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 o rganic (Oh) and mineral (Bs) horizons of a well developed podzol were regularly sampled at a moorland catchment in Mid-Wales. Samples were a nalysed for pH, major cations, major anions, and dissolved organic car bon (DOC). Acid neutralizing capacity (ANC) was estimated by the charg e balance method. Average monthly ANC of soil solutions from the Oh ho rizon varied seasonally, with a maximum in July and a minimum in Febru ary. 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) co ncentrations led to release of H+ through ion exchange. ANC declined i n the absence of any buffering mechanism. In summer, the depletion of exchangeable acidity that occurred in winter, was replenished by H+ pr oduced by the dissociation of organic acids. During this period, organ ic anions contribute to an increase in ANC, while H+ concentrations re mained similar to those in winter. These processes probably influenced the acidity and ANC of Bs horizon soil solutions but to a lesser exte nt than in the Oh horizon. Other mechanisms such as weathering and ion exchange involving H+ and Al may buffer solution acidity in the miner al soil.