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
Two small experimental headwater catchments (approx. 1500 m2) were set up in an alpine spruce forest in the Alptal valley (central Switzerland). They are located at an altitude of 1200 m and receive an average of 2300 mm precipitation a year. The site is moderately impacted by atmospheric nitrogen deposition (12 kgN/ha/a bulk deposition of inorganic N). Two different gley soils occur as patches atop a Flysch substratum. The spatial heterogeneity of the soil is related with both the microtopography of the site and the vegetation.
The catchments were made by digging 80 cm deep trenches into the shallow gleyic soils. Because of the low permeability of the clay-rich subsoil, these trenches are able to collect the lateral water flow generated in the plots. The catchments were equipped with gauging stations (V-notch weirs) and automatic, runoff-proportional water samplers. Along with meteorological parameters and deposition measurements, the water and solute discharge were monitored during one year.
A correct water balance was obtained from the experimental catchments. The leaching of inorganic nitrogen was estimated to be 4 kgN/ha/a, mainly as nitrate. It is, however, not clear if this nitrate is leached because the ecosystem is nitrogen saturated (N in surplus of the maximal ecosystem retention). Quick preferential water flow occurs through the soil and this may prevent nitrate from rain or snowmelt to be removed by the soil matrix before entering the water runoff pathways. Both mini-catchments showed similar biogeochemical cycles, especially for nitrogen. The time series obtained will be used as reference period for a paired-catchment experiment simulating an increased nitrogen (ammonium nitrate) deposition.
Keywords: nitrogen cycling, nitrogen deposition, forest ecosystem, headwater catchment, NITREX project