SOIL DISSOLVED ORGANIC-MATTER SORPTION AS INFLUENCED BY ORGANIC AND SESQUIOXIDE COATINGS AND SORBED SULFATE

The sorption of dissolved organic carbon (DOC) and its fractions as in fluenced by organic C (OC), sesquioxides [amorphous Al(OH)(3), ferrihy drite, goethite], and extractable SO42- was investigated by modifying the properties of the Bs horizon of an Entic Haplorthod. Coating the B s material with OC extracted from the Oa horizon of a mor forest boor layer decreased the sorption of total DOC. The effects on the sorption of the hydrophobic DOC fraction were little, whereas the sorption of the hydrophilic fraction was strongly reduced. At the highest OC coati ng levels, a net release of hydrophilic DOC occurred, indicating a dis placement of indigenous hydrophilic substances from the soil by the so rbing hydrophobic dissolved organic matter (DOM). Coating of the soil material with sesquioxides clearly increased the sorption of DOC and i ts fractions. The strongest effects occurred when amorphous Al(OH)(3) was added, followed by ferrihydrite and goethite. Increased contents o f extractable SO42- did not affect the sorption of DOC on soil materia l. With increasing addition of DOG, however, the concentrations of SO4 2- in the equilibrium solution increased. Adsorbing DOM seems to displ ace SO42- bound to soil materials. Thus, DOM shows a higher affinity t o sorb on soil material than SO42-.