IMPACT OF CRUDE OIL IN SOIL ON THE NITROGEN-FIXING FREE-LIVING MICROBIOTA

Abstract

Petroleum oil spills affect the physical properties of the soil and overall the natural populations of microorganisms. They cause a particular decrease in nitrogen-fixing free-living bacteria, responsible to absorb and recycle nutrients in biogeochemical cycles; by altering these, soil fertility can be affected. The aim of this study was to evaluate the impact of crude oil on nitrogen-fixing free-living bacteria in soil under controlled conditions. Experiments were performed in soil, contaminated with five petroleum oil concentrations (1200, 3500, 10,000, 20,000 and 40,000 mg kg-1 on dry weight) in a completely randomized design. Nitrogen-fixing free-living bacteria populations were measured on each concentration. The index of microbial impact of bacteria showed that at the end of the test (219 h), there was a highly significant (P ≤ 0.01) exponential increase of bacterial colonies in all five oil concentrations, 2 to 3.5 times more abundant than the control. A self-resettling and adjustment to the environment phenomenon was inferred. Macroscopically, there was lower diversity of colonies, but significantly higher populations on treatments with higher oil concentration. Also, bacterial colonies were observed on treatments with major concentration of the hydrocarbon, but these were not present on the control. Nitrogen-fixing free-living bacteria seem to adapt to adverse conditions. Selection of the most suitable might be useful in bioremediation or reparation of impacted areas by crude oil.