This research investigated the effects of varying concentrations of crude oil on the population of crude oil degrading microorganisms in crude oil polluted agricultural soils from Igodan- Lisa, Oba-Ile and Ido-Ani areas of Ondo State, Nigeria. The soil samples were exposed to 1- 4% (w/w) crude oil and analyzed monthly for six periods using standard microbiological techniques for the cultivation and enumeration of crude oil degrading bacteria and fungi. Results indicated that the crude oil degrading microbial populations were significantly altered. The population of crude oil degrading microbes were higher (1.03 x 105 – 1.10 x 106 cfu/g for bacteria and 1.07 x 104 – 8.67 x 105 sfu/g for fungi) in polluted than unpolluted (1.53 x 104 – 9.40 x 105 cfu/g for bacteria and 1.17 x 103 – 5.17 x 105 sfu/g for fungi) soils and also varied with increase in the amount of crude oil spilled and time. The mean count indicated that the microbiological status of the soil samples were not negatively impacted at 1-4% crude oil contamination and the effect on soil micro flora is a function of both concentration and contact time.
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