Carbon loss in drained forestry peatlands in Finland, estimated by re-sampling peatlands surveyed in the 1980s

The total area of boreal peatlands is about 3.5 million km(2) and they are estimated to contain 15-30% of the global soil carbon (C) storage. In Finland, about 60 000 km(2), or 60% of the original peatland area, has been drained, mainly for forestry improvement. We have studied C inventory changes on forestry-drained peatlands by re-sampling the peat stratum in 2009 at the precise locations of quantitative peat mass analyses conducted as part of peatland transect surveys during the 1980s. The old and new profiles were correlated mainly by their ignition residue stratigraphies; at each site we determined a reference level, identifiable in both profiles, and calculated the cumulative dry mass and C inventories above it. Comparison of a total of 37 locations revealed broad variation, from slight increase to marked decrease; on average the 2009 results indicate a loss of 7.4 (SE +/- 2.5) kg m(-2) dry peat mass when compared with the 1980s values. Expressed on an annual basis, the results indicate an average net loss of 150 g C m(-2) year(-1) from the soil of drained forestry peatlands in the central parts of Finland. The C balance appeared not to correlate with site fertility (fertility classes according to original vegetation type), nor with post-drainage timber growth.