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 o n the acid-base balance of any ecosystem. Because of the absence of mi neral soils, ion exchange and accretion in biomass are the major inter nal processes governing cation accumulation in peatlands. It is widely believed that peatlands have a large capacity to store cations, and t hus to affect the acidity of surface waters. A long-term record of hyd rologic and chemical inputs and outputs to the S-2 peatland in the Mar cell 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 ov er the period 1971-1984; wet-only and dry bucket measurements do not s how 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. Neverthe less, ion exchange represents a relatively minor source of acidity to this peatland, and accumulation of cations in woody biomass accounts f or the majority of the cation retention. The major mechanism of retent ion in the peatland is different for each cation; accumulation in wood y biomass is most important for Ca, storage on ion exchange sites in p eat is most important for Mg, green plant tissues are the dominant sit e of K storage, and Na is bound by unknown mechanisms in the peat. Ret ention efficiencies show large annual variations but average 50% for C a, 22% for Mg, 29-44% for Na, and 21% for K.