Concentrations and forms of heavy metals in Slovak soils

The risk assessment of heavy‐metal contamination in soils requires knowledge of the controls of metal concentrations and speciation. We tested the relationship between soil properties (pH, CEC, C_org, oxide concentrations, texture) and land use (forest, grassland, arable) and the partitioning of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn among the seven fractions of a sequential extraction procedure in 146 A horizons from Slovakia. Using a cluster analysis, we identified 92 soils as representing background metal concentrations while the remaining 54 soils showed anthropogenic contamination. Among the background soils, forest soils had the lowest heavy‐metal concentrations except for Pb (highest), because of the shielding effect of the organic layer. Arable soils had the highest Cr, Cu, and Ni concentrations suggesting metal input with agrochemicals. Grassland soils had the highest Cd and Zn concentrations probably for geogenic reasons. Besides the parent material (highest metal concentrations in soils from carbonatic rock, lowest in quartz‐rich soils with sandy texture), pedogenic eluviation processes controlled metal concentrations with podzols showing depletion of most metals in E horizons. Partitioning among the seven fractions of the sequential extraction procedure was element‐specific. The pH was the overwhelming control of the contributions of the bioavailable fractions (fractions 1–4) of all metals and even influenced the contributions of Fe oxide‐associated metals (fractions 5 and 6) to total metal concentrations. For fractions 5 and 6, Fe concentrations in oxides were the most important control of contributions to total metal concentrations. After statistically separating the pH from land use, we found that the contributions of Cu in fractions 1–4 and of NH₄NO₃‐extractable Al, Cd, Pb, and Zn to total metal concentrations were significantly higher under forest than under grassland and in some cases arable use. Our results confirm that metal speciation in soils is mainly controlled by the pH. Furthermore, land use has a significant effect.