Vertical distribution patterns of bacterial communities along soil profiles in a copper smelting area

Metal smelting has considerably contaminated the soil environment, while most studies focus on microbial responses to heavy metals in surface soil, leaving microbial variations in deep soil relatively understudied. We selected six 6-meter-long soil profiles from a copper smelting area to characterize the depth-dependent bacterial communities and investigate the effects of multi-metal contamination on bacterial adaptation. The results demonstrated that bacterial α-diversity showed no significant difference throughout the soil profiles at both low- and high-contaminated sites, while bacterial β-diversity was significantly different between upper-layer soil (0–2 m) and subsoil (2–6 m). The Nemerow Pollution Index, As levels and pH were significantly associated with the bacterial communities across the soil profiles. The co-occurrence networks exhibited loose connectivity in the upper-layer soil but became tighter-connected in the subsoil at low-contaminated sites. Conversely, the opposite pattern was observed at high-contaminated sites. Overall, our results indicated that multi-metal contamination can greatly shape the soil bacterial communities and affect depth-resolved bacterial adaptation along the soil profiles.