Production activity and resulting pollution, not depth, driven microbial community structure, function, and assembly mechanisms in an aged petroleum hydrocarbon contaminated soil

The degradation activity of soil microorganisms remains a crucial pathway for the reduction of petroleum hydrocarbons (PHs) pollutants in industrial site soils. Stimulating the metabolic activity of indigenous soil microorganisms to achieve pollutant removal is a promising remediation strategy in remediation practice. A vertical and horizontal investigation covering four production areas, four pollution levels, and five sampling depths at a PHs contaminated site from a long-term coking plant was launched to study soil PHs and bacterial community structure, function potential, and assembly. The results showed that both surface and subsurface soils (up to 6 m deep) at the contaminated site exhibit PHs contamination, with the most severe levels found between 4.5 and 5 m below the surface. The total petroleum hydrocarbon (TPH) concentrations in the soil ranged from 20 to 34,000 mg/kg. Potential PHs degrading bacteria, specifically the phylum Proteobacteria and the genera Pseudomonas and Sphingomonas, demonstrated positive correlations with TPH levels. Non-metric multidimensional scaling (NMDS) results showed that microbial community structure significantly differed among different production area and TPH pollution level. Redundancy analysis (RDA) results indicated that total nitrogen, moisture content, and available potassium in the soil exert the most substantial influence on the microbial community, while TPH, pH, soil organic carbon (SOC), sulfate content, and available phosphorus have comparatively minor effects. Analysis of community assembly mechanisms revealed that, with the exception of soils at the crude benzene, cooling tower (LQT), and L3 pollution levels-which were predominantly shaped by stochastic processes-most soil microbial communities were primarily influenced by deterministic factors such as environmental conditions and biotic interactions. Thus, production activity and the resulting pollution overwhelmed soil depth in shaping the structure, function potential, and assembly mechanisms of the microbial community in the aged, PHs polluted soil of the coking plant site. The relevant research results can provide theoretical support for the decision-making of PHs contaminated site remediation technologies and the regulation of remediation projects.