Biogeochemical modelling of EXMAN research sites: A comparison

Abstract

The Experimental Manipulation of Forest Ecosystems in Europe (EXMAN) project was conducted to assess the effects caused by atmospheric deposition on forest ecosystems. The dynamic biogeochemical model, Soil Acidification in Forest Ecosystems (SAFE), was used to simulate the soil and soil solution chemistry at Ballyhooly (IR), Höglwald (DE), Klosterhede (DK) and Solling (DE) from 1800 to 2000. The model considers soil processes (cation exchange, weathering, solution equilibria) and external forcing function (deposition, hydrology, nutrient uptake). The model was successfully calibrated to all EXMAN sites. Base cation budgets were evaluated, and they demonstrate the importance of nutrient cycling for the element fluxes in the rooting zone. The vital stands Ballyhooly and Höglwald have higher litterfall rates and the lowest canopy leaching rates than Klosterhede and Solling. SAFE suggests that the soils at Höglwald and Ballyhooly are still being depleted of base cations, while Klosterhede and Solling are close to a steady-state or show slight recovery. Comparison with soil solution data show that SAFE reproduces the pH insolution in the most soil layers. One exception was the O-layer in Höglwald, where underpredicted pH resulted from overestimation of nitrification. Base cation concentrations were well reproduced for all layers for all sites. SAFE predicts that base cation concentrations at all sites have reached a peak, and will decline. Calculated aluminium concentrations were within the range of the observations. The model shows that nutrient cycling dominates O-layer chemistry. N-uptake appears to follow the fine root biomass distribution. To reproduce base cation fluxes correctly, base cation uptake had to be allocated to lower soil layers. Al-solubility was similar for comparable sites. The effects of shortcoming in the SO 4 2− chemistry of SAFE on model predictions was discussed. The model predicted the major effects of the ‘clean rain’ manipulation on soil solution at Solling, decreased Al and base cation concentrations and increased pH. The model application to the Klosterhede manipulation was difficult to evaluate due to the temporal variability in experimental data, caused by the marine influence.