Abstract: Arbuscular mycorrhizal fungi (AMF) and rare bacterial taxa are crucial for maintaining ecosystem stability and function. However, the underlying mechanisms by which AMF influence endophytic rare bacterial (ERB) communities in plants remain unclear. Using a gradient phosphorus (P) supply experiment designed to capture dynamic symbiotic responses, we characterized the physiological effects of AMF inoculation on Moso bamboo (Phyllostachys edulis) seedlings and examined how AMF regulate ERB community composition, assembly processes, and potential ecological functions by modulating host carbon dynamics in a P-dependent manner. We demonstrated that AMF exert a filtering effect on ERB communities through modulation of soluble sugar and starch concentrations via P-dependent pathways. This carbon-regulated restructuring increases the influence of stochastic processes, particularly dispersal limitation, and promoted niche specialization. These changes alter ERB network connectivity and co-occurrence network modularity, ultimately affecting community functional potential. AMF not only contribute to plant nutrient acquisition but also alter host carbon dynamics, thereby reshaping the composition and connectivity of plant-associated microbial communities. These findings support a “resource-mediated community assembly” framework that links belowground symbiosis to the mechanisms governing community formation, and they offer a basis for improving ecosystem functioning and sustainable agroforestry management by manipulating subsurface symbiotic networks.

Key words: Arbuscular mycorrhizal fungi, Phyllostachys edulis, Phosphate, Community assembly, Endophytic rare bacteria, Sustainable agriculture