MODELING THE IMPACT OF ACID DEPOSITION AND NUTRIENT CYCLING ON FORESTSOILS

The long-term (60-yr) impact of a reducing atmospheric deposition scen ario on the soil and soil solution chemistry of a representative acid forest soil in the Netherlands was evaluated using RESAM (Regional Soi l Acidification Model), a process-oriented soil acidification model. T he model simulates the major biogeochemical processes occurring in the forest canopy, litter layer and mineral horizons including canopy int eractions, element cycling processes, nitrogen transformation processe s, and geochemical weathering and exchange reactions. The deposition s cenario used was based on expected policy measures in the Netherlands. At high inputs of S and N, model results showed (i) a dominant role o f (N transformations by) mineralization, root uptake and nitrification and of Al mobilization in the uppermost soil layers on the proton bud get, (ii) tracer behaviour of SO42- and retention of N, (iii) a strong relationship between leaching of Al3+ and that of SO42-,2- plus N O-3 (-) and (iv) a dominant role of AI hydroxide dissolution in Al mobiliz ation, which are an in agreement with field and/or laboratory measurem ents. At reduced deposition levels RESAM predicted (i) an inversion fr om net N retention to net N mobilization followed by net N retention a gain, (ii) a strong decrease in Al hydroxide dissolution and (iii) a r elative fast de-acidification of the soil, reflected by an increase in pH and base saturation and a decrease in Al3+ concentration. The reli ability of these predictions is discussed in view of available data.