Mobilization and immobilization of dissolved organic matter in forest soils

Field and laboratory studies combined with destructive and non-destructive analytical methods were used to characterize dissolved organic matter (DOM) in acid forest soils. DOM is produced in significant amounts in the forest canopy and in the forest floor. A major part of the organic solutes are lignocellulose-degradation products being strongly microbially altered in the course of ligninolysis. The release of lignin-derived moieties into the soil solution is controlled by their degree of biooxidation. Microorganisms contribute also directly to the organic solutes through the release of microbial metabolites. DOM released from the forest floor passes the upper mineral soil almost conservatively, whereas in the subsoil most DOM is removed from solution. Immobilization of DOM is mainly due to sorption on Fe and Al oxides. The highly oxidized lignin-derived moieties are preferentially removed from the soil solution whereas the saccharides are relatively enriched. We conclude that DOM in the forest soil output to the hydrosphere is a result of (1) the release of microbially degraded ligno-cellulose compounds and of microbial metabolites into the forest floor solution and (2) selective sorptive removal of the lignin-derived constituents in the subsoil.