Crude oil and kerosene have useful applications in the society and are, therefore, used as sources of income and energy. However, the spills arising from crude oil and kerosene in our environment are becoming a visible problem in our ecosystem. Crude oil and kerosene pollute our farmlands and other arable lands and may negatively affect the growth and activities of nitrifying bacteria responsible for nitrification process, leading to loss of soil fertility. Considering the important roles played by nitrifying bacteria in soil fertility in our ecosystem, the assessment of the response of the nitrifying bacteria to pollution stress exerted by crude oil and kerosene on them was investigated. Two genera of nitrifying bacteria were isolated from the soil and were identified asNitrosomonas sp and Nitrobacter sp based on their morphological, biochemical and physiological characteristics. Nitrosomonas sp was isolated using Winogradsky medium for nitrification phase I while Nitrobacter sp was isolated using Winogradsky medium for nitrification phase II. A range of crude oil and kerosene concentrations (in triplicates) were prepared and introduced into sterile Winogradsky salt broth to arrive at final concentrations of 0.5%, 1%, 2%, 5% and 10% (v/v) respectively. The flasks were thereafter inoculated with standardized pure cultures of Nitrosomonas sp and Nitrobacter sp using a uniform inoculum size of 1ml of 1.5 x106 cells for Nitrosomonas sp and 1.2 x106 cells for Nitrobacter sp and incubated for 120 hours. The result of the toxicity test carried out on these isolates showed that the sensitivity of each test organism was a function of both the contact time and concentrations of crude oil and kerosene. The effect of different concentrations of crude oil and kerosene on the oxidation of nitrite to nitrate showed that oxidation of nitrite to nitrate was hindered at higher concentrations of the two pollutants, but, at lower concentrations, crude oil and kerosene did not hinder oxidation of nitrite to nitrate. Information generated from various tests can be of use in the management of pollution for the purpose of prediction of environmental effects of crude oil and kerosene, comparison of toxicants or organisms or test conditions and regulation of crude oil and kerosene spills. For instance this study will help us to provide information for regulatory purposes that a soil contaminated with certain percentage of crude oil or kerosene should not be used for agricultural purposes or practices.

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