1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 Forest Ecology, Swiss Fed. Inst. of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland
3 Paul Scherrer Institute (PSI), CH-5232 Villigen-PSI, Switzerland
4 Institute of Plant Sciences, Swiss Fed. Inst. of Technology, ETH Zentrum, CH-8092 Zurich, Switzerland
In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of 15NO3 and 15NH4 were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600 m2, were delimited by trenches in the Gleysols. K15NO3 was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, 15NH4Cl was applied as a second one-year pulse, followed by a second sampling campaign. Trees (needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well.
Most of the added tracer remained in both ecosystems. More NO3- than NH4+ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event- based runoff analyses showed an immediate response of 15NO3 - in runoff, with sharp 15N peaks corresponding to discharge peaks. NO3- leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C:N ratio.
Keywords: mountain forest, mountain meadow, 15N tracer, nitrate leaching, nitrogen deposition