Retention and leaching of elevated N deposition in a forest ecosystem withGleysols

The responses of nitrogen transformations and nitrate (NO3-) leaching to ex perimentally increased N deposition were studied in forested sub-catchments (1500 m(2)) with Gleysols in Central Switzerland. The aim was to investiga te whether the increase in NO3- leaching, due to elevated N deposition, was hydrologically driven or resulted from N saturation of the forest ecosyste m. 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-extr actable N concentrations in the soil, and on net nitrification rates. In co ntrast, N losses from the ecosystem through denitrification and NO3- leachi ng increased significantly. Nitrate leaching was 4 kg N ha(-1) yr(-1) at an ambient N deposition of 18 kg N ha(-1) yr(-1). Leaching of NO3- at elevate d N deposition was 8 kg N ha(-1) yr(-1). Highest NO3- leaching occurred dur ing snowmelt. Ammonium was effectively retained within the uppermost centim etres of the soil as shown by the absence of NH4+ in the soil solution coll ected with micro suction cups. Quantifying the N fluxes indicated that 80% of the added N were retained in the forest ecosystem. Discharge and NO3- co ncentrations of the outflow from the sub-catchments responded to rainfall w ithin 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 pr ecipitation and from water which had interacted only with the topsoil. This suggests a dominance of near-surface flow and/or preferential transport th rough this soil. The contact time of the water with the soil matrix was suf ficient to retain NH4+, but insufficient for a complete retention of NO3-. At this site with soils close to water saturation, the increase in NO3- lea ching 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-inte rnal N surplus.