Petroleum Pollution And The Microbial Community Structure In The Soil Of Shengli Oilfield

Oil and gas are considered as the world’s most important resources. The petroleum industry plays a critical role in driving the global economy. On the other hand, the petroleum industry holds a major potential of hazards for the environment and ecosystem, due to pollution from exploration and production of crude oil and natural gas. In this study, soil samples were collected at depth of0-20cm around four oil wells with different exploitation times (1960s,1990s、2000s、2010s) in Shengli Oilfield and determined for physico-chemical properties, total petroleum hydrocarbons (TPHs) and polycyclic aromatic hydrocarbons (PAHs), providing an opportunity to assess spatial distribution of these contaminants and potential sources of PAHs. Microbial community structure and diversity were also studied by PCR-DGGE and clone sequencing techniques to analyse the soil dominant species, microbial community diversity and its relation with soil environmental factors, in order to provide theoretical bases and background information for construction of oil degradation bacteria and in situ bioremediation in Shengli Oilfield. The main results were as follow:(1) The concentrations of TPHs ranged from693mg/kg to5550mg/kg, which exceeded the threshold of the environmental quality standards of soil500mg/kg and decreased with increasing distance away from the oil wells. The TPHs concentrations were significantly positively correlated with the contents of organic carbon (P<0.05) but had no effect on moisture content, pH, total nitrogen, available nitrogen and available potassium. Petroleum pollution in the soils led to a decrease in pH but an incresae in C:N and C:P.(2) PAHs concentration of well head in different oil wells was1960s>1990s>2000s>2010s. As for the same oil well, PAHs concentration decreased with increasing distance from the oil wells (except2010s). Among the individual PAHs, the dominant PAH compounds were3-rings, while the2-rings and5-6rings were reletively low. The soils around four oil wells were heavily polluted by PAHs, a combination of petroleum and combustion sources was the dominant source of PAHs. Organic carbon made an important contribution to the distribution of PAHs in soils. (3) Principal component analysis (PCA) was used to evaluate the relationship between microbial community composition and environmental factors in the soils. The results showed that microbial community diversity was significantly and negatively correlated with TPHs,∑PAHs,6-rings PAHs, organic carbon, C:N (P<0.05or P<0.01). There were no significant correlations between microbial community diversity and moisture content, pH, total nitrogen, available nitrogen, available phosphorus, available potassium.(4) The method of PCR-DGGE was used to investigate the changes of microbial community structure and diversity under the stress of petroleum pollution. The results showed that abundant microbial species existed in the petroleum-contaminated soils around new oil wells (2000s、2010s) and old oil wells (1960s、1990s), and obvious differences were found between new oil wells and old oil wells. Heavy petroleum pollution resulted in the decrease of evenness index, but the increase in the number of dominant species. Soils around the new oil well (2010s) had higher microbial diversity than other oil wells. The maximum of similarity indice were observed in the soils around the same oil well with similar petroleum pollution levels.(5) Sequence analysis of the prominent bands in DGGE profiles showed that some dominant species (Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria and Chloroflexi) were observed in the petroleum-contaminated soils around new oil wells and old oil wells. Most of them were oil-associated and hydrocarbon degrading bacteria. Streptomyces, Mycobacterium, Bacillus and Clostridium were unique to old oil wells, while Dietzia and Microbacterium were unique to new oil wells. Flavobacterium and γ-Proteobacteria were observed in new oil wells and old oil wells. They were most likely to be the functional microflora involved in situ bioremediation in Shengli Oilfield.