Response of a forested catchment over the last 25 years to past acid deposition assessed by biogeochemical cycle modeling (Strengbach, France)

A numerical coupling of a dynamic biogeochemical model (B-WITCH) and a model of forest balance water (BILJOU) was used to simulate the concentration of major species within soil profiles from two contrasted ecosystems: beech and spruce stands located in the Vosges Mountains and their evolution during decades. This coupled modelling allows an important vertical discretization of water and cation cycle and a direct comparison with the hydrological and geochemical data available over twenty-five years and on a seasonal basis. The processes controlling the past evolution of soil solution chemistry have been identified and tested. The biogeochemical cycles of cations estimated on the two sites shows that the biological fluxes control up to 70% (for Ca and Mg) and up to 95% (K) of the chemical composition of the soil solutions. The Ca and Mg concentrations decrease over the last decades, which can be explained by the evolution of atmospheric inputs and by the behavior of the exchange complex. This paper highlights the weak contribution of mineral dissolution and the key role of biological recycling and cation exchange processes in the soil solution signatures. The future sustainability of forest, in mountainous environment on base poor bedrock, depends strongly on the forest management and evolution of exchangeable pool nature.