Modeling Time Patterns of Forest Soil Acidification for Various Deposition Scenarios

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. Major model outputs include base saturation, pH and the Al/BC ratio, where BC stands for divalent base cations. The processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and aluminum hydroxides, cation exchange and CO2 equilibria. With the exception of silicate weathering, all processes are described by equilibrium reactions. Model parameters mainly refer to equilibrium constants for the various processes, while model variables include the amounts of base cations and aluminum in carbonates, hydroxides, the exchange complex and in soil solution. The model behavior in the different buffer ranges between pH 7 and pH 3 has been evaluated by analyzing the response of an initially calcareous soil to a constant high acid load (5000 molc ha−1 yr−1) over a period of 500 years. In calcareous soils, weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. In non-calcareous soils the response in the range between pH 7 and pH 4 mainly depends on the initial amount of exchangeable base cations. Although the model cannot be strictly verified for its ultimate use of long-term predictions, it can be used as a tool to obtain a conceptual understanding of long-term soil responses for the broad concern of resource managers and policy makers. Therefore it will be linked to the Regional Acidification Information and Simulation model (RAINS) to analyze the impact of various emission scenarios on a European scale.