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 wer e mechanically separated into an aggregate surface fraction and an agg regate core fraction. Seven operationally defined fractions of Al, Cd, Cu, Cr, Ni, Pb, and Zn were determined by a sequential extraction pro cedure 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 l ithogenic metals (Al, and, within a serpentine derived soil, Ni and Cr ) showed lower total concentrations in the surface than in the core fr action. Metals originating primarily from atmospheric deposition form two groups. Cadmium, Pb, and Zn usually showed higher total metal conc entration could be extracted by the residual extraction step (mainly b ound within silicates) in the surface than in the core fraction. Accor dingly, 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 conce ntrations of organic matter and oxides as well as the CEC were lower t han 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 th e identification of external metal inputs. Based on these results, a n ew method to identify external inputs is proposed.