FORMATION AND MOBILIZATION PATHWAYS OF DISSOLVED ORGANIC-MATTER - EVIDENCE FROM CHEMICAL STRUCTURAL STUDIES OF ORGANIC-MATTER FRACTIONS IN ACID FOREST FLOOR SOLUTIONS

Dissolved organic matter (DOM) is well recognized to influence the geochemistry of ecosystems. This study was conducted to determine the pathways of DOC mobilization in the forest floor of coniferous forests. DOM from the forest floor of two acid forest soils in the Fichtelgebirge (Germany) was fractionated into hydrophobic acids and neutrals, and hydrophilic acids and neutrals. Carbon distribution at both sites was similar: 53 and 52% hydrophobic acids, 23 and 22% hydrophilic acids, 10 and 11% hydrophobic neutrals, and 7 and 8% hydrophilic neutrals, respectively. Structural composition of the DOM fractions was determined using chemical degradation, FT-IR and C-13 NMR spectroscopy, as well as pyrolysis-field ionization mass spectrometry. Results indicate different chemical composition of the DOM fractions within each site. Hydrophobic acids show high concentrations of carboxyl and hydroxyl groups, and a high ratio of vanillic acid to vanillin (ca 1.0), both indicating a high degree of biodegradation of plant-derived compounds. Carbohydrates are covalently bound to apolar moieties and from carbohydrate composition a lignocellulose nature of the hydrophobic acid fraction is suggested. Hydrophilic acids can be differentiated from the hydrophobic acids by their higher degree of oxidative biodegradation. The hydrophilic acid fraction also exhibits a higher ratio of microbially released polysaccharides versus plant-derived polysaccharides. Hydrophobic neutrals show the closest relationships to the refractory soil humin, with less degraded dimeric 'condensed' lignin subunits and relatively high contents of non-carbohydrate aliphatics. Hydrophilic neutrals are enriched in carbohydrates mobilized by enzymatic cellulose and hemicellulose breakdown, as well as from microbial origin. We conclude that DOM release into forest floor solution is related to microbial activity by oxidative degradation of plant-derived organic matter (e.g. water-soluble lignin and lignocellulose fragments) and by production of microbial metabolites (e.g. polysaccharides). Overall evidence suggests that hydrophobic acids (i) represent intermediates in organic matter decomposition, which can be further degraded to hydrophilic acids and CO2, and (ii) are precursors of humic substances in illuvial horizons after precipitation/adsorption.