Polycyclic aromatic hydrocarbons (PAH) in different forest humus types

The behavior of 20 PAH in the organic layers of a L mull, an Of mull and a mor was assessed by a combined approach of a soil profile study, and the analysis of particle-size separates. Increasing PAH concentrations with depth in the mor profile (L, 866 mu g kg(-1); Of, 2902 mu g kg(-1); Oh, 10489 mu g kg(-1)) were assigned to selective enrich ment during organic matter decomposition. PAH were further highly enriched within the finer separates. For the L horizons, significant positive correlations were established between the enrichment of individual PAH (as observed from the decomposition gradient between the >2-mm fraction and the <0.05-mm fraction), and the K-OW for each compound. The slope of the regression line, m, described the degree of differentiation between low- and high-molecular PAH during litter decomposition. Since m was greatest in the most biologically active humus type (L mull, 0.33) and smallest in the most inactive (mor, 0.20), microbial breakdown was assumed as the dominating process for this differentiation. The results also indicated char decomposition processes had already taken place in the L horizons, leading to morphological and chemical changes of organic matter, and to an enrichment of high molecular PAH.