Assessment of Microbial and Physicochemical Responses to Spent Engine Oil Contamination across Different Time Intervals in Mechanic Workshops

Abstract

Spent engine oil (SEO) contamination is a persistent and underregulated source of soil degradation in urban and peri-urban environments, particularly within mechanic workshop settings. This study provides a chronosequence-based assessment of the long-term impacts of SEO on soil physicochemical properties and microbial community dynamics across four workshops with operational histories of 1, 5, 10, and 20 years in Choba, Rivers State, Nigeria. Comprehensive analyses were conducted on total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), total heavy metals (THMs), pH, electrical conductivity (EC), total organic carbon (TOC), total organic nitrogen (TON), and culturable bacterial populations, including total heterotrophic bacteria (THB) and hydrocarbon-utilising bacteria (HUB). Results revealed a pronounced temporal contamination gradient, with significantly elevated concentrations of TPH (up to 3,272 mg/kg), PAHs (up to 46.02 mg/kg), and THMs in older workshop soils (p < 0.05), indicating cumulative pollutant loading over time. Contaminated soils exhibited progressive acidification and depletion of TON, alongside increased TOC, reflecting altered nutrient balance and organic matter accumulation. Microbial assessments showed a marked decline in THB abundance with increasing contamination age, whereas HUB populations were selectively enriched, indicating adaptive shifts toward hydrocarbon-degrading taxa. Correlation analyses demonstrated strong positive associations among TPH, PAHs, and THMs (r = 0.99), alongside a negative relationship between TPH and TON, suggesting tightly coupled chemical accumulation and biological response processes. Regression modelling further indicated that TPH exerts a significant inhibitory effect on general microbial biomass, while TOC contributes positively but insufficiently to microbial resilience. Collectively, these findings highlight that prolonged SEO contamination promotes the co-accumulation of hydrocarbons and heavy metals, disrupts soil nutrient cycling, and drives microbial community restructuring toward specialised degraders without achieving effective natural attenuation. This study identifies mechanic workshops as long-term contamination hotspots and underscores the urgent need for systematic monitoring, improved waste-oil management practices, and targeted remediation strategies to mitigate escalating ecological and public health risks.