Abstract

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Microbial response to plants' roots and contaminants in diesel-oil impacted soils were assessed in a greenhouse study. Two plants, Pueraria spp. and Panicum maximum were planted in 6% w/w polluted and control soil samples to obtain contaminated planted and uncontaminated planted pots. Contaminated control and uncontaminated control pots were also maintained. Periodically, from week zero to 8 weeks after planting (WAP), the total bacterial, fungal and hydrocarbon utilizing microbial counts of the soil samples were determined. The results showed that total bacterial and fungal counts at week zero decreased from an average of 5.8 x 106 CFU/g soil and 3.42 x 104 CFU/g soil in uncontaminated soil to 3.04 x 106 CFU/g soil and 2.27 x 104 CFU/g soil respectively, in the contaminated control samples. This could be in response to the direct toxic effects of volatile hydrocarbons, limitations in oxygen and water resulting from hydrophobic oil. At 4 WAP, the bacterial counts increased to 16.33 x 106 CFU/g soil and 12.67 x 106 CFU/g soil in the contaminated samples planted Pueraria spp. and P. maximum respectively. This could be as a result of the response of adapted species to the rhizosphere effect. At the end of the study, the percentage oil in soil were reduced by 55.63%, 64.38% and 65.45% in contaminated unvegetated sample, contaminated sample planted Pueraria spp. and in contaminated sample planted P. maximum, respectively. This study showed that microbial growth was sustained in the rhizosphere of the plant species studied. This has implications for bioremediation especially in the tropics where warm temperatures favour plants' growth and microbial activities.

Keywords: Phytoremediation, plant derived hydrocarbons, rhizosphere effect and toxic volatile hydrocarbons