Retention and mobility of cations in a small peatland: trends and mechanisms

Inputs and net storage or release of cations can have a large impact on the acid-base balance of any ecosystem. Because of the absence of mineral soils, ion exchange and accretion in biomass are the major internal processes governing cation accumulation in peatlands. It is widely believed that peatlands have a large capacity to store cations, and thus to affect the acidity of surface waters. A long-term record of hydrologic and chemical inputs and outputs to the S-2 peatland in the Marcell Experimental Forest was used to construct annual and monthly mass balances for the four major cations. Bulk deposition and throughfall measurements suggest that deposition of Na, Ca and Mg has increased over the period 1971–1984; wet-only and dry bucket measurements do not show such a change. Seasonal and annual variations in cation inputs are not visible in the cation export from the peatland because the large cation reservoir on exchange sites in peat buffers the surface waters and prevents them from responding rapidly to changing inputs. Nevertheless, ion exchange represents a relatively minor source of acidity to this peatland, and accumulation of cations in woody biomass accounts for the majority of the cation retention. The major mechanism of retention in the peatland is different for each cation; accumulation in woody biomass is most important for Ca, storage on ion exchange sites in peat is most important for Mg, green plant tissues are the dominant site of K storage, and Na is bound by unknown mechanisms in the peat. Retention efficiencies show large annual variations but average 50% for Ca, 22% for Mg, 29–44% for Na, and 21% for K.