1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 Centre for Development and Environment, University of Bern, CH-3012 Bern, Switzerland
3 School of Natural Resources & Environment, University of Michigan, Ann Arbor, USA
4 Forest Ecology, Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland
Despite numerous studies on nitrogen (N) cycling in forest ecosystems, many uncertainties remain, particulary 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 15N tracer experiments provide unique datasets for testing the model. We describe model parameterization for this spruce forest, and then test the model with 14- and 9-year time series of 15N tracer recovery from N-amended and control catchments, respectively. Finally, we use the model to project the fate of ecosystem N accumulation over the next 70 years. Field 15N 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 on both plots to ca. 10%. 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 of 15N 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 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 expected to be able to immobilize a constant fraction of 70-77% of deposited N.
Keywords: N cycling, 15N tracer, simulation model, mountain forest, Norway spruce, N deposition