MODELING TRENDS IN SOIL SOLUTION CONCENTRATIONS UNDER 5 FOREST-SOIL COMBINATIONS IN THE NETHERLANDS

The influence of forest and soil properties on soil solution concentra tion and the response of soil solution concentrations upon a reduction in deposition was examined with the dynamic simulation model RESAM. R uns were performed for five different forest-soil combinations in the Netherlands, i.e, for Douglas fir, Scots pine and oak on a Cambic podz ol and for Douglas fir on a Carbic podzol and a Gleyic Arenosol. Predi cted concentrations generally decreased in the direction Douglas fir > Scots pine > oak. This was mainly caused by differences in filtering of dry deposition (decreasing in the direction Douglas fir > Scots pin e > oak) and in precipitation excess (decreasing in the direction oak > Scots pine > Douglas fir). However, concentrations of some elements, especially NH4 and Ca, were strongly influenced by biocycling. The in fluence of the considered non calcareous sandy soil types on the soil solution concentrations was relatively small. The influence was mainly caused by differences in N transformation rates. In the Gleyic Arenos ol the predicted mineralization and nitrification fluxes were relative ly low, whereas the denitrification fluxes were relatively high compar ed to the podzol soils. Concentrations which are not strongly influenc ed by biocycling (SO4, Cl) were somewhat higher in the Gleyic Arenosol due to a lower precipitation excess, Al concentrations decreased in t he direction Carbic podzol > Gleyic Arenosol > Cambic podzol due to a decreasing amount and dissolution rate of Al-(hydr)oxides in that same direction and a lower acidification in the Gleyic Arenosol. In all co nsidered forest-soil combinations, a reduction in deposition level cau sed an increase in pH and a decrease in SO4, NO3, NH4 and Al concentra tions. Differences between the rate of de-acidification under the cons idered tree species occurred due to differences in biocycle which resu lt in differences in the ratio between internal and external acid inpu t. In the Gleyic Arenosol a faster de-acidification took place compare d to the well-drained podzol soils due to a rise in denitrification up on a rise in pH.