Use of a coupled equilibrium model to describe the buffering of protons and hydroxyl ions in some acid soils

The buffering of protons and hydroxyl ions in acid soils was studied by the addition of small amounts of HCl, H₂SO₄, and NaOH in consecutive batch experiments using surface soils and subsoils from two Cambisols and one Podzol. A chemical equilibrium model was used to study the main buffer processes. The model included inorganic complexation and multiple cation exchange, and also the solubility of jurbanite and Al(OH)₃ for the subsoils. Buffering of protons was predicted quite well by the model for the surface soil of the Spodi-Dystric and Spodic Cambisols, suggesting that multiple cation exchange was the main buffer process. For the Podzol surface soil, however, the model overestimated proton buffering by cation exchange considerably. Hydroxyl buffering in acid surface soils could be described well by the model for the Podzol soil only. For the Cambisols, hydroxyl buffer reactions included not only cation exchange, but also solubilization of large amounts of organic matter and presumably deprotonation of dissolved organic carbon (DOC). Modelling proton and hydroxyl buffering in subsoils suggested that equilibrium with AJ(OH)₃ was not maintained for the Podzol and spodic Cambisol. Sulphate sorption had to be considered to describe titration experiments in all three soils. The assumption of jurbanite being in equilibrium with soil extracts was useful only for the Spodi-Dystric Cambisol.