Synthetic microbial community promotes seedling growth of Chinese fir via dissolving phosphorus and modifying rhizosphere microbial community

Phosphorus (P) is an essential nutrient for plant growth, yet its availability in soil is severely constrained by fixation into insoluble forms that plants cannot directly utilize. Although phosphate-solubilizing microorganisms (PSM) represent a promising strategy to mobilize soil P, the functional potential of endophytic PSM and their synthetic consortia in promoting tree growth remains largely underexplored. In this study, endophytic bacteria were isolated from the roots of Chinese fir (Cunninghamia lanceolata). Among them, 42 isolates were screened for phosphate-solubilizing activity on media containing calcium phosphate, iron phosphate, aluminum phosphate, and organic phosphorus. Six strains with strong solubilizing capacity and no antagonistic interactions were selected to construct a synthetic microbial community (SynCom-P6), which was then applied to local soil and Chinese fir seedlings. Soil available P content initially decreased but subsequently increased from day 7 to day 21 after SynCom-P6 inoculation, indicating effective mobilization of insoluble P in soil. Inoculation with SynCom-P6 significantly enhanced root elongation and biomass accumulation, with both root fresh and dry weights showing marked increases compared to the control. Absolute quantification 16S amplicon and metagenomic sequencing revealed that SynCom-P6 reshaped the rhizosphere bacterial community, enriching beneficial genera such as Massilia and Dyadobacter, and altered functional profiles, including upregulation of hormone signaling and nitrogen fixation related genes. These results demonstrated that the synthetic community not only improved soil P availability but also promoted root growth and modified the rhizosphere microbiome in a beneficial direction. Our findings highlight the potential of endophytic phosphate-solubilizing SynCom-P6 as a promising bio-inoculant for sustainable forestry, reducing the need for chemical P fertilizers while enhancing Chinese fir productivity. Future research should focus on field validation and mechanistic exploration of microbial interactions and functional gene expression.