Pnet model

The model used here is relatively simple, but the results make sense given our understanding of controls on decomposition and mineralization in ecosystems. Also, more complex models yield similar results. Year-to-year variations in NPP in Century are controlled by interactions between precipitation and precipitation-induced variations in N mineralization (Burke et al, 1997) these year-to-year variations can drive losses of N even when it limits NPP in most years. The Pnet model also predicts year-to-year variations in nitrate leaching from deciduous forest watersheds as a consectuence of variations in precipitation these predictions are strongly supported by watershed-level observations (Aber and Driscoll, 1997). This mechanism could help to explain observations that the N cycle appears to be more open in semiarid areas than in mesic forest ecosystems, in the sense that both inputs and outputs of N are larger relative to N pools within ecosystems (Austin and Vitousek, 1998). 

Gbondo-Tugbawa SS, Driscoll CT, Aber JD, Likens GE. 2001. Evaluation of an integrated biogeoctiemical model (PnET-BGC) at a northern hardwood forest ecosystem. Water Resour Res 37 1057-1070. 

J. (1999) Application of the forest-soil-water model (PnET-BGC/CHESS) to the Lysina catchment, Czech Republic. Ecol. Model. 120, 9-30. 

We used the model PnET-BGC (Gbondo-Tugbawa et al. 2001) to compare current emissions reductions required by the 1990 CAAA with an additional 55% and 75% cut in emissions of sulfur dioxide, and 20% and 30% decreases in nitrogen oxides by 2010. These scenarios are based on the electric utility emission reductions embodied in bills recently introduced to the U.S. Congress. PnET-BGC considered changes in sulfur dioxide and nitrogen oxide emissions. It was assumed that base cation and ammonium deposition and climate would remain unchanged. 

Scientists have developed computer models that depict the physical, chemical and biological processes within forest watersheds. Watershed acidification models can be used as research and management tools to investigate factors responsible for the historical acidification of soil and water as well as the ecosystem response to anticipated future changes in acidic deposition. In order to effectively predict the pH, ANC and aluminum concentrations in streams, all major chemicals must be accurately simulated (e.g., sulfate, nitrate, calcium, magnesium). The acidification model PnET-BGC was used for this assessment because it has been rigorously tested at Hubbard Brook and other sites in the northeastern United States, and it allows the user of the model to consider the ecosystem response to multiple chemicals simultaneously. Other frequently used acidification models include MAGIC (Cosby et al. 2001), and NuCM (Lui et al. 1992). 

Figure 3.10. Time series of predictions with the acidification model PnET-BGC of changes in stream chemistry at Hubbard Brook to changes in past and potential future emissions of sulfur dioxide and nitrogen oxides, including the 1990 Amendments of the Clean Air Act and moderate and aggressive emission control scenarios. Shown are model-predicted stream concentrations of sulfate, nitrate, acid neutralizing capacity, pH and dissolved inorganic aluminum, and soil percent base saturation. Measured values are indicated for comparison...

Time series of predictions with the acidification model PnET-BGC of... 

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