Regional-scale biogeographical patterns of soil- and root-associated microbial communities across nine planted Chinese fir forests.

Chinese fir () is an economically important plantation tree species. Gaining insights into the belowground microbiome of Chinese fir is critical for optimizing plantation management and enhancing timber production. In this study, we investigated microbial community structures in both rhizospheric soil and root samples from nine Chinese fir plantations (sites) at a regional scale. Moreover, we analyzed relationships between tree growth and microbial community structures and soil properties. Our results revealed that significantly higher bacterial and fungal richness was observed in rhizospheric soils than in tree roots. Differing distribution patterns of soil- and root-associated bacterial and fungal community compositions were observed across different sites. Soil nitrate was the key factor shaping root-associated bacterial diversity, and both soil total nitrogen and nitrate were the critical drivers influencing root-associated fungal community composition. There were apparent geographical variations in the biomass and growth increment of Chinese fir trees, with soil moisture emerging as the strongest predictor for these two parameters. Moreover, soil-associated bacterial community composition, root-associated bacterial diversity, and root-associated fungal community composition were identified as the primary determinants of tree biomass. Our findings highlight the critical but different contributions of soil- and root-associated bacterial and fungal communities to the productivity of trees in subtropical plantations.IMPORTANCEChinese fir plantations are widely distributed in Southeast China and characterized by their considerable economic significance. Belowground microbial communities play pivotal roles in shaping forest ecosystem functions. Nevertheless, knowledge of the relationship between microbial communities and tree growth is scarce. Here, we investigated soil- and root-associated bacterial and fungal communities and their relationships with the tree growth of nine Chinese fir plantations in subtropical regions. We found that both compartment and site factors influenced bacterial and fungal diversity and community composition. Apparent geographical variations in the biomass and growth increment of Chinese fir trees were observed. Moreover, soil-associated bacterial community composition, root-associated bacterial diversity, and fungal community composition were identified as the primary determinants of tree biomass. Altogether, this study provides a comprehensive analysis of microbial communities in mature Chinese fir planted forests, offering new insights into their roles in supporting forest productivity.