Polycyclic aromatic hydrocarbons in different forest soils: Mineral horizons

The objective of this study was to assess the behavior of PAH in mineral soil horizons of different forest soils (Allersdorf, All: Inceptisol, mull humus type; Geisberg, Gels: Entisol, mull; Hohe Matzen, HoM: Spodosol, mor). At the mor site, the highest PAH loading was observed in the forest floor (HoM L to Oh, Sigma 20 PAH: 829 g ha(-1)), whereas at the mull sites the humified mineral soil horizons were the main link for PAH (All aAxh, Sigma 20 PAH: 522 g ha(-1)). In all soils,there was a significant PAH translocation into subsoil horizons (Sigma 20 PAH in ;the subsoil: 76-195 g ha(-1)). In order to delineate possible transport mechanisms, double-logarithmic relationships were established between the translocation of the distinct PAH from the surface soil to the subsoil and the PAH's K-ow values. The data suggested that transport of low-molecular PAH into the subsoil was primarily a function of the water solubility of each compound. In the biologically active All and Geis soils, high-molecular PAH were translocated independently from their K-ow value, and particle-bound transport probably by soil burrowing animals was assumed to control translocation of the penta- and hexacyclic PAH. In contrast, at HoM transfer of high-molecular PAH increased with increasing hydrophobicity, suggesting dissolved organic matter (DOM)-mediated transport of PAH. Fractionation of soil into a floatable fraction and into sand- (20-2000 mu m), silt- (2-20 mu m), coarse clay- (0.2-2 mu m), and fine clay sized (< 0.2 mu m) separates revealed that more than 80% of the PAH loading could be assigned to silt- and coarse clay-sized separates, irrespective of the soil's texture (loamy sand to silty clay loam). Silt generally showed the highest C-org-related PAH concentrations. PAH profiles (relative proportion of each PAH on the sum of 20 PAH) revealed increasing proportions of high-molecular, more refractory PAH from the floatables and the sand-sized separates to the finer particles, corresponding with an increasing degree of SOFA alteration in the same direction. At HoM, depth gradients of high-molecular PAH suggested co-transport of penta- and hexacyclic PAH with DOM and subsequent co-sorption selectively to the silt- and coarse-clay sized separates of the Bsh horizon.