1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstr. 111, CH-8903 Birmensdorf
2 Institute of Terrestrial Ecology, ETH Zurich, Grabenstr. 11, CH-8952 Schlieren
In a Gleysol of an alpine forest site in Alptal, Switzerland, nitrogen transformations are investigated within the NITREX project. The paper presented here aims at studying the relation between microbial N transformation and drainage processes. Both microbial immobilisation of N and the contact time between soil solution and soil matrix could affect the rate of nitrate leaching. Throughfall and soil solution of 5, 10, and 30 cm depth are collected in 10 plots (20 m2), 5 plots with simulated increased N deposition (+ 30 kg NH4NO3- N/ha/a) and 5 plots with ambient deposition (17 kg N/ha/a). Furthermore, we collect the runoff of two forest catchments (1500 m2), with and without nitrogen addition. Dissolved organic nitrogen (DON) is the major N-species in the soil solution, the proportion increasing with soil depth. The NO3/Cl ratio decreases characteristically from the throughfall to the Gr-horizon (30 cm depth) indicating losses via denitrification. Redox potentials measurements confirm these findings. In the runoff of the forest catchments the NO3/Cl ratio as well as conductivity and element concentrations are similar to those in the soil solution at 5 cm depth. This points to solute transport by preferential flow and a short contact time between soil solution and soil matrix. Simulation of increased N-deposition increases the nitrate concentrations only in the topsoil and in the runoff, but most added N appears to be retained in the system.