MODELING LONG-TERM CATION SUPPLY IN ACIDIFIED FOREST STANDS

A dynamic soil chemistry model was used to explain the observed decrea se in soil base saturation between 1949 and 1984 at three stands in so uthern Sweden. The results show that acid deposition has caused soil a cidification. The model, SAFE (Soil Acidification in Forest Ecosystems ), includes the fundamental physical processes such as leaching and ac cumulation, and chemical processes such as cation exchange, mineral we athering, nutrient uptake and solute equilibrium reactions. The source s and sinks of base cations in the soil system were quantified, showin g that weathering, deposition of base cations and depletion of exchang eable base cations supply cations to the soil solution in similar amou nts in the upper 1 m during the acidification phase. This demonstrates that budget studies alone cannot be used to distinguish between long- term capacity to resist acidification, represented by weathering, from short-term buffering caused by cation exchange.