We estimated the soil CO2 balance of 68 forestry-drained boreal peatland sites in Finland by subtracting the litter input to soil from the CO2 efflux from soil. We also measured soil-atmosphere fluxes of CH4 and N2O and the CO2 sink of the growing tree stand in order to assess the current greenhouse gas impact of the study sites. The soil was, on average, a CO2 source of +190 +/- 70 g m(-2) year(-1) at the fertile Herb-rich and Vaccinium myrtillus type sites, but a CO2 sink of -70 +/- 30 g m(-2) year(-1) at the poor Vaccinium vitis-idaea and Dwarf shrub type sites. The source increased at the fertile and the sink decreased at the poor sites as the water table deepened. The source at the fertile sites also increased by increasing temperature sum, the highest CO2 sources being around +1000 g m(-2) year(-1) at well drained sites in Southern Finland. Both fertile and poor sites had a climate cooling impact. The sink in CO2 equivalents at the fertile sites was -690 +/- 90 g m(-2) year(-1) and at the poor sites -540 +/- 70 g m(-2) year(-1). The greater sink at the fertile sites was due to clearly better tree growth, their tree stand CO2 sink being -880 +/- 60 g m(-2) year(-1) compared to the -490 +/- 60 g m(-2) year(-1) at the poor sites. Ditching-based forestry can be climatically sustainable at nutrient-poor boreal peatlands since the peat soil continues to be a CO2 sink even after drainage. At the fertile sites, forestry will inevitably lead to loss of carbon in the long term, unless the tree biomass is stored after cuttings, for example in wooden buildings or as biochar in agricultural soils. (C) 2012 Elsevier B.V. All rights reserved.
