Heavy metal distribution between soil aggregate core and surface fractions along gradients of deposition from the atmosphere

Abstract

This study tests the hypothesis that heavy metals deposited from the atmosphere are bound preferentially to soil aggregate surfaces mainly in forms of higher solubility than in aggregate cores. Soil aggregates (2–20 mm in diameter) were sampled along two gradients of heavy metal deposition from the atmosphere. Near Dornach/Switzerland (DOR) heavy metal deposition originates from a brass foundry, and at the second site near Ziar nad Hronom/Slovakia (ZNH) from the coal-based power station of an aluminum smelter. The soils at the DOR-sites have a neutral to weakly acid soil reaction (pH 6–7), those at the ZNH-sites have an acid soil reaction (pH 3–4). At DOR the 0–20 cm layer and at ZNH the organic layer, A and B horizons of three soils with decreasing depositional burden, were sampled. Aggregates were divided mechanically into core and surface fractions. We sequentially extracted seven fractions of Cd, Cu, Zn (DOR and ZNH) and Ni (ZNH) from both aggregate fractions. At DOR the higher the depositional burden, the more pronounced is the enrichment of heavy metals in the aggregate surface compared with the core fractions. At the acid ZNH-sites preferential leaching of heavy metals from the aggregate surfaces dominates, resulting in depletions of Cd>Ni>Zn>Cu in the aggregate surface compared with the core fractions according to the different element mobilities. The preferential leaching is, however, compensated in part by preferential sorption of deposited heavy metals at aggregate surfaces. Thus, the higher the depositional burden, the less pronounced is the depletion. Generally, at both locations greater proportions of the total metal concentrations were found in the bioavailable metal fractions in the aggregate surface than in the core fractions. This indicates that deposited metals are mainly bound in forms of higher solubility. From these results implications for the availability of deposited heavy metals to plants and for leaching may arise.