Potentially toxic elements and their impact on minesoil biota: a pioneering chronosequence study at the Moatize coal mine in Mozambique

This study is the first to investigate the relationship between potentially toxic elements (PTEs) and soil fauna abundance across a chronosequence of minesoils in Moatize coal mine, Mozambique. Soil samples were collected (0.00−0.10 m layer) across a chronosequence of minesoils afforested for 0.5, 1.4, 5.3, and 10.6 years. One adjacent unmined soil was sampled as reference. The samples were analyzed for bioavailable Cr, Pb, Cu, Zn, and Ba contents, and their ecological risk factor and potential ecological risk index were assessed. Soil organic carbon content in the form of fulvic acid (C-fulvic), humic acid (C-humic) and humin (C-humin) as well as soil macro- and mesofauna organisms were quantified. The ecological risk factor and index values indicated low levels of contamination, with individual risk factors ranked as Cr>Cu>Ba>Pb>Zn, regardless of restoration time. Nevertheless, principal component analysis revealed antagonist interactions between Ba, Cu and Zn contents and fauna recolonization in the minesoil. Soil fauna abundance remarkably increased over restoration time, especially Acari, Collembola and Hymenoptera organisms, reaching 642, 227, and 143 organisms, respectively, after 10.6 years. This was accompanied by increased C-humic and C-humin concentration. Nevertheless, Acari, Collembola and Hymenoptera abundance in the 10.6-year restored minesoil were still 84%, 91% and 68% lower, respectively, compared to the unmined soil. Elapsed 10.6-years of restoration, minesoil fauna abundance and diversity increased, but this process seems to be hindered by residual contamination of the minesoil with PTEs, mainly Cu, Zn, Ba, and Cr. Our data may serve as a baseline for long-term environmental monitoring, as well as guidelines for policymakers defining restorations strategies in Moatize. This is critical as legislative reference values of PTEs are not established in this region.