Aapa mires as a carbon sink and source during the Holocene

1. The vertical and horizontal dynamics of peat carbon accumulation in Ruosuo aapa mire, north-central Finland, were investigated in an attempt to understand variations in carbon accumulation over time. 2. Paludification began around 8700 BP (9600 cal BP). The long-term net carbon accumulation rate, based on vertical and lateral growth, was 8.0 g C m(-2) year(-1). Variations in carbon accumulation rates can be explained to a great extent by natural succession and variations in local conditions, but the role of climate cannot be ignored. 3. The marked decline in carbon accumulation rates after 6100 BP (7000 cal BP) may indicate a relatively dry and warm climate. The levelling out and subsequent increase in carbon accumulation rates after 4000 BP (4500 cal BP) indicates the development of a cooler and moister climate. 4. The low long-term net carbon accumulation rate and the high carbon content per unit volume in the layer underlying the surficial peat is typical for a wet aapa mire. The continuous penetration of roots, mostly sedge, results in carbon input into the underlying peat layer and contributes to the intensive decay of the surficial peat due to high biological activity. 5. Although one-third of the apparent carbon has accumulated during the last 2000 years, most of Ruosuo is approaching the stage where primary production does not adequately compensate for overall carbon loss. The declining net carbon accumulation rates from the base up to the depth of 80 cm may indicate a real delayed trend in net carbon accumulation rates. 6. We suggest that under the present climatic conditions the northern circumpolar wet mires are significant sources of greenhouse gases. In addition to high methane production, northern aapa mires also have lower carbon accumulation rates than more southern raised bogs. 7. Our results, which are based on horizontal and vertical carbon accumulation rates, suggest that the earlier published carbon accumulation rates in Finnish aapa mires are overestimated.