ACCUMULATION AND RELEASE OF ORGANIC-MATTER IN OMBROTROPHIC BOG HUMMOCKS - PROCESSES AND REGIONAL VARIATION

Dry bulk density (BD) and concentrations of N and C in the organic matter have been studied in the top layers (0-40 cm) of Sphagnum-dominated hummocks on ombrotrophic bogs selected to form three gradients from hyperoceanic to weakly oceanic conditions in areas of different latitude in NW Europe and North America. In such hummocks N is conserved during the decay process and can, therefore, be used as an innate marker to calculate mass balances and the rate of the decay process and also to establish an approximate time scale. The variation in BD and N by depth in the living moss layer is explained by the growth pattern of the mosses, and the litter formed at the bottom of the layer (the litter depositon level, LDL) becomes depleted in N. Further down in the litter and peat layers the variation in BD and N is explained by the decomposition and the subsequent losses of C and disintergration of the matrix of the organic matter in the acrotelm. Below the LDL the concentration of N increases with depth down to the decay decrease level (DDL) as does BD down to the compaction decline level (CDL), a few cm below the DDL. In both cases the increase can be described by an exponential function of the cumulative amount of N and cumulative weigth of organic matter, respectively. The increase in BD dependes more on the matrix of organic matter losing resistance than on the load of the overlying peat. The DDL is regarded as the limit between the two functional layers, which are the acrotelm and catotelm. Below the DDL and CDL, the irregular variation in N and BD indicates that the processes of decay and compression have become interrupted at various stages. The annual transfer of organic matter to the litter layer at the LDL is about ten times greater on the bogs in the hyperoceanic, temperate regions, than in the sub-arctic region. However, the depth of the DDL and amount of organic matter contained above it does not correlate with the degree of oceanity. The age of the organic matter at DDL (range 40->400 yr) is less in the hyperoceanic areas than in the less oceanic areas at the same latitude. However, higher decay rates on the bogs in the hyperoceanic areas offset the higher transfer of organic matter at the LDL. As a result, the final accumulation of slowly decaying organic material in the catotelm does not correlate with the degree of oceanity but might, instead, depend more on the hydrology of the bog.