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In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of 15 NO 3 and 15 NH 4 were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600 m 2 , were delimited by trenches in the Gleysols. K 15 NO 3 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, 15 NH 4 Cl 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 NO 3 À than NH 4 + 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. Eventbased runoff analyses showed an immediate response of 15 NO 3 À in runoff, with sharp 15 N peaks corresponding to discharge peaks. NO 3 À 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.

Nitrate leaching was measured over seven years of nitrogen (N) addition in a pairedcatchment experiment in Alptal, central Switzerland (altitude: 1200 m, bulk N deposition: 12 kg ha −1 a −1 ). Two forested catchments (1500 m 2 each) dominated by Picea abies) were delimited by trenches in the Gleysols. NH 4 NO 3 was added to one of the catchments using sprinklers. During the first year, the N addition was labelled with 15 N. Additionally, soil N transformations were studied in replicated plots. Pre-treatment NO − 3 -N leaching was 4 kg ha −1 a −1 from both catchments, and remained between 2.5 and 4.8 kg ha −1 a −1 in the control catchment. The first year of treatment induced an additional leaching of 3.1 kg ha −1 , almost 90% of which was labelled with 15 N, indicating that it did not cycle through the large N pools of the ecosystem (soil organic matter and plants). These losses partly correspond to NO − 3 from precipitation bypassing the soil due to preferential flow. During rain or snowmelt events, NO − 3 concentration peaks as the water table is rising, indicating flushing from the soil. Nitrification occurs temporarily along the water flow paths in the soil and can be the source of NO − 3 flushing. Its isotopic signature however, shows that this release mainly affects recently applied N, stored only between runoff events or up to a few weeks. At first, the ecosystem retained 90% of the added N (2/3 in the soil), but NO − 3 losses increased from 10 to 30% within 7 yr, indicating that the ecosystem became progressively N saturated.

Despite numerous studies on nitrogen (N) cycling in forest ecosystems, many uncertainties remain, particularly regarding long-term N accumulation in the soil. Models validated against tracer isotopic data from field labeling experiments provide a potential tool to better understand and simulate C and N interactions over multiple decades. In this study, we describe the adaptation of the dynamic process-based model TRACE to a new site, Alptal, where long-term N-addition and 15 N-tracer experiments provide unique datasets for testing the model. We describe model parameterization for this spruce forest, and then test the model with 9-and 14-year time series of 15 Ntracer recovery from control and N-amended catchments, respectively. Finally, we use the model to project the fate of ecosystem N accumulation over the next 70 years. Field 15 N recovery data show that the major sink for N deposition is the soil. On the control plot, tracer recovery in the soil increased from 32 % in the second year to 60 % in the ninth year following tracer addition, whereas on the N-saturated plot, soil recovery stayed almost constant from 63 % in the third year to 61 % in the twelfth year. Recovery in tree biomass increased over the decadal time scale in both treatments, to ca. 10 % over 9 years on the control plot and ca. 13 % over 14 years on the N-amended plot. We then used these time series to validate TRACE, showing that the adaptation and calibration procedure for the Alptal site was successful. Model-data comparison identified that the spreading method of 15 N tracers needs to be considered when interpreting recovery results from labeling studies. Furthermore, the ground vegetation layer was recognized to play an important role in controlling the rate at which deposited N enters soil pools. Our 70-year model simulation into the future underpinned by a Monte-Carlo sensitivity analysis, suggests that the soil is able to immobilize a constant fraction of 70 and 77 % of deposited N for the treated and the control plot, respectively. Further, the model showed that the simulated increased N deposition resulted in a relatively small elevated C sequestration in aggrading wood with an N use efficiency of approximately 7 kg C per kg N added. Keywords N cycling Á 15 N tracer Á Simulation model Á Mountain forest Á Norway spruce Á N deposition Communicated by R. Matyssek.