Sphagnum species have cell walls rich in uronic acids that in aqueous solution readily exchange a hydrogen ion for a base cation. The base cations that are in solution in bogs and poor fens are received by the peatland from atmospheric deposition or inflowing water and are always associated with an inorganic anion (HCO-, SO4-, Cl-). When the base cation is exchanged for the organically produced H+, acidity of the peatland waters is produced. This acidity thus originated through the exchange of an inorganic base cation for an H+ produced by Sphagnum growth - hence this is termed inorganic acidity. Inorganic acidity relies on the presence of base cations and can only produce acidity when base cations are present in the pore water to exchange. Inorganic acidity is an extremely powerful process when abundant base cations are present such as in rich fens transitional to poor fens and in poor fens. In bogs, with limited supplies of base cations due to their ombrogenous water supply, inorganic acidity is less important.
Organic material produced by plants is decomposed and carbon mineralized through bacterial and fungal respiration. Under aerobic conditions, bacteria break down long cellulose chains and in doing so eventually produce short-chained molecules that are small enough to be dissolved in the pore waters. This dissolved organic carbon (DOC) may be lost to the peatlands via runoff or may remain suspended in the pore waters for some length of time. These decompositional processes produce acidity through dissociation of humic acids, acidity that is completely produced via organic processes; hence, peatland acidity produced via decompositional processes, and extremely important in ombrotrophic bogs, is termed organic acidity.
Rich fens, with pH above 7.0, also accumulate deep deposits of peat and are well buffered by large inputs of bicarbonate alkalinity. With continued inputs of bicarbonate, rich fens may remain stable for millennia, dominated by brown mosses that have little capacity for inorganic acidification, but strong tolerance for the alkaline peat-land waters. However, as rich fens accumulate peat to depths of several meters, there is the possibility that the active surface layer will become more isolated from the bicarbonate inputs and alkalinity may decrease to the point that some tolerant species of Sphagnum may invade. If Sphagnum species establish, then cation exchange proceeds, acidity increases while alkalinity decreases, and rich fen plant species are replaced by poor fen species tolerating acidic conditions. This acidification of rich fens has been documented in the paleorecord wherein the change from rich fen to poor fen vegetation takes place extremely rapidly, perhaps in the order of 100-300 years. As a result, these transitional rich fen-poor fen communities are short-lived on the landscape and among the most rare of peatland types.
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