Concentrations and properties of dissolved organic matter in forest soils as affected by the redox regime

Changes in concentrations and properties of dissolved organic matter (DOM) caused by oxygen deficiency are poorly understood. We estimated the influence of redox conditions on DOM dynamics in the field, sampling soil solutions at different depths of three soils (Humic and Histic Gleysol, Chromic Cambisol) along a soil catena in the cool-humid Black Forest (Germany) over a period of 2 years. We measured dissolved organic carbon (DOC) and determined the specific absorbance at 280 nm and two humification indices derived from fluorescence spectra to describe the aromaticity and complexity of DOM. Redox potential (Eh) was monitored continuously in situ. In the forest floor, DOC concentrations ranged independent of soil organic matter content and redox regime between 40 and 60 mg C L-1. DOC concentrations in all soils decreased with depth, accompanied by a decrease in DOM aromaticity and complexity. In the mineral subsoil, DOC concentrations, aromaticity, and DOM complexity were smallest in the aerobic soil (Chromic Cambisol; Eli > 500 mV) and largest in the most anaerobic soil (Histic Gleysol; Eli < 100 mV). Large DOM retention in the aerobic soil could be related to high contents of Fe oxides, highlighting their importance for DOM adsorption. Despite significantly reduced DOM retention under anaerobic conditions, it remains relatively large because the main DOM adsorbents changed from Fe oxides under oxic conditions to clay minerals, which were about 100 times more abundant under anaerobic conditions than Fe oxides. We found indications that biodegradation of DOM contributes more to DOM retention under anaerobic conditions, and we conclude that large DOM fluxes from anaerobic forest soils are the result of limited DOM adsorption in the subsoil rather than large DOM release from the topsoil.