DISSOLVED ORGANIC-CARBON IN FOREST FLOOR LEACHATES - SIMPLE DEGRADATION PRODUCTS OR HUMIC SUBSTANCES

Dissolved organic carbon (DOC) in forest soils exerts a profound influence on soil internal carbon fluxes. We focused our research on whether DOC mobilized in the forest floor can be already classified as humic material. Our approach comprises chemical structural studies of DOC fractions by degradative methods, CPMAS C-13-NMR spectroscopy, and pyrolysis-field ionization mas spectrometry. Hydrophobic acids are mainly composed of plant-derived compounds which become water-soluble by oxidative biodegradation. A part of the carbohydrate moieties in this fraction is probably covalently bound to phenolic structures in lipocellulose-degradation products. As indicated by the higher degree of biooxidation the hydrophilic acids appear to be genetically downstream from the hydrophobic acids. Hydrophobic neutrals are mainly composed of only slightly altered plant-derived material, with less degraded lignin moieties and relatively high contents of non-carbohydrate aliphatics. Hydrophilic neutrals are characterized by the highest concentrations of carbohydrates derived from plant and microbial origin. DOC represents a continuum of organic substances from only slightly modified plant-derived oligosaccharides over highly altered lignin-derived compounds to products of microbial resynthesis. In contrast to the recalcitrant organic matter in humic soil horizons, hydrophobic acids do not show two main characteristics of humification, which can be addressed as enrichment of C-C linked aromatic moieties and as cross-linking of aliphatic compounds. Nevertheless, the hydrophobic acids in particular can be attributed as precursors of humic substances in illuvial soil horizons.