Aapa mires as a carbon sink and source during the Holocene

1The vertical and horizontal dynamics of peat carbon accumulation in Ruosuo aapa mire, north-central Finland, were investigated in an attempt to under stand variations in carbon accumulation over time. 2 Paludification began around 8700 BP (9600 cal BP). The long-term net carb on 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 B P) may indicate a relatively dry and warm climate. The levelling out and su bsequent 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 conten t per unit volume in the layer underlying the surficial peat is typical for a wet aapa mire. The continuous penetration of roots, mostly sedge, result s in carbon input into the underlying peat layer and contributes to the int ensive 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 producti on does not adequately compensate for overall carbon loss. The declining ne t carbon accumulation rates from the base up to the depth of 80 cm may indi cate a real delayed trend in net carbon accumulation rates. 6 We suggest that under the present climatic conditions the northern circum polar wet mires are significant sources of greenhouse gases. In addition to high methane production, northern aapa mires also have lower carbon accumu lation rates than more southern raised bogs. 7 Our results, which are based on horizontal and vertical carbon accumulati on rates, suggest that the earlier published carbon accumulation rates in F innish aapa mires are overestimated.