MOBILIZATION AND IMMOBILIZATION OF DISSOLVED ORGANIC-MATTER IN FORESTSOILS

Field :and laboratory studies combined with destructive and non-destru ctive analytical methods were used to characterize dissolved organic m atter (DOM) in acid forest soils. DOM is produced in significant amoun ts in the forest canopy and in the forest floor. A major part of the o rganic solutes are lignocellulose-degradation products being strongly microbially altered in the course of ligninolysis. The release of lign in-derived moieties into the soil solution is controlled by their degr ee of biooxidation. Microorganisms contribute also directly to the org anic solutes through the release of microbial metabolites. DOM release d from the forest floor passes the upper mineral soil almost conservat ively, whereas in the subsoil most DOM is removed from solution. Immob ilization of DOM is mainly due to sorption on Fe and Al oxides. The hi ghly 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 re sult of (1) the release of microbially degraded lignocellulose compoun ds and of microbial metabolites into the forest floor solution and (2) selective sorptive removal of the lignin-derived constituents in the subsoil.