Assembly processes and ecological dynamics of root-associated bacterial communities during phytoremediation of vanadium-titanium mine tailings using Millettia pinnata

The vanadium-titanium magnetite (V-Ti) mine tailings are a reservoir of heavy metals, posing a grave peril to the adjacent soil environment. Millettia pinnata has emerged as a proficient pioneer tree species for the purpose of phytoremediation of V-Ti tailings. In the course of phytoremediation, the soil microbiota play an indispensable role within the rhizosphere ecosystem. The aim of this study was to dissect the underlying mechanisms governing the assembly processes of the bacterial community. The beta NTI and RCbray analyses revealed that while the overall community assembly manifested deterministic attributes, rare taxa were chiefly impacted by stochastic processes (73.7%), inclusive of dispersal limitation and undominated mechanisms. In contrast, the abundant subcommunity was primarily subject to deterministic forces (84.3%), notably heterogeneous selection. Mantel test, regression analysis and ecological niche analysis showed that soil nitrogen availability emerged as a pivotal driver of the microbial assembly processes, exerting substantial influence on both rare (Mantel's r = 0.6817, p = 0.0001) and abundant taxa (Mantel's r = 0.5103, p = 0.0001) by amplifying their diversity and community turnover, constricting habitat niche breadth and overlap, and fostering the proliferation of ecological specialists. However, the heavy metal content was not found to wield significant influence in shaping the structure and assembly of the rhizosphere community (p > 0.05). The present results affirm the proposition that rare taxa are important in upholding ecological stability and providing essential microbial ecosystem services, while abundant taxa serve to fortify the entirety of the community. These insights advance our understanding of the community dynamics pertaining to the rhizosphere community associated with M. pinnata, holding paramount theoretical implications for the endeavour of plant-assisted ecological restoration in polluted tailings sites.