Appraisal of current and post-mine status of salt mines in a Guinea Savanna ecosystem : implications for natural recovery

Abstract

Generally, concerns have been raised about increasing mining activities, their impacts on ecosystems and the possibility of site recovery in different parts of the world. This study appraised the status of plant diversity and soil conditions in a popular salt mine of Keana, Nigeria. Ten plots of size 30 m × 20 m were established randomly at each active and abandoned salt mines (SM) and within each plot, five subplots of size 2 m × 2 m were used for the plant diversity assessments. This gave rise to a total of 100 subplots in all the sites. The diversity indices were quantified and dominant species identified. Principal component analysis with Bray-Curtis dissimilarity test was used to establish and compare the species compositions between the sites. Soil samples were collected at four different depths (5, 10, 15 and 20 cm depths) and the chemical properties were determined. Soil analysis revealed high concentrations of heavy metals such as lead, mercury, arsenic and nickel at both sites, but metal concentrations were significantly higher at the active SM using the one-way analysis of variance. The results showed a significantly higher species richness (Taxa_S = 59) and diversity (Simpson index = 0.96 and Shannon index = 3.57; all p < 0.01) at the active SM compared to the abandoned SM (Taxa_S = 32, Simpson index = 0.95 and Shannon index = 3.04). The species compositions between the two sites were significantly different, although with Bray-Curtis value of 0.77 revealing a high shared species compositions. The abandoned SM showed signs of natural recovery, with increased species richness and diversity, and lower metal concentrations. Plant species such as Dactyloctenium aegyptica and Mimosa pudica which were found to be associated with the abandoned SM could be identified as active players in the natural restoration of that site. Some plant species including Hyptis suaveolens and Azadirachta indica that were tolerant to varying high metal levels could be candidates for phytoremediation purposes. The findings of this study highlight the impact of salt mining on plant diversity and metal concentrations in the soil and plants and the potential for natural recovery at abandoned mine sites, insights for post-mining land management and restoration strategies in similar savanna ecosystems.