Evaluation of Residual Total Petroleum Hydrocarbon (Tph) in Rhizosphere of Cowpea (Vigna Unguiculata) and Non-Rhizosphere Soil Contaminated with Crude Oil Under Different Organic Manure Treatments

Abstract

James Onyebuonyeoma Ordu and Tombari Bodo

This study was undertaken to evaluate residual Total petroleum hydrocarbon (TPH) in rhizosphere of Vigna unguiculata (cowpea) and non-rhizosphere soils contaminated with crude oil under different organic manure treatments, with the rhizosphere microbial community acting as plant-microbe interaction at subsurface environment. Enumeration of total heterotrophic bacteria and total saprophytic fungi were carried out using plant rhizosphere soil samples. The rhizosphere soil samples were collected from crude oil contaminated soil amended with organic nutrients: poultry droppings, goat faeces, cow dung and fish pond waste water i.e. water from pond containing Clariesgariepenus mixed with its faeces and feeds. The plant rhizosphere sample was cowpea (Vigna unguiculata). A non-rhizosphere soil was used as control. The representative strains or genera of both bacteria and fungi were identified. Total heterotrophic bacterial count range from 3.5x104 to 8.21x106 colony forming units per log10 CFU/g log 10of sample while total fungal counts ranged from 4.30x105 to 8.67x106 log 10 CFU/g; Non-rhizosphere soil (control) samples, had colony counts within the range of 4.30x103 to 4.71x105 for both bacteria and fungi; the rhizosphere were also screened quantitatively for the presence ofhydrocarbon utilizing fungi (HUF) andhydrocarbon utilizing bacteria (HUB) and to evaluate the rate of degradation of crude oil by pure culturesover a period of 28 days in mineral salt broth with crude oil as sole source of carbon. While an experimental set-up mineral salt broth with crude oil without any inoculums served as control. The analysis of pH and total viable counts were carried out at weekly intervals while total petroleum hydrocarbon content was determined at bi weekly interval. Results showed percentages of crude oil degradation in the control and crude oil polluted bioremediated rhizosphere soils, which percentages of crude oil degraded range in polluted soils amended with organic nutrients were as follows: poultry droppings (19.53%), cow dung (13.8%), Goat faeces (12.30%), fish pond waste water (10.69%) and polluted soil without addition of nutrients (9.40%). The result also showed that there was a constant decrease in PH from range 6.08 to 5.04. Total Petroleum Hydrocarbon (TPH) also followed the same pattern of steady decrease. Within days 7, the total TPH was degraded from 28600mg/kg to 1502.877mg/kg, and at day 14 it was reduced to 943.871mg/kg. Thus, this study has succeeded in evaluating the residual TPH levels in the rhizosphere of (cowpea) vigna unguiculata and also in the non- rhizosphere soils contaminated with crude oil under different organic manure treatments.

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