The seasonal variation in soil water acid neutralizing capacity in peaty podzols in Mid-Wales

Between 1985 and 1990, bulk precipitation and soil solution from the organic (Oh) and mineral (Bs) horizons of a well developed podzol were regularly sampled at a moorland catchment in Mid-Wales. Samples were analysed for pH, major cations, major anions, and dissolved organic carbon (DOC). Acid neutralizing capacity (ANC) was estimated by the charge balance method. Average monthly ANC of soil solutions from the Oh horizon varied seasonally, with a maximum in July and a minimum in February. In contrast, H⁺ concentrations varied little. Solute deposition, dominated by sodium and chloride, also varied seasonally with a winter maximum, which is reflected in the soil solution chemical composition. In the Oh horizon during winter, the increase in base cation (Na) concentrations led to release of H⁺ through ion exchange. ANC declined in the absence of any buffering mechanism. In summer, the depletion of exchangeable acidity that occurred in winter, was replenished by H⁺ produced by the dissociation of organic acids. During this period, organic anions contribute to an increase in ANC, while H⁺ concentrations remained similar to those in winter. These processes probably influenced the acidity and ANC of Bs horizon soil solutions but to a lesser extent than in the Oh horizon. Other mechanisms such as weathering and ion exchange involving H⁺ and Al may buffer solution acidity in the mineral soil.