ABSTRACT
Contamination of soil by waste oil discharge is prevalent in oil producing and industrialized countries of the world and prolonged exposure at high concentrations may lead to health problems in man as well as environmental deterioration. Remediation activities are targeted at reducing contamination level to the least minimum in order to enhance environmental development. Some of the factors considered during selection of microorganisms for bioremediation is the presence of plant growth promoting factors, and their interactions which are geared towards enhanced environment remediation. The aim of this study was to isolate and identify plant growth promoting rhizobacteria (PGPR) from two selected ruderals in automechanic workshops in Benin City. Fifteen rhizospheric soil samples were collected around the roots of two selected ruderals (Axonopus compressus, family Poaceae and Eleusine Indica, family Poaceae) from Auto-mechanic workshops in four local government areas (Egor, Oredo, Ikpoba-Okha and Ovia North-East) of Edo State. The soils were transported to the laboratory for physicochemical analyses which included pH, electrical conductivity, organic carbon, organic matter, exchangeable acidity and cation exchange capacity as well as total hydrocarbon content of the soil samples. These were based on standard methods. Minerals and heavy metals were determined using atomic absorption spectrophotometry. Bacteria were isolated using dilution technique and cultured on appropriate culture media followed by in-vitro screening for plant growth promotion activities such as phosphate solublization, Indole acetic acid production (IAA) and ammonia production. Bacterial samples with positive plant growth promotion characteristics were further identified using molecular technique. Findings from this study showed no significant differences in the determined physicochemical parameters of all rhizospheric soil samples from various study locations (P > 0.05) except for electrical conductivity (P < 0.05). The mineral contents were different except for phosphorus, calcium and magnesium were not significantly different (P > 0.05). There were significant changes in the heavy metal concentrations. Zinc concentration was highest (33.84±8.51ml/L) while cadmium concentration was the least (0.07±0.02 ml/l) from rhizospheric soil samples from various study locations. Total hydrocarbon content ((1188.7±65.50) was highest in E. indica soil from automechanic workshops in Egor L.G.A. as well as organic carbon and organic matter contents. However, total hydrocarbon content was least (775±75.00 mg/L) in A. compresssus soil from xi automechanic workshop in Ovia North L.G.A. Bacteria count was highest in ruderal soil of A. compresssus (157 ± 5.00 × 103 cfu/g) and least in ruderal soil of Eleusine indica (30.00 ± 1.00 × 103 cfu/g) in all automechanic workshop from Ovia North East Local Government Area. Eight bacterial isolates were obtained from the ruderalssoil. These were three (3) Gram positive bacteria; Bacillus spp., Staphylococcus spp. and Micrococcus spp. and five (5) Gram negative bacteria; Klebsiella spp., Pseudomonas spp., Enterobacter spp., Salmonella spp. and Escherichia spp. Upon screening for plant growth promotion Enterobacter spp., Escherichia spp. and Salmonella spp. were found to exhibit positive characteristics for nitrogen fixation, Phosphate solubilization, ammonia production and indole acetic acid production. Molecular identification of isolates revealed the presence of PGPRs such as Pseudomonas aeruginosa, Pseudomonas putida, and Alcaligenes faecalis. Soils from the various automatic workshop contain rhizobacteria which are capable of enhancing plant growth and survival even under severe environmental stress conditions as in hydrocarbon and heavy metal pollution.
