Modelling the impact of acid deposition and nutrient cycling on forest soils

The long-term (60-yr) impact of a reducing atmospheric deposition scenario on the soil and soil solution chemistry of a representative acid forest soil in the Netherlands was evaluated using RESAM (Regional Soil Acidification Model), a process-oriented soil acidification model. The model simulates the major biogeochemical processes occurring in the forest canopy, litter layer and mineral horizons including canopy interactions, element cycling processes, nitrogen transformation processes, and geochemical weathering and exchange reactions. The deposition scenario used was based on expected policy measures in the Netherlands. h inputs of S and N, model results showed (i) a dominant role of (N transformations by) mineralization, root uptake and nitrification and of Al mobilization in the uppermost soil layers on the proton budget, (ii) tracer behaviour of SO2−4 and retention of N, (iii) a strong relationship between leaching of Al3+ and that of SO2−4 plus NO−3 and (iv) a dominant role of Al hydroxide dissolution in Al mobilization, which are all in agreement with field and/or laboratory measurements. At reduced deposition levels RESAM predicted (i) an inversion from net N retention to net N mobilization followed by net N retention again, (ii) a strong decrease in Al hydroxide dissolution and (iii) a relative fast de-acidification of the soil, reflected by an increase in pH and base saturation and a decrease in Al3+ concentration. The reliability of these predictions is discussed in view of available data.