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Pathways and dynamics of NO3 and NH4 applied in a mountain Picea abies forest and in a nearby meadow in central Switzerland |
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| Authors: | Isabelle Providoli Rolf Siegwolf Patrick Schleppi |
| Institution: | a Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland b Forest Ecology, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland c Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland d Institute of Plant Sciences, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zurich, Switzerland |
| Abstract: | 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. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha−1 y−1 equally divided between NH4+ and NO3−. Pulses of 15NH4+ and 15NO3− were applied separately as tracers on plots of 2.25 m2. Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground 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 1 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 1 h, 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 1 year later were surprisingly small. Overall, more NO3− tracer than NH4+ tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH4+ by the ground vegetation, especially by mosses. |
| Keywords: | Ammonium Gleysol 15N tracer Nitrate Nitrogen cycle Nitrogen deposition |
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