NITROGEN BUDGETS OF 2 SMALL EXPERIMENTAL FORESTED CATCHMENTS AT ALPTAL, SWITZERLAND

Two small experimental headwater catchments (approx. 1500 m(2)) were s et up in an alpine spruce forest in the Alptal valley (central Switzer land). They are located at an altitude of 1200 m and receive an averag e of 2300 mm precipitation a year. The site is moderately impacted by atmospheric nitrogen deposition(12 kg N ha(-1) yr(-1) bulk deposition of inorganic N). Two different gley soils occur as patches atop a Flys ch substratum. The spatial heterogeneity of the soil is related with b oth the microtopography of the site and the vegetation. The catchments were made by digging 80-cm deep trenches into the shallow gleyic soil s. Because of the low permeability of the clay-rich subsoil, these tre nches collect the lateral water flow generated in the plots. The catch ments were equipped with gauging stations (V-notch weirs) and automati c runoff-proportional water samplers. Meteorological parameters, depos ition, water and solute discharge were monitored during one year. A co rner water balance was obtained from the experimental catchments. The leaching of inorganic nitrogen was estimated to be 4 kg N ha(-1) yr(-1 ), mainly as nitrate. It is, however, not clear if this nitrate is lea ched because the ecosystem is nitrogen saturated (N in surplus of the maximal ecosystem retention). Quick preferential water flow occurs thr ough the soil, and this may prevent nitrate from rain or snowmelt to b e removed by the soil matrix before entering the water runoff pathways . Both mini-catchments showed similar biogeochemical cycles, especiall y for nitrogen. The time series obtained will be used as reference per iod for a paired-catchment experiment simulating an increased nitrogen (ammonium nitrate) deposition. (C) 1998 Elsevier Science B.V.