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The process-oriented model PnET-N-DNDC describing biogeochemical cycling of C-and N and N-trace gas fluxes (N sub 2 O and NO) in forest ecosystems was tested for its sensitivity to changes in environmental factors (e. g., temperature, precipitation, solar radiation, atmospheric N-deposition, soil characterstics). Sensitivity analyses revealed that predicted N-cycling and N-trace gas emissions varied within measured ranges. For model validation, data sets of N-trace gas emissions from seven different temperate forest ecosystems in the United States, Denmark, Austria, and Germany were used. Simulations of N sub 2 O emissions revealed that field observations and model predictions agreed well for both flux magnitude and its seasonal pattern. Differences between predicted and measured mean N sub 2 O fluxes were smaller than 27 %. An exception to this was the N-limited pine stand at Harvard Forest, where predictions of fluxes deviated by 380 % form field measurements. This difference is most likely due to a missing mechanism in PnET-N-DNDC describing uptake of atmospheric N sub 2 O by soils. PnET-N-DNDC was also validated for its capability to predict NO emission from soils. Predicted and measured mean NO fluxes at three different field sites agreed within a range of plus/minus 13 %. The correlation between modeled and predicted NO emissions from the spruce and beech stand at the Höglwald Forest was r high 2 is equal 0.24 (spruce) and r high 2 is equal 0.35 (beech), respectively. The results obtained from both sensitivity analyses and validations with field data from temperate forest soils indicate that PnET-N-DNDC can be successfully used to predict N sub 2 O and NO emissions from a broad range of temperate forest sites.

To predict emissions of nitrous oxide (N2O) and nitric oxide (NO) from forest soils, we have developed a process-oriented model by integrating several new features with three existing models, PnET, Denitrification-Decomposition (DNDC), and a nitrification model. In the new model, two components were established to predict (1) the effects of ecological drivers (e. g., climate, soil, vegetation, and anthropogenic activity) on soil environmental factors (e. g., temperature, moisture, pH, redox potential, and substrates concentrations), and (2) effects of the soil environmental factors on the biochemical or geochemical reactions which govern NO and N2O production and consumption. The first component consists of three submodels for predicting soil climate, forest growth, and turnover of soil organic matter. The second component contains two submodels for nitrification and denitrification. A kinetic scheme, a so-called "anaerobic balloon," was developed to calculate the anaerobic status of the soil and divide the soil into aerobic and anaerobic fractions. Nitrification is only allowed to occur in the aerobic fraction, while denitrification occurs only in the anaerobic fraction. The size of the anaerobic balloon is defined by the simulated oxygen partial pressure which is calculated based on oxygen diffusion and consumption rates in the soil. As the balloon swells or shrinks, the model dynamically allocates substrates (e. g., dissolved organic carbon, ammonium, nitrate, etc.) into the aerobic and anaerobic fractions. With this approach, the model is able to predict both nitrification and denitrification in the same soil at the same time. This feature is important for soils where aerobic and anaerobic microsites often exist simultaneously. With the kinetic framework as well as its interacting functions, the PnET-N-DNDC model links ecological drivers to trace gas emissions. Tests for validating the new model are published in a companion paper.