Evaluation of the pattern of changes in basal and substrate-induced respiration in oil-contaminated soils (Case study: Naft-Shahr Kermanshah)

One of the most critical environmental pollutants is oil contamination. This pollution affects biological characteristics as well as the physical and chemical properties of soil. Soil is a habitat for microbial communities whose abundance and diversity can be affected by petroleum hydrocarbons. Soil biological indicators including microbial respiration, are highly sensitive to environmental stresses and respond to them quickly. Microbial respiration is one of the most common biological indicators which is used to investigate the quality and health of the soil. Since petroleum hydrocarbons are toxic and persistent in soil, studying the pattern of changes in soil biological characteristics is important in effective soil management. The aim of this study was to investigate changes in the basal respiration (BR) and substarte induced respiration (SIR) of microbial communities in the presence of oil, and how petroleum hydrocarbons can disrupt microbial respiration. For this purpose, 120 samples of crude oil-contaminated soils were collected in the oil-rich area of Naft-Shahr (located in the west of Kermanshah province) which had natural and long-term oil pollution. After measuring the physicochemical properties of soil samples microbial respiration was measured by titration method.

In this research 120, oil-contaminated soil samples were used. According to the factors included in this experiment, a nested design was used to analyze the data. The test factors included locations (4 locations) and 3 different levels of oil pollution (L: low, M: moderate, and H: high). It should be mentioned that 10 replications were considered in three levels of oil pollution and a total of 120 soil samples were gathered in this study (4×3×10). The collected soils were analyzed for soil texture, pH, EC and organic carbon (OC), and carbonate calcium equivalent (CCE) using standard methods. The concentration of petroleum pollutants, were determined by the Soxhlet extractor. In order to investigate the abundance of culturable microbial population, bacterial counting was carried out in nutrient agar (NA) and carbon-free minimal medium (CFMM)+crude oil media. Basal and substrate-induced respiration were measured by the titration method. Backward regression coefficients were used in order to identify important independent variables affecting changes in BR and SIR. Finally, the results of measuring chemical, physical and biological parameters were analyzed using principal component analysis (PCA).

The experiments showed that the percentage of oil measured by the Soxhlet method for oil pollution levels (L, M, and H) were 4.03%, 9.95%, and 22.50%, respectively. The obtained results showed that basal and stimulated breathing increased with the increase in the intensity of pollution. Also, the microbial population showed a direct relationship with the increasing of the oil pollution. The highest measured BR and SIR respiration were obtained with values of 0.053 and 0.234 mgCO2/g.h, respectively, in heavily polluted soils .Multiple regression analysis of independent variables on BR and SIR showed that the most influential variable was oil percentage, which individualy explained 59% of BR variance and 72% of SIR variance. Principal components analysis (PCA) was also done and 73% of the density variance of the samples can be justified by the first two components (biochemical component and physical component).

In a summary, according to the microbial respiration results in oil-contaminated soil, the microbial population followed by microbial respiration increased with increasing oil pollutant concentration. It seems that long-term, aged and natural oil pollution has caused the selection of resistant microbial communities to the oil compounds, hence we can observe their positive response to the presence of oil compounds, and an increase in microbial respirations (BR and SIR).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .