectively isolate the phosphate-solubilizing bacteria, in order to delineate their composition, solubilization capacity, and operative mechanisms. Efficient strains were discerned via halo formation in solid media and liquid culturing, followed by quantification of organic acids, pH levels, and alkaline phosphatase activity within the potent phosphate-solubilizing bacteria cultures. Subsequent morphological, physiological, biochemical, and phylogenetic analyses were carried out for the precise characterization and identification of efficient strains. Pearson correlation analysis was used to elucidate the interplay between phosphate solubilization capacity and the culturing environment. The results are as follows: 1) A total of sixty-one strains of phosphate-solubilizing bacteria were isolated from both rhizosphere and non-rhizosphere soil of Artemisia ordosica, belonging to three phyla and five genera. Among them, Pseudomonas had the highest isolation rate (80.33%), followed by Streptomyces (11.47%). 2) Only seven strains (named R4, R5, R6, R7, NR1, NR2, and NR3) were produced in the transparent halos, with NR3 at the largest ratio of halo diameter to colony diameter. 3) There was an effective phosphorus increment of the seven strains with transparent halos after seven days of liquid culture. Three additional strains (named R1, R2, and R3) without transparent halos had better growth in the range from -5.40 to 800.28 mg/L. Specifically, four strains (R1, R4, NR1, and NR3) demonstrated phosphorus increments of 501.24, 67.78, 55.48, and 800.28 mg/L, respectively, indicating the efficient phosphate-solubilizing bacteria. 4) All four strains were growing on a medium with a pH of 11 and a 5% NaCl concentration. NR3 and R1 were then identified as Pantoea and Amycolatopsis, respectively, while R4 and NR1 were identified as Pseudomonas. 5) R4 and NR1 secreted seven organic acids (oxalic, tartaric, malic, lactic, acetic, citric, and succinic acids), R1 secreted six organic acids except for succinic, and NR3 secreted only four. Moreover, R1 and NR3 secreted the smaller quantities of each organic acid. 6) The pH values of NR3, R1, and R4 were significantly lower than those of the blank control after seven days of incubation, whereas, there was no change in the alkaline phosphatase activity of the four strains, compared with the blank control. 7) The amount of phosphorus solubilization by the four strains was significantly positively correlated with their secretion of oxalic and acetic acids, while significantly negatively correlated with the amount of tartaric acid and pH. The study showed that: 1) Many strains of phosphate-solubilizing bacteria were isolated and cultured in the soil of Artemisia ordosica from the Mu Us desert. 2) The four highly efficient phosphate-solubilizing bacteria strains were screened with high resistance to salinity and alkalinity. The content of effective phosphorus was significantly enhanced in the secretion of organic acids and the lowering of the environmental pH value. Four strains with significant solubilization capacity were successfully screened under defined experimental conditions. These strains can be expected to develop into functional microbial agents in soil amelioration and ecological rehabilitation in desert regions.