Forb and soil microbe diversity patterns of ultramafic tailings facilities at Phalaborwa

Abstract

Palabora Mining Company (PMC) in the Limpopo Province has exploited unique ultramafic rock formations in the Phalaborwa Igneous Complex. Subsequently, mining activities has created ‗mountains‘ of processed materials (tailings and dumps). Efficient reclamation approaches have stabilized the degraded landscape and improved the ecosystem functionality of these facilities. However, successful rehabilitation of copper mine tailings requires an in depth understanding of the biotic and abiotic factors most limiting to vegetation establishment and growth. Knowledge of the factors that promote effective rehabilitation is important, as it allows mine management to make informative decisions to address rehabilitation shortcomings by means of appropriate mitigation measures. Therefore, to achieve sustained rehabilitation success, knowledge of aboveground and belowground factors form a crucial link in assessing rehabilitation progress on post-mining sites. The objectives of this study were therefore to determine the effect of different post rehabilitation ages, aspects and topographic positions on; i) soil microbial biomass and community structure, ii) species composition, diversity, biomass and cover of the herbaceous layer, and iii) physical and chemical soil properties of tailings facilities. The herbaceous layer of two of the PMC tailings facilities, namely the Rock Dump (RD) and Tailings Dam (TD), were sampled by means of the fixed quadrat method. Quadrats were placed in a stratified manner to sample the different age levels, aspects and topographic positions of both the facilities. A total of 174 quadrats were sampled, and the herbaceous plant richness, abundance, cover and biomass was documented for each quadrat. Soil sampling was conducted simultaneously to vegetation sampling. A total of 91 soil, and soil microbial, samples were collected and analysed. PRIMER 6, PAST and STATISTICA 11 were used for data analyses, which included Non-metric Multidimensional Scaling (NMDS) ordinations based on the Bray-Curtis index, One-way Analysis of Similarities (ANOSIM), Similarity Percentage Analysis (SIMPER), One-way Analysis of Variance (ANOVA) and Tukey‘s post-hoc HSD for unequal N. Results indicated high levels of vegetation cover on the oldest and intermediate post rehabilitation ages on the RD and TD respectively, while the intermediate age on the RD and youngest age on the TD had the highest biomass. Regarding aspect and topography, the RD and TD reacted similarly, with cover highest on the slope positions on the eastern aspects, and biomass highest on the terrace positions on the western aspects. Terrace positions revealed the highest species richness and diversity, especially on the eastern aspect of the RD and southern aspect of the TD. Highest species richness and diversity was recorded for the oldest age on the RD and youngest age on the TD. Benchmark vegetation results indicated that all tested vegetation indices of natural Mopaneveld were significantly higher than both the RD and TD. Slope positions of eastern aspects on the oldest post rehabilitation ages maintained the highest microbial biomass; however, precarious patterns of microbial community structure were identified. Highly variable physical and chemical soil properties were noticeable on both the RD and TD. Best performing soils were found on the oldest soils on slopes with an eastern aspect on the RD and the oldest soils on terraces of the western aspect on the TD. This study revealed that different post rehabilitation ages, aspects and topographic positions do not affect floristic composition to such an extent that a significant dissimilarity could be identified across the RD and TD. Most of the tested vegetation variables, although with erratic occurrence of significant differences, were recorded between terrace and slope topographic positions, eastern and western aspects, and oldest and youngest post rehabilitation ages of the RD and TD. Overall, enhanced species richness, diversity, physical and chemical soil and soil microbial properties were revealed under combined conditions of slope positions on the RD and terrace positions on the TD on the eastern aspects of the oldest post rehabilitation ages. This study provides valuable information regarding patterns of herbaceous species diversity, microbial community structures and physical and chemical soil characteristics on copper mine tailings and serves as benchmark for long-term monitoring of biotic and abiotic environmental factors