SMALL-SCALE HETEROGENEITY OF SOIL CHEMICAL-PROPERTIES .2. FRACTIONS OF ALUMINUM AND HEAVY-METALS

Soil aggregates from nine horizons of five forest sites in Bavaria were mechanically separated into an aggregate surface fraction and an aggregate core fraction. Seven operationally defined fractions of Al, Cd, Cu, Cr, Ni, Pb, and Zn were determined by a sequential extraction procedure within both aggregate fractions. The CEC, C-org, and oxides of Mn and Fe also were measured. The differences in metal concentrations between surface and core fractions were small but consistent: mainly lithogenic metals (Al, and, within a serpentine derived soil, Ni and Cr) showed lower total concentrations in the surface than in the core fraction. Metals originating primarily from atmospheric deposition form two groups. Cadmium, Pb, and Zn usually showed higher total metal concentration could be extracted by the residual extraction step (mainly bound within silicates) in the surface than in the core fraction. Accordingly, higher proportions could be extracted by the sum of all other steps (bound to cation exchangers, organic matter, and oxides) in the surface fraction, although in the aggregate surface fraction the concentrations of organic matter and oxides as well as the CEC were lower than in the core fraction. The fact that the sorptions capacity of the surface is lower than of the core fraction has to be considered for the identification of external metal inputs. Based on these results, a new method to identify external inputs is proposed.