Effects of Amendments on Tropical Mining Soils: Geochemical, Toxicological and Microbiological Approaches

Soil amendments have emerged as a crucial remediation strategy. However, the effects of improvements on tropical soils contaminated by potentially toxic metals and metalloids (PTMs) in mining areas remain largely unknown. This paper reports on a pot experiment conducted with peat, compost, and biochar, together with soil and slag containing high concentrations of PTMs from an abandoned mine site (Ribeira Valley, Brazil). Mixtures of slag, soil, and amendments altered the geochemical fractionation and, in most improvements, resulted in positive changes in maize (Zea mays) growth, which may be due to the lower bioavailability of contaminants and/or the greater supply of nutrients. The germination rate was less sensitive to the effect of the improvements, with values greater than 75%. However, the addition of 10% biochar resulted in 0% germination. Contaminants were present in the roots of all samples, but they were observed exclusively in the shoots of certain plants. The combination of bioconcentration factor (BCF) < 1 and translocation factor (TF) < 1 suggests mechanisms that limit the uptake of metals and metalloids from the soil, with minimal translocation to the shoots of plants. Regarding ecological risk, improvements were notable with the addition of 10% compost (resulting in lower risks for Cd, Pb, Zn, and Co) and with 5% peat + compost + biochar (resulting in lower risks for Pb, Zn, As, and Ni). The results of ACE and Chao1 for the improvements were equal to or greater than those for soil and slag, indicating the amendments tended to increase species richness. Proteobacteria, Bacteroidota, and Actinobacteriota phyla were identified in the improvements and the presence of amendments altered the soil bacterial community at the genus level. Although peat, compost, and biochar (including their combined use) are promising alternatives for PTMs contaminated soils, the addition rates must be adjusted correctly for avoiding undesirable ecotoxicological effects.

Graphical Abstract