Isolation, screening, and characterization of dominant plant growth-promoting rhizobacterial species in Zoige alpine grassland

Determining the dominant species distribution of plant growth-promoting rhizobacteria in the Zoige alpine grassland and screening for plant growth-promoting rhizobacteria (PGPR) can be resourceful in developing microbial agents suitable for alpine grasslands. In this study, PGPR from the rhizospheres of Poa annua and Elymus sibiricus in the Zoige alpine grassland were screened using different selective media at low temperature. The phosphorus solubilization abilities of the strains were determined using the molybdenum antimony resistance colorimetric method, nitrogen fixation ability was determined using the acetylene reduction method, and the taxonomic status of the strains was determined using 16S rRNA amplification and phylogenetic analysis. We isolated 49 strains of PGPR, namely 28 and 21 strains of phosphate-solubilizing and nitrogen-fixing bacteria, respectively. Among the phosphate-solubilizing bacteria, 17 strains had inorganic phosphate-solubilizing capacities of 290.32～457.77 μg·mL⁻¹, with strain PBRS2 showing the highest capacity at 457.77 μg·mL⁻¹. Organic phosphate was solubilized by 11 strains at 1.64～6.95 μg·mL⁻¹, with strain MCRS15 showing the highest capacity of 6.95 μg·mL⁻¹. The nitrogenase activity of the 21 strains of nitrogen-fixing bacteria was 93.33～234.35 nmol·(h·mL)⁻¹ and strain NARS10 had the strongest nitrogenase activity at 234.35 nmol·(h·mL)⁻¹. Most strains were isolated and screened from the rhizosphere of E. sibiricus. The PGPR in the soil adhering to the root region exhibited strong growth-promoting ability. Nine strains of PGPR with the highest growth-promoting ability were screened and identified based on 16S rRNA. Seven strains belonged to Pseudomonas and one strain each belonged to Rhodoferax and Massilia. Pseudomonas was the dominant species of the PGPR isolated and screened. These nine strains of PGPR with strong growth-promoting ability can be resourceful in developing microbial agents suitable for grassland in the Zoige alpine.