MODELING THE EFFECTS OF ACID DEPOSITION ON THE BIOGEOCHEMISTRY OF THEHOEGLWALD SPRUCE STAND, FRG

The intensively monitored Hoeglwald spruce site in southern Bavaria, G ermany is located in a region with high atmospheric inputs of acidifyi ng substances. In order to interpret the experimental data and to perf orm model validation, the biogeochemical model SAFE was parameterized for the site, SAFE is a dynamic, multilayer model that includes chemic al weathering, cation exchange and soil solution equilibria. Reconstru cted historic deposition and nutrient uptake and cycling patterns were made. To predict the general chemical characteristics of the upper pa rt of the soil profile it was necessary to introduce two important ele ments in the parameterization of the model, gaseous uptake of nitrogen , and to assign a base cation uptake distribution different from fine root distribution. The model predicted the general characteristics of soil solution chemistry. Soil solution pH was well predicted for the m ineral soil, but underpredicted for the O layer, due to the model assu mption that nitrification is complete. The model could not reproduce h igh fluxes of SO42-, Al and acidity in the A layer, while modeled base d cation fluxes agree with data, since SAFE does not include SO42- dyn amics that could explain the mobilization in the A layer, or immobiliz ation in the A/B layer. Fluxes and concentrations of SO42-, NO3-, Al a nd base cations at the 40 cm level (A/B horizon) were all well reprodu ced by the model, The modeled decreasing trends in base cation concent rations are opposite to the increase shown by the data, The model clea rly shows the importance of nutrient cycling for the soil chemistry in rooting zone, The SAFE model suggests that the Hoeglwald spruce stand has been subject to severe acidification due to acid inputs, and that the Hoeglwald spruce stand is showing signs of recovery from acidific ation, At present, model uncertainties may be too large to draw defini te conclusions regarding recovery rate. (C) 1998 Elsevier Science B.V.