Objectives This study screened functional strains with nitrogen-fixing and drought-tolerant properties from the rhizosphere soil of Sophora moorcroftiana in the North and South Mountains of Lhasa, thereby providing microbial resources and theoretical support for vegetation restoration in arid regions.

Methods Rhizospheric soil of Sophora moorcroftiana was used to isolate nitrogen-fixing bacteria using modified Ashby medium, followed by preliminary screening for drought-tolerant strains using liquid LB medium supplemented with 25% PEG 6000. Plant growth-promoting (PGP) traits—including K solubilization (silicate medium), inorganic phosphate solubilization (Montagna medium), siderophore production (Chrome Azurol S medium), ACC deaminase activity (ADF medium), and indole-3-acetic acid (IAA) secretion (Salkowski assay)—were assessed. Drought tolerance was further evaluated via seed germination assays simulating drought stress with PEG 6000. Nitrogenase activity and drought resistance were verified using the acetylene reduction assay (ARA) and multi-gradient drought tolerance tests, respectively. Taxonomic identification was conducted through morphological observation combined with 16S rDNA and housekeeping gene (rpoB/gyrB) sequencing. A pot experiment was established with two controls: sterile water (CK) and LB medium (LB). The effects of eight selected strains on the growth of S. moorcroftiana seedlings and soil total N content under drought stress were evaluated.

Results (1) A total of 226 rhizospheric nitrogen-fixing isolates were obtained, from which 37 potential drought-tolerant strains exhibiting at least three PGP traits were preliminarily screened. Comprehensive evaluation using weighted scoring and subordinate function values from germination assays yielded eight target strains. These were identified as seven species across three genera: Pantoea agglomerans (strains SN1-23 and SN3-34); Acinetobacter pittii (strain SN3-30); and five Pseudomonas species—P. lurida (SN3-36), P. azerbaijanorientalis (SN4-4), P. kilonensis (SN2-1), P. yamanorum (SN3-33), and P. glycinae (SN4-19). All eight strains maintained stable growth under PEG-induced drought stress; notably, SN3-30, SN3-33, and SN3-36 grew stably even under 35% PEG stress, indicating superior drought tolerance. Nitrogenase activities ranged from 20.21 to 73.82 nmol/(h·mL), with SN3-36 and SN4-19 exhibiting the highest rates. Strain SN3-30 showed the highest potassium solubilization index (D/d = 9.40 ± 0.80), while SN1-23 produced the highest IAA concentration (52.02 ± 2.84 mg/L). (2) Seed germination assays demonstrated that all eight strains significantly promoted the germination and seedling growth of S. moorcroftiana and maize under drought stress. Compared to CK, the Germination Index (GI) and Vigor Index (VI) increased by an average of 108.41% and 500.59%, respectively, while root length and dry weight increased by 189.20% and 30.70%, respectively. Pot experiments confirmed that all strains enhanced seedling growth under drought stress. Strains SN3-33 and SN4-19 showed the strongest overall growth promotion, while SN1-23 exhibited outstanding comprehensive PGP effects. Soil total nitrogen content was significantly elevated in all inoculated treatments under drought stress. Specifically, the SN3-34 treatment reached 0.81 ± 0.05 g/kg, representing increases of 125.00% and 107.69% over CK and LB, respectively.

Conclusion  The eight selected drought-tolerant nitrogen-fixing strains exhibit significant potential for mitigating drought stress and promoting plant growth. Pseudomonas glycinae SN4-19, P. yamanorum SN3-33, and P. lurida SN3-36 demonstrate superior drought tolerance and growth promotion. Pseudomonas kilonensis SN2-1 shows exceptional root-promoting effects, while Pantoea agglomerans SN1-23 displays outstanding comprehensive PGP traits. Notably, SN3-34 is highly effective in improving soil total nitrogen in arid environments, and Acinetobacter pittii SN3-30 exhibits the strongest potassium solubilization and siderophore production, making it suitable for potassium-deficient drylands. However, except for the two Pantoea agglomerans strains listed in safety directories, the biosafety of the remaining six species requires further verification before commercial development as microbial inoculants.