Significance of DOM in the translocation of cations and acidity in acid forest soils

We estimated the contribution of dissolved organic matter (DOM) to cation leaching and the translocation of acidity in three acid forest soils. The analysis was based on monitored (2 years) concentrations of dissolved organic carbon (DOC) in the field, measured total acidities of DOM, and measured as well as predicted weighted mean dissociation constants of the organic acids. Although the forest floor solutions were strongly acidic (pH 3.47-4.10), a considerable proportion of the organic acids were dissociated and organic anions represented 22-40% of the total anions in the mineral soil input. The flux of DOM-associated exchangeable protons from the forest floor to the mineral soil ranged from 0.35 (Wulfersreuth) to 3.72 (Hohe Matzen) kmol ha(-1) yr(-1). In the subsoil, this organic acidity may be neutralized by microbial decomposition of the organic acids, but a part of the hydrogen ions may dissociate and contribute to acidification of the soil solution and to weathering processes. Due to the pronounced retention of DOM in the mineral subsoil horizons, the contribution of DOM to the output of cations and acidity from the soil is much lower than in the surface horizons but still significant.