Effects of Long-term Oil Pollution on Soil Microbial Respiration and β-glucosidase Activity

Oil pollution is one of the most critical environmental contaminations that can affect soil's biological, physical, and chemical properties. To investigate the effects of long-term and natural oil pollution on soil microbial respiration, including basal respiration (BR) and substrate-induced respiration (SIR), and beta-glucosidase enzyme activity, 120 oil-contaminated soil samples were collected from 0-15 cm depth in the oil-rich region of Naft-Shahr in Kermanshah province with three pollution levels: high (H), moderate (M), and low (L). After measuring the physical and chemical properties of the soils, the total bacteria and oil-degrading bacteria were counted on NA and CFMM culture media, respectively. This finding showed a positive and significant correlation between microbial population and oil concentration. The average oil percentage measured by Soxhlet extraction was 4.03%, 9.95%, and 22.50% for L, M, and H levels, respectively. The results showed that with the increase in oil concentration in soil samples, BR and SIR increased, and the highest BR and SIR respiration rates were 0.053 and 0.234 (mgCO2.g-1.h-1) in H soils, respectively. The measured beta-glucosidase activity was also higher in the presence of oil pollutants, with the highest activity (24.78 µgPNP.g-1.h-1) in H soils and the lowest (6.09 µgPNP.g-1.h-1) in L soils. Finally, a Principal Component Analysis (PCA) was conducted, and the results showed that 72% of the variance among samples could be explained by the first two components (biochemical and physical components). Oil pollutants that are naturally present in the soil for an extended period lead to the adaptation of pollution-resistant microbial communities over time, increasing their abundance, microbial respiration, and the activity of enzymes such as beta-glucosidase.