A new paper by Leifeld et al. 2011 calculates the amount of carbon lost from a temperate peatland after drainage. A convenient method of estimating the amount of carbon lost has previously been to assume that it is roughly 50% of the total amount of volume lost, but the authors show that this can vary a lot even within the narrow climate zone of their study in Switzerland. After drainage, water goes out, and the ground subsides. Part of it is due to simply the fact that the water is lost and the peat is compressed. The other part is due to oxidation of the organic matter, that previously had been preserved in the very anoxic condition (think bogmen).
The authors show that in their fen that was drained 140 years ago, the annual subsidence of the ground has been 0,8-1,6 cm per year. So what does this mean for carbon loss. Well, it turns out that the amount of lost carbon is between 2,5-5,5 metric ton annually per hectare. Now assuming that the one who drained the peatland would have to pay for these emissions we would have to multiply the amount of C with 3,7 to get the amount of CO2. We are here making the assumption that all C go out as CO2. Then we end up with an annual release of 8,25-18.15 ton CO2 per hectare. Offsetting that CO2 would today cost roughly 53-117 euro per hectare annually. Considering that the amount of drained peatlands in the world are large (think Netherlands), it ends up being quite a lot. Hopefully we can be able to turn these carbon sources back to carbon sinks by restoring them, but unfortunately it is not always successful. One of the most important lessons of this is therefore to conserve the functioning peatlands we have. It is also of importance to not allow further drainage. In Sweden forestry owners are allowed to restore the ditch to the original depth, which is false thinking. The peatland has subsided since the drainage, and therefore making the ditch as deep as it was originally will take it even further down into the peat. It ought to be self evident that FSC forestry should not include “ditch-rinsing” but unfortunately it is allowed.
Leifeld, J., Müller, M., & Fuhrer, J. (2011). Peatland subsidence and carbon loss from drained temperate fens Soil Use and Management DOI: 10.1111/j.1475-2743.2011.00327.x