Bacterial community in semiarid hydrocarbon contaminated soils treated by aeration and organic amendments

• Both, hydrocarbon contamination and organic amendments increase soil bacterial abundance but decrease bacterial diversity.
• Functional and structural soil bacterial community change with hydrocarbon pollution.
• Actinobacteria, Proteobacteria and Cloroflexi are the most dominant phylum in treated contaminated arid soils.
• Hydrocarbons rather than organic amendments are responsible for changes in bacterial community.

Bioremediation of oil sludge contaminated soils may enhance the rate of petroleum hydrocarbons (PHs) biodegradation. Changes in bacterial community structure and functionality may be expected as a result of hydrocarbon contamination and bioremediation processes.A 6 month microcosm experiment was performed in which an oil sludge contaminated semi-arid soil was treated by aeration (AE), and aeration plus organic amendment (dried biosolid, DB) and composted biosolid, CB)). Hydrocarbon degradation and functional and structural changes in microbial communities were examined after submitting the soil to these bioremediation treatments. AE reduced the amount of PHs in soil from the initial 5% to 2.7% whereas the amount of PHs left in the soil with the CB and DB treatments were 2.5% and 2.4%, respectively. Bacterial abundance, measured by real-time PCR, was significantly higher in bioremediated soils (AE, DB and CB) than in the non-contaminated soil (control soil). Functional and structural differences in bacterial community measured by BIOLOG and denaturing gradient gel electrophoresis (DGGE), respectively, were observed between bioremediated soils and control soil, both at the start and end of the bioremediation process. After 6 months of bioremediation, 16S rDNA clone libraries showed less phyla in bioremediated soils than in the control soil. The most dominant phylum observed in bioremediated soils was Actinobacteria whereas in the control soil Proteobacteria was the most dominant.This study suggests that regardless of the type of bioremediation carried out and the degree of hydrocarbon degradation, the bioremediation process leads to an increase in soil bacterial abundance accompanied by a decrease in microbial diversity, as well as, to structural and functional changes in the bacterial community.