La dynamique du carbone dans les tourbières à Sphagnum, de la sphaigne à l'effet de serre

Sphagnum mires are sink ecosystems for carbon, C-fixation by photosynthesis being higher than organic decay. Total primary production is generally lower than 500 g C·m⁻²·yr⁻¹, moss layer contribution reaching sometimes 80% of the whole production. Mean annual accumulation in Sphagnum peatlands peat is about 30 g C·m⁻², which corresponds approximately to 10% of primary production. Accumulation/production ratio increases from fen to bog and varies with vegetation communities. Methane emissions in the atmosphere from mires are about 39 T g·yr⁻¹ (1 Tg = 10¹² g), i.e. 35% of the whole contribution of wetlands. They fluctuate with the types of mires and vegetation and reach sometimes 3 g CH₄·m⁻²·day⁻¹. Mires have been used for agriculture and peat mining. The consequences of drainage and other managements to practise these different uses were carbon emissions of 6 500 Mt from peat, since the beginning of the 19^th century. In these conditions, fitting of carbon emissions in the atmosphere shows that disturbed mires could contribute to greenhouse effect as much as coal mining. The influence of increasing atmospheric depositions is not yet well known but seems to be site- and species-dependent. In some European regions, great area of mires have disappeared under the increasing of nitrogen and sulfur depositions. Restoration experiments of mires show that it is possible to regenerate turfigenous process but the time responses for total recovery differ with sites and degrees of disturbance.