A process-oriented model of N2O and NO emissions from forest soils. 2. Sensitivity analysis and validation

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.