1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 University of Bayreuth, D-95440 Bayreuth, Germany
3 Institute of Terrestrial Ecology, ETH Zurich, CH-8952 Schlieren, Switzerland
In forest ecosystems, organic solutes play prominent roles in pollutant and nutrient transport. This study, conducted in subalpine forested Humaquepts in Switzerland, investigated the influence of the redox conditions and flow processes on dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). In the mineral soil, concentrations of DOC were higher under reduced than under oxidized conditions. They averaged to 1.2 mm DOC L-1 in the reduced mineral soil at a 100-cm depth. A close correlation between DOC and dissolved Fe concentrations (r2 = 0.83; p < 0.001) suggests that reductive dissolution of Fe-oxides was the major reason for the low retention. However, during aerobic sampling of soil solution from the reduced mineral soil, DOC coprecipitated with Fe. This suggests that the DOC input from reduced mineral soils to aerobic stream water is lower than expected from the high DOC concentrations in the soil. Organic N was the major form of dissolved N in the soil solution at all soil depths. In the reduced subsoil, dissolved N was completely organically bound. This was probably due to both an immobilization of organic N and a low retention of DON. During storms, DOC concentrations increased rapidly with increasing discharge in the subsurface flow. At peak flow, DOC concentrations were doubled, compared with base flow. The molar UV absorptivity of DOC in the subsurface flow corresponded closely to that of the topsoil. These findings suggest that DOC concentrations and properties are sensitive to flow velocities, and that DOC is preferentially transported to the subsoil at high discharge.