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 years spruce needles became larger, with higher N concentrations, but so far without effect on tree growth.