SIMULATION OF THE LONG-TERM IMPACT OF ATMOSPHERIC DEPOSITION ON DUNE ECOSYSTEMS IN THE NETHERLANDS

1. The long-term impact of present levels of acid deposition on sandy soils in dune areas was simulated for calcareous and non-calcareous (coastal) dune ecosystems in the Netherlands for a 100-year period. 2. Simulations were made with a process-orientated Regional Soil Acidification Model (RESAM). The model - uses mechanistic descriptions for processes in the vegetation canopy, litter layer and mineral soil horizons which significantly influence the soil chemistry (amounts in minerals and at the adsorption complex) and the soil solution chemistry (ionic concentrations). 3. Simulations indicated that the impact of atmospheric deposition on a calcareous dune soil becomes very important when the calcium carbonate content has fallen below 0-3%. In these soils, the pH in the topsoil (first 10 cm) will decline from about 6-5 to 3.0 over several decades, and acid deposition will become the dominating acidifying mechanism (more than 95%). This is because the weathering rate of silicates is very low and the buffering capacity of the exchange complex and of amorphous Al hydroxides is almost negligible in dune soils. Under natural conditions the pH will not fall below 4. 4. Simulations for a non-calcareous dune soil indicated that present deposition levels cause a depletion of amorphous Al3+ hydroxides, which leads to a pH decline from about 3.5-4.5 to 3.0-3.5 over a depth of more than 50 cm during the simulation period. Evidence is given that this will lead to a decrease in plant species. 5. Finally, various management practices, designed to minimize the effects of acidification, are discussed.