Ecological impact of mining on soils of Southwestern Nigeria

This paper investigated the current status of soils from three mine sites (Awo, Itagunmodi and Ijero-Ekiti) in Southwestern Nigeria. Composite soil samples were collected at 0 to 15 cm depth, air-dried and analysed for physical, chemical properties and heavy metal contents (cadmium, copper, lead, arsenic and iron) using routine procedures with atomic absorption spectrophotometer. Microbial analyses were carried out on freshly collected soil samples for total heterotrophic bacteria (THB) and total heterotrophic fungi (THF) followed by identification of the isolated microorganisms. Data were analysed using ANOVA, with means separated using Duncan’s Multiple Range Test at 95% significance level, correlation and cluster analysis. The results showed low soil pH in mine soils (Awo, 5.1; Itagunmodi, 5.3 and Ijero-Ekiti, 3.5) with soil chemical properties (organic carbon, total nitrogen, available phosphorus and exchangeable cations), and THB and THF populations significantly differing ( p < 0.05) to those of their corresponding undisturbed sites. In soils of the mine sites, copper concentration had positive significant correlation ( p < 0.05) with THB and THF, while manganese, zinc, copper and sodium concentration showed significant correlation ( r = 0.666, p = 0.05) with THB and THF. Heavy metal concentrations in mine soils were higher than the FAO guidelines for agricultural soils. In total, five heterotrophic bacteria species ( Pseudomonas aeruginosa, Acinetobacter calcoaceticus, Klebsiella edwardsii, Pseudomonas pseudomallei and Klebsiella pneumonia) and 10 heterotrophic fungi species (Aspergillus fumigatus, Aspergillus glacus, Rhizopus stolonifer, Rhizopus japonicus, Penicillium expansium, Trichoderma viride, Fusarium sp., Microsporium audouinii, Cladosporium werneckii and Scopulariopsis brevicaulis) were identified. The study concluded that soils of mine sites were ecologically degraded, as soil had low pH, reduced organic carbon, total nitrogen, available phosphorus, exchangeable cations, elevated heavy metal contents and higher THF counts. These results underline the need for strict mining operation policies in Nigeria and suggest immediate remediation strategies.