Mass balance and nitrogen accumulation in hummocks on a South Swedish bog during the late Holocene

Two peat cores from the Store Mosse mire in the central part of South Sweden have been analyzed for dry bulk density, carbon, and nitrogen. They cover the development of the peat mound from the time of the conversion of the initial fen to an ombrotrophic bog at 5450 BP through three different bog stages, the Fuscum, the Rubellum-Fuscum and the Magellanicum bog stages, each one characterized by a specific macrofossil assemblage. All N supplied to the bog surface is assumed to be contained in the organic matter. At the beginning of the Magellanicum bog stage, 1000 BP, the nitrogen accumulation rate increased from an earlier value of ca 0.4 g m(-2) yr(-1) to 0.8 g m(-2) yr(-1). These accumulation rates for N have been used to establish time scales for the periods between the C-14-datings. The estimated litter deposition rate in the hummocks is 120 g m(-2) yr(-1) in the two older bog stages and 270 g m(-2) yr(-1) in the Magellanicum bog stage. The decay losses in the acrotelm increased, as a proportion of the addition, with time through each one of the bog stages. The rang of variation in the cores for the acrotelm decay losses was 25-80%, and the annual input of organic matter to the catotelm, 30-130 g m(-2). These ranges are greater than those found among recent bog hummocks in NW Europe and North America. The decay losses during 5000 yr in the catotelm may not have exceeded 20% of the original input. The over-all net rate of accumulation of C was highest, ca 40 g m(-2) yr(-1), at the beginning of the Fuscum bog stage. The changes in the macrofossil assemblages are all associated with rapid increases in the peat accumulation rate, but decreases in accumulation rate are not. At the conversion from fen to bog the increased input of organic matter to the catotelm depended on expansion of Sphagnum fuscum which formed a decay resistant litter. Later increases depended on rapid rises of the mean water table, resulting in shorter residence times and smaller decay lasses from the acrotelm. The periods of decreases in input of organic matter to the catotelm depended on longer residence times in the acrotelm when the water table fell relative to the bog surface. However, comparisons with recent conditions suggest that the variation in mean water level relative to the surface may not have exceeded 10-15 cm.