[Effects of the nitrogen-fixing and phosphate-solubilizing bacterial strain T4 on the growth and rhizosphere environment of Astragalus membranaceus var. mongholicus].

Plant growth promoting rhizobacteria (PGPR) play an important role in promoting plant growth and improving soil environment. This is particularly relevant in the context of the current high-quality development of the traditional Chinese medicine industry, where the application of PGPR in the cultivation of medicinal plants contributes to ensuring the quality and safety of medicinal materials and promoting sustainable and high-quality industrial development. In this study, Astragalus membranaceus var. mongholicus was used as the object to explore the effects of nitrogen-fixing and phosphate-solubilizing bacterial strain T4 soaking on the growth of A. membranaceus var. mongholicus seedlings, rhizosphere soil nutrients and enzyme activities, bacterial community structure and assembly mechanism, in order to reveal its growth-promoting mechanism. Therefore, the seeds of A. membranaceus var. mongholicus were inoculated with the nitrogen-fixing and phosphate-solubilizing bacterial strain T4 via seed soaking prior to sowing. After 60 days of growth, the seedlings and rhizosphere soil samples were collected. Seedling growth indices, soil nitrogen, phosphorus and potassium content, as well as soil enzyme activities were determined. A soil microbial library was constructed using 16S rDNA gene sequencing, followed by taxonomic annotation and diversity analysis to compare the differences in species composition among groups. The microbial community assembly process was elucidated using infer community assembly mechanisms by phylogenetic-bin-based null model analysis (iCAMP). The relationship between soil redundancy analysis (RDA), and a species co-occurrence network was constructed. The metabolic functional potential of the microbial community was predicted using PICRUSt2 (phylogenetic investigation of communities by reconstruction of unobserved states). The results showed that inoculation with the nitrogen-fixing and phosphate-solubilizing bacterial strain T4 promoted the growth of A. membranaceus var. mongholicus and improved the soil quality. Compared with the control, the root length, shoot dry weight and root dry weight increased by 32.08%, 64.56% and 84.36%, respectively, while the soil alkali-hydrolyzable nitrogen content, available phosphorus, and soil urease activity increased by 134.09%, 27.00% and 136.39%, respectively. Furthermore, the strain T4 altered the soil bacterial community structure, driving a shift in community assembly from stochastic processes to deterministic processes, and significantly enriched functional bacterial genera such as Arthrobacter, Acinetobacter and Mesorhizobium (P<0.05). Functional prediction showed that the metabolic functions of the microbial community were significantly enhanced in four aspects (P<0.05): reduction in nitrogen loss, enhancement of detoxification capacity, induction of plant resistance, and augmentation of microbial vitality. In summary, the nitrogen-fixing and phosphate-solubilizing bacterial strain T4 promoted the growth of A. membranaceus var. mongholicus by regulating the rhizosphere microecology and improving soil quality. It lays a theoretical and application foundation for the development of special bacterial fertilizer that can effectively improve soil nutrition level and promote the growth of A. membranaceus var. mongholicus.