Spatial distribution and geochemical background of quartzitic and ferruginous rupestrian field soils

Abstract

Recent changes in Minas Gerais (Brazil) environmental legislation increased the threats caused by mining activities on the conservation of ferruginous rupestrian fields (FRF). The permission to compensate areas to be degraded by others with different vegetation, if there is “ecological equivalence”, now allows that quartzitic rupestrian fields (QRF) can be used to compensate the suppression of FRF. This work provides scientific basis for discussing this issue using soil data (one of the main determinants of vegetation structure and diversity) from the Iron Quadrangle (IQ), a unique region where QRF and FRF share the same geological evolution that defined two deformational domains. Al, As, Ba, Cd, Cu, Fe, Mn, K, P, Pb and Zn geochemical background values were calculated in the QRF and FRF soils throughout the entire IQ and separately in its domains. FRF soils have significantly higher Fe and P average concentrations and background values, at least twice as those found in QRF. In turn, QRF soils present higher Ba, Cu and K average concentrations, with background values one order of magnitude higher. The high standard deviation of background values of some elements reflects the great heterogeneity of each lithotype soil, leading to non-significant differences, as for Al (FRF=51,025 mg.kg⁻¹, QRF=18,100 mg.kg⁻¹). Geological evolution that created IQ domains seems to have exert a strong influence on the geochemistry of rupestrian fields (RF) soils, as first demonstrated in this study. In the western, Al, Ba, K, Mn, Pb and Zn average concentrations are significantly higher. FRF in mid-eastern IQ region stand out for the highest Ba, Cd, Pb and Zn concentrations. Considering differences observed between QRF and the FRF soils and the influence of edaphic parameters on the vegetation structure and species composition of RF, it becomes evident that these areas should not be automatically defined as ecological equivalents.