Natural and anthropogenic acidification of peatlands

Peatlands are ubiquitous in northern landscapes, and decomposition of their plant remains produces complex, colored organic acids that acidify their waters and those of the steams and lakes into which they drain. Fens with weakly acid surface waters (pH about 6) and low alkalinity (about 40 ueg/L) are vulnerable to rapid change, and may be acidified by invasion of carpet-forming Sphagnum mosses that bring about major alterations in their biotic communities. The plant communities of such fens include a mixture of species characteristic of both minerotrophic and ombrotrophic peatlands. Because mosses exhibit widely differing pH tolerances, stratigraphic examination of their remains in peat profiles (coupled with dating by various techniques) can reveal anthropogenic and natural acidification of peatlands. Decreasing concentrations of metals upward in peat profiles indicate concurrent impoverishment of lithophile elements (calcium, iron, etc.). Acid deposition falling upon peatlands is largely neutralized (except where unusually heavy) by plant uptake and -- beneath the water table -- by microbial reduction of associated nitrate and sulfate. Whether fen peats above the water table can be leached sufficiently by acid deposition to initiate or accelerate invasion by Sphagnum and consequent acidification remains to be seen, but is to be expected at least under exceptionally severe conditions of acid loading.