Suppression of dissolved organic carbon by sulfate induced acidification during simulated droughts.

The relationship between dissolved organic carbon (DOC) and the acidification of soils and freshwaters by sulfate (SO4(2-)) has been a topic of great debate over the last few decades. Most interest has focused on long-term acidification. Few have considered the influence of episodic drought-induced acidification in peatlands on DOC mobility, even through the increased acidity and ionic strength associated with the oxidation of reduced sulfur to SO4(2-) are known to reduce DOC solubility. Reduced DOC concentrations during droughts have often been attributed to: (i) reduced hydrological export; (ii) physicochemical changes in the peat structure; or (iii) changes in the biological production and/or consumption of DOC. Our experimental drought simulations on peat cores showed that SO4(2-) induced acidification reduced DOC concentrations during droughts. However, the relationships between SO4(2-)/pH/ ionic strength and DOC were only apparent when the reductions in observed DOC were expressed as a fraction of the estimated DOC concentration in the absence of SO4(2-), which were derived from soil depth, temperature, and watertable data. This analysis showed that a pH fall from 4.3 to 3.5, due to a SO4(2-) rise from < 2.5 to 35 mg L(-1), caused a 60% reduction in DOC concentrations. In contrast, poor correlations were recorded between S042-/pH/ionic strength and the observed DOC data. As DOC both influences acidity and is influenced by acidity, the relative change in DOC needed to be considered to disentangle the effect of inputs of mineral acids into a system naturally dominated by variable concentrations of organic acids.