BIOGEOCHEMICAL MODELING OF EXMAN RESEARCH SITES - A COMPARISON

The Experimental Manipulation of Forest Ecosystems in Europe (EXMAN) p roject was conducted to assess the effects caused by atmospheric depos ition on forest ecosystems. The dynamic biogeochemical model, Soil Aci dification in Forest Ecosystems (SAFE), was used to simulate the soil and soil solution chemistry at Ballyhooly (IR), Hoglwald (DE), Kloster hede (DK) and Selling (DE) from 1800 to 2000. The model considers soil processes (cation exchange, weathering, solution equilibria) and exte rnal forcing function (deposition, hydrology, nutrient uptake). The mo del was successfully calibrated to all EXMAN sites. Base cation budget s were evaluated, and they demonstrate the importance of nutrient cycl ing for the element fluxes in the rooting zone. The vital stands Bally hooly and Hoglwald have higher litterfall rates and the lowest canopy leaching rates than Klosterhede and Selling. SAFE suggests that the so ils at Hb;Hoglwald and Ballyhooly are still being depleted of base cat ions, while Klosterhede and Selling are close to a steady-state or sho w slight recovery. Comparison with soil solution data show that SAFE r eproduces the pH in solution in the most soil layers. One exception wa s the O-layer in Hoglwald, where underpredicted pH resulted from overe stimation of nitrification. Base cation concentrations were well repro duced for all layers for all sites. SAFE predicts that base cation con centrations at all sites have reached a peak, and will decline. Calcul ated aluminium concentrations were within the range of the observation s. The model shows that nutrient cycling dominates O-layer chemistry. N-uptake appears to follow the fine root biomass distribution. To repr oduce base cation fluxes correctly, base cation uptake had to be alloc ated to lower soil layers. Al-solubility was similar for comparable si tes. The effects of shortcoming in the SO42- chemistry of SAFE on mode l predictions was discussed. The model predicted the major effects of the 'clean rain' manipulation on soil solution at Selling, decreased A l and base cation concentrations and increased pH. The model applicati on to the Klosterhede manipulation was difficult to evaluate due to th e temporal variability in experimental data, caused by the marine infl uence. (C) 1998 Elsevier Science B.V.