Is a universal model of organic acidity possible:: Comparison of the acid/base properties of dissolved organic carbon in the boreal and temperate zones

The acid/base properties of dissolved organic carbon (DOC) are an important feature of soil and surface waters. Large differences in the acid/base properties of DOC found by different studies might be interpreted as spatial and temporal differences in these properties. Different analytical techniques, however, may explain some of the differences. We used a combination of ion-exchange techniques, titration, and surface water chemistry data to evaluate DOC character from two substantially different areas-the relatively pristine boreal zone of Sweden and the heavily acidified temperate zone of the Czech Republic. We found a significantly higher site density (amount of carboxylic groups per milligram of DOC) for the Swedish sites (10.2 muequiv/mg of DOC +/- 0.6) as compared to the Czech sites (8.8 muequiv/mg of DOC +/- 0.5). This suggests a slightly higher buffering capacity for Swedish DOC. A triprotic model of a type commonly incorporated in biogeochemical models was used for estimating the DOC dissociation properties. For Swedish sites, the following constants were calibrated: pK(a1) = 3.04, pK(a2) = 4.51, and pK(a3) = 6.46, while the constants for Czech sites were pK(a1) = 2.5, pK(a2) = 4.42, and pK(a3) = 6.7. Despite differences in site density values, both models predict very similar dissociation and thus pH buffering by DOC in the environmentally important pH range of 3.5-5.0. This can be incorporated into models to make reliable estimates of the effect of organic acids on pH and buffering in different regions.