The responses of nitrogen transformations and nitrate (NO₃^-) leaching to experimentally increased N deposition were studied in forested sub-catchments (1500 m²) with Gleysols in Central Switzerland. The aim was to investigate whether the increase in NO₃^- leaching, due to elevated N deposition, was hydrologically driven or resulted from N saturation of the forest ecosystem. Three years of continuous N addition at a rate of 30 kg NH₄NO₃-N ha^-1 yr^-1 had no effects on bulk soil N, on microbial biomass N, on K₂SO₄-extractable N concentrations in the soil, and on net nitrification rates. In contrast, N losses from the ecosystem through denitrification and NO₃^- leaching increased significantly. Nitrate leaching was 4 kg N ha^-1 yr^-1at an ambient N deposition of 18 kg N ha^-1 yr^-1. Leaching of NO₃^- at elevated N deposition was 8 kg N ha^-1 yr^-1. Highest NO₃^- leaching occurred during snowmelt. Ammonium was effectively retained within the uppermost centimetres of the soil as shown by the absence of NH4⁺ in the soil solution collected with micro suction cups. Quantifying the N fluxes indicated that 80% of the added N were retained in the forest ecosystem. Discharge and NO₃^- concentrations of the outflow from the sub-catchments responded to rainfall within 30 min. The water chemistry of the sub-catchment outflow showed that during storms, a large part of the runoff from this Gleysol derived from precipitation and from water which had interacted only with the topsoil. This suggests a dominance of near-surface flow and/or preferential transport through this soil. The contact time of the water with the soil matrix was sufficient to retain NH4⁺, but insufficient for a complete retention of NO₃^-. At this site with soils close to water saturation, the increase in NO₃^- leaching by 4 kg N ha^-1 yr^-1 through elevated N inputs appeared to be due to the bypassing of the soil and the root system rather than to a soil-internal N surplus.