The Effect of Petroleum-Derived Compounds in Soil on Microbiological Activity and the Physiological State of Plants

Petroleum contamination significantly impacts soil microbial communities and vegetation; however, the long-term effectiveness of phytoremediation remains poorly understood. This study evaluated soil microbiological activity, polycyclic aromatic hydrocarbon (PAH) concentrations, and physiological responses five years after the remediation of a petroleum spill site in central Poland. Following a pipeline failure in June 2020 that released diesel fuel and gasoline into the riparian habitat, the contaminated area underwent remediation using Urtica dioica L. as the primary phytoremediator. Soil samples from five plots along a contamination gradient were analyzed for microbial abundance (total bacteria, fungi, fluorescent Pseudomonas sp.), PAH fractions (C6–C12, C13–C16, C17–C35), and physicochemical properties. Chlorophyll fluorescence (JIP test) on two species was used to assess plant photosynthetic efficiency. Results revealed that successful PAH degradation required high fungal abundance rather than optimal soil fertility. Plots with 8–9-fold higher fungal populations achieved 69–81% reduction in heavy PAHs (C17–C35), while the Control plot, despite superior physicochemical properties, maintained high contamination due to low fungal colonization. Urtica dioica exhibited exceptional tolerance (stable maximum quantum yield of PSII (Fv/Fm) and elevated photosynthetic performance index (PIabs)) across all contamination levels, whereas Poa trivialis L. showed significant stress responses. The principal component analysis confirmed that soil texture influences fungal establishment, with sandy soils favoring aerobic degradation despite lower nutrient retention. These findings demonstrate that phytoremediation success depends critically on fungal-mediated biodegradation rather than baseline soil quality alone.