1 Institut fédéral de recherches sur la forêt, la neige et le paysage (WSL), CH-8903 Birmensdorf
2 Ecole polytechnique fédérale de Lausanne (EPFL), CH-1015 Lausanne
Nitrogen is added since 10 years to simulate a 25 kg ha-1 year-1 increase in atmospheric deposition to a small catchment in a Norway spruce forest at Alptal (central Switzerland). Compared to a control catchment, this addition doubled nitrate losses in runoff water to the brook within weeks. Especially by 15N labelling, these losses could be attributed to an insufficient contact between soil and precipitation water because of a fast lateral flow. This effect of the addition increased then with the year, in parallel with an accumulation of most deposited N in the soil, lowering its C/N ratio. The vegetation was not much affected. After several year spruce needles became larger, with higher N concentrations, but so far without effect on tree growth.
Keywords: nitrogen deposition, nitrate leaching, forest, 15N, Gleysol, immobilisation