[en] Release of CO₂ from peat was studied using IR analyzer in a range of boreal peatlands under varying nutrient status and moisture conditions. Root associated CO₂ efflux was separated from the total release by experiments both in the field and in a greenhouse. Emissions of CO₂ and CH₄ (the latter by gas chromatography) were measured during the snow-covered period and their contribution to the annual fluxes of these gases was inspected. Ecosystem exchange of CO₂ under varying irradiation, temperature and moisture conditions was measured at different microsites at two peatland sites with different nutrient ecology. One site represented minerotrophic conditions during a wet growing season and the other site ombrotrophic conditions during an exceptionally dry growing season. Annual carbon balances were compiled for the two sites, and the role of the microsites in the annual carbon balance and CH₄ release was studied. The Holocene history of CO₂ sequestration and CH₄ emission dynamics in a raised mire were simulated using lateral and vertical growth rates derived from radiocarbon ages of peat samples from mire bottom and vertical cores. The model was formulated for a geographic information system (GIS). Artificial or natural lowering of water table increased CO₂ release from peat. A drought lasting from late May to July caused a 90 g C m² net loss in the annual C balance of a natural ombrotrophic bog. In drained forested sites the increase in peat CO₂ release could be even 100 %, but the development of the tree layer at least partially compensated for these losses. Wet conditions induced a net accumulation of 67 g C m^-2a^-1 in the minerotrophic fen site, while the long term average accumulation rate is estimated to be only 15 g C m^-2a^-1 for Finnish fens. Carbon balance in boreal peatlands is thus extremely sensitive to year-to-year climatic variations. Root activity of vascular plants contributed to the total peat CO₂ efflux by 10-40 % as root respiration and root associated heterotrophic CO₂ release. Much of the spatial variability in the gas fluxes was attributed to the microsite properties in natural peatlands. Winter CO₂ and CH₄ emissions were important components in the C balance, comprising 10Ae30 % of the annual gas release from peat. According to the simulation results, the CH₄ release from expanding peatlands could have contributed to the early interglacial atmospheric warming during several millennia, at least prior to the ombrotrophication and increased peat accumulation from about 3500 years BP onwards. The atmospheric cooling effect by peat accumulation is less clear. (orig.)