SIMULATION OF THE LONG-TERM SOIL RESPONSE TO ACID DEPOSITION IN VARIOUS BUFFER RANGES W. DE VRIES The Winand Staring Centre for Integrated Land, Soil and Water Research, P.O. Box 125, NL-6700 AC Wageningen, The Netherlands M. POSCH International Institute for App#ed Systems Analysis, A-2361 Laxenburg, Austria and J. KAMARI Water and Environment Research Institute, P.O. Box 250, SF-O0101 Helsinkg Finland (Received April 28, 1989; revised August 9, 1989) Abstract. A soil acidification model has been developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. Its major outputs include base saturation, pH and the molar A1/BC ratio, where BC stands for divalent base cations. Apart from net uptake and net immobilization of N, the processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and A1 oxides and hydroxides, cation exchange and CO 2 equilibriums. First, the model's behavior in the different buffer ranges between pH 7 and pH 3 is evaluated by analyzing the response of an initially calcareous soil of 50 cm depth to a constant high acid load (5000 molc ha -t yr -1) over a period of 500 yr. In calcareous soils weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. Results indicate a time lag of about 100 yr for each percent CaCO3 before the pH starts to drop. In non-calcareous soils the response in the range between pH 7 and 4 mainly depends on the initial amount of exchangeable base cations. A decrease in base saturation by H/BC exchange and A1/BC exchange following dissolution of AP + leads to a strong increase in the A1/BC ratio near pH 4. A further decrease in pH to values near 3.0 does occur when the A1 oxides and/or hydroxides are exhausted. The analyses show that this could occur in acid soils within several decades. The buffer mechanisms in the various pH ranges are discussed in relation to Ulrich's concept of buffer ranges. Secondly, the impact of various deposition scenarios on non-calcareous soils is analyzed for a time period of 100 yr. The results indicate that the time lag between reductions in deposition and a decrease in the A1/BC ratio is short. However, substantial reductions up to a final deposition level of 1000 molc ha 1 yr-1 are needed to get A1/BC ratios below a critical value of 1.0. 1. Introduction Forest canopies can be affected directly by the deposition of 802, NO x and NH 3 but forest ecosystems are also influenced by indirect, soil-mediated effects on the roots. The most notable effect is the inhibition of nutrient uptake (especially of Ca and Mg) either by mobilization of A1 (acidification) or by accumulation of ammonium (eutrophication), which leads to unfavorable ratios of these compounds to base cations (Ulrich and Matzner, 1983; Roelofs et al., 1985). As to acidification, the development of unfavorable ratios of A1 to divalent base Water, Air, and Soil Pollution 48: 349-390, 1989. © 1989 Kluwer Academic Publishers. Printed in the Netherlands.