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
To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of 15N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce ( Picea abies) and a nearby meadow, at the Alptal research site in central Switzerland. Pulses of 15NH4+ and 15NO 3- were applied separately to trace deposited inorganic N originating from emissions from either agricultural or combustion processes. The tracers were applied on plots of 2.25 m2, and several ecosystem pools were sampled at short to longer term intervals (from a few hours to one year), above and below-ground biomass (excluding trees), litter layer, soil LF horizon (approx. 5 - 0 cm), A horizon (approx. 0 - 5 cm) and gleyic B horizon (5 - 20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to one week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added 15NO3- was immobilised just after one hour, and probably chemically bound to soil organic matter. 16% of the NH4+ tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost 15N over time, a long-term increase in 15N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and one year later were surprisingly small. Overall, more NO3- tracer than NH 4+ tracer was recovered in the soil. This was due to a strong above-ground uptake of the deposited NH4+ by the ground vegetation, especially by mosses.
Keywords: ammonium, Gleysol, 15N tracer, nitrate, nitrogen cycle, nitrogen deposition