1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 Institute of Terrestrial Ecology, ETH Zurich, CH-8952 Schlieren, Switzerland
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-1y-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-1y-1 at an ambient N deposition of 18 kg N ha-1y-1. Leaching of NO3- at elevated N deposition was 8 kg N ha-1y-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 minutes. 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-1y-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.
Keywords: catchment, end-member mixing analysis, forest ecosystem, gleysol, nitrate leaching, nitrogen deposition, nitrogen transformation