‘Petroleum derivatives’ toxicity: influence on the growth of soil nitrifying bacteria

Abstract

Over dependence on crude oil and its derivatives to drive our daily activities for energy and economy results in frequent discharge of petrochemicals into the environment causing pollution of varying magnitude with worrisome ecological and environmental impact of global concern. This study aimed to determine the toxicity of petroleum derivatives on soil nitrifying bacteria. Soil samples were collected into sterile polythene bag and transported to the laboratory for analyses. Microbiological analyses of the soil samples were performed using conventional and standard microbiological techniques. Kerosene, diesel and petrol at 1%, 3%, 5% and 10% concentrations were used as toxicants on nitrogen-fixing bacteria. Results revealed the mean counts of 2.7 x 107 (CFU/g), 4.3 x 106 (CFU/g), 8.7 x 104 (CFU/g) and 1.2 x 103 CFU/g of total heterotrophic bacteria, bacterial growth on Okon red, Ashby medium and Yeast extract mannitol agar respectively. The identities of nitrifying bacteria were Azospirillum spp., Azotobacter spp. and Rhizobium spp. Toxicity results revealed that while Azospirillum spp. exhibited complete cell death to petrol from 3%, all the other bacteria showed log survival of 70.29 – 86% to the toxicants and Azospirillum, spp., Azotobacter spp. and Rhizobium spp. had 76.63%, 79.63% and 86% log survival to diesel and 77.76%, 79.63 and 77.6% respectively to kerosene. Significant difference (p-value = 0.000) was observed among the three toxicants across all concentration levels. Therefore, this study concludes that petroleum derivatives are toxic to nitrifying bacteria at any concentration and the microbial response is dependent on concentration and hydrocarbon types. Soil contaminated with petroleum derivatives should of necessity be treated in order to ameliorate the consequences of pollution and restore soil productive capacity.