Properties of organic matter in soil solution in a German fen area as dependent on land use and depth

Abstract

The aim of this study was to identify the differences in the composition of dissolved organic matter (DOM) in soil dependent on both land use and soil depth. Soil solutions of six differently used sites in a German fen area were sampled using ceramic suction cups at three depths (25, 55 and 95 cm) and analyzed for dissolved organic carbon (DOC) over a 2 1/2-year period. Studies on DOM composition included UV and fluorescence spectroscopy as well as a fractionation in hydrophobic and hydrophilic components. The DOC concentration was affected by land use in topsoil only. Degraded peatlands had a high intensity of land use and therefore a low soil carbon content and low DOC concentrations. The DOC concentrations decreased significantly with increasing soil depth. The DOM retention through adsorption or decomposition was much higher at intact peatlands than at degraded peatlands. The degraded peatlands soil DOM had a higher specific absorption at 285 nm and a higher humification index derived from fluorescence spectra than DOM of intact peatlands (low intensity of land use) down to the subsoil. The higher DOM humification was related to strong peat decay. The DOM of degraded peatlands preserved its higher degree of aromaticity and polycondensation down to the subsoil, although highly condensed and substituted aromatic compounds were retained preferentially with increasing soil depth. Therefore, effects of land use on the soil DOM composition are stronger than the effects dependent on increasing soil depth. The DOM fractionation in hydrophobic and hydrophilic components was not sensitive enough to detect the small differences in DOM composition caused by land use and depth.