The association between the bacterial community variations and co-contamination with PAHs and heavy metals in different zones and soil layers of the steel industrially contaminated site

Co-contamination by polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs) is frequently observed in contaminated sites, but its effects on soil bacteria, especially in different soil layers, are still unclear. This study assessed the associations between co-contamination and microbial communities in different zones of the fill soil layer at an old steel plant site, and in different soil layers at its coking zone. The results showed that the most abundant contaminants at the site were PAHs, arsenic (As), and chromium (Cr), with maximum concentrations of 624.08, 25.5, and 306.04 mg/kg, respectively. Bacterial diversity was highest in the low-risk zone and the off-site zone, followed by the coking zone of the fill soil layer. The dominant phyla were identical, but the dominant genera were different in different zones and soil layers. PAHs, As, and soil moisture were key factors affecting the bacterial community in the fill soil layer. In contrast, pH was the critical factor shaping the microbial community in different soil layers of the coking zone. In addition, the annotation of functional genes associated with bacterial metabolism indicated that PAH stimulation did not increase the abundance of PAH-related genes in co-contaminated soil. These results provide valuable insights into the association between microbial communities and different co-contaminated zones, and especially different soil layers, potentially contributing to the remediation of deep soil layers in contaminated sites.