Land-use and greenhouse gas balances of peatlands in the Nordic countries – present knowledge and gaps

This article provides an overview of the effects of land-use on the fluxes of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) of peatlands in the Nordic countries presented in about 100 studies. In addition, the article identifies the gaps in the present knowledge on the greenhouse gas (GHG) balances associated with the land-use of these northern ecosystems. Northern peatlands have accumulated, as peat, a vast amount of carbon from the atmosphere since the last glaciation. However, past land-use and the present climate have evidently changed their GHG balance. The mean annual GHG balances of undisturbed ombrotrophic and minerotrophic peatlands were surprisingly positive (net sources) of 140 and 380 g CO2 eq m−2, respectively, even if the former was a sink of 63 g CO2 eq m−2 when only the CO2 balance was considered. Drainage of such peatlands for agriculture resulted in the most disadvantageous land-use option for the atmosphere, with the net annual GHG balance increasing to 2190, 2280 and 3140 g CO2 eq m−2 for areas drained for grass swards, cereals or those left fallow, respectively. Even after ceasing of the cultivation practices, N2O emissions remained high and together with the other GHGs resulted in net emissions of 1570 and 500 g CO2 eq m−2, in abandoned and afforested peatlands, respectively. Peat extraction sites were also net sources, 730 g CO2 eq m−2. The cultivation of reed canary grass turned the site to net sink of −330 g CO2 eq m−2 but restoration did not (source of 470 g CO2 eq m−2). Data for afforested extraction sites is lacking. Peat soils originally drained for forestry may act as net sinks of 780 g CO2 eq m−2, and when those sites were restored the sink was 190 g CO2 eq m−2. However, more data is needed to confirm this point. Peat soils submerged under water reservoirs had a mean annual emission of 240 g CO2 eq m−2. In general, there is a lack of studies where all three GHGs have been measured at an ecosystem level, especially in the forested peatlands.