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 (N2O and NO) in forest ecosystems was test ed for its sensitivity to changes in environmental factors (e.g., temperatu re, precipitation, solar radiation, atmospheric N-deposition, soil characte ristics). Sensitivity analyses revealed that predicted N-cycling and N-trac e gas emissions varied within measured ranges. For model validation, data s ets of N-trace gas emissions from seven different temperate forest ecosyste ms in the United States, Denmark, Austria, and Germany were used. Simulatio ns of N2O emissions revealed that field observations and model predictions agreed well for both flux magnitude and its seasonal pattern. Differences b etween predicted and measured mean N2O fluxes were <27%. An exception to th is was the N-limited pine stand at Harvard Forest, where predictions of flu xes deviated by 380% from field measurements. This difference is most likel y due to a missing mechanism in PnET-N-DNDC describing uptake of atmospheri c N2O by soils. PnET-N-DNDC was also validated for its capability to predic t NO emission from soils. Predicted and measured mean NO fluxes at three di fferent field sites agreed within a range of +/-13%. The correlation betwee n modeled and predicted NO emissions from the spruce and beech stand at the Hoglwald Forest was r(2) = 0.24 (spruce) and r(2) = 0.35 (beech), respecti vely. The results obtained from both sensitivity analyses and validations w ith field data sets from temperate forest soils indicate that PnET-N-DNDC c an be successfully used to predict N2O and NO emissions from a broad range of temperate forest sites.