Long-term co-contamination with arsenic and PAHs reshapes soil microbial interactions and functional allocation

Arsenic (As) and polycyclic aromatic hydrocarbons (PAHs) frequently co-occur in industrial soils and act as persistent stressors that reshape soil ecological processes. However, it remains unclear how long-term As–PAH co-contamination influences pollutant redistribution, microbial community organization, and microbial functional allocation under chronic stress. In this study, gravity-driven soil column systems were established to simulate PAH contamination (30 mg/kg) with different As levels (0, 300, and 600 mg/kg) and operated continuously for 270 days. Long-term co-contamination altered the soil-profile redistribution of both PAHs and As. Compared with the PAHs-only treatment, co-contaminated treatments showed greater retention of PAHs in deeper soil layers (>30 cm), while As underwent pronounced vertical redistribution and a progressive shift toward As(V)-dominated profiles during prolonged incubation. Microbial community analysis showed that co-contamination reduced microbial abundance, strengthened deterministic community assembly, and increased network size, connectivity, clustering, and robustness. The dominant genus shifted from Micrococcaceae _unclassified in the PAHs-only treatment to Rhodococcus in the co-contaminated treatments, indicating selection toward more stress-tolerant and contaminant-responsive taxa. Metagenomic analysis further showed coordinated enrichment of stress-response genes, As metabolism genes, and PAH degradation genes, especially under high As loading. These results indicate that long-term As–PAH co-contamination acts as a profile-scale ecological filter that reshapes soil microbial communities and redirects microbial functional allocation toward stress tolerance, detoxification, and contaminant-associated metabolism.