Greenhouse gas balances of managed peatlands in the Nordic countries - present knowledge and gaps

This article provides an overview of the effects of land-use on the fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and from peatlands in the Nordic countries based on the field data from about 100 studies. In addition, this review aims to identify 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, the past land-use and present climate have evidently changed their GHG balance. Unmanaged boreal peatlands may act as net sources or sinks for CO2 and CH4 depending on the weather conditions. Drainage for agriculture has turned peatlands to significant sources of GHGs (mainly N2O and CO2). Annual mean GHG balances including net CH4, N2O and CO2 emissions are 2260, 2280 and 3140 gCO(2) eq. m(-2) (calculated using 100 year time horizon) for areas drained for grass swards, cereals or those left fallow, respectively. Even after cessetion of the cultivation practices, N2O and CO2 emissions remain high. The mean net GHG emissions in abandoned and afforested agricultural peatlands have been 1580 and 500 gCO(2) eq. m(-2), respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 gCO(2) eq. m(-2). Cultivation of a perennial grass (e. g., reed canary grass) on an abandoned peat extraction site has been shown to convert such a site into a net sink of GHGs (-330 gCO(2) eq. m(-2)). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 gCO(2) eq. m(-2), mostly from high CH4 emissions). Peatland forests, originally drained for forestry, may act as net sinks (mean -780 gCO(2) eq. m(-2)). However, the studies where all three GHGs have been measured at an ecosystem level in the forested peatlands are lacking. The data for restored peatland forests (clear cut and rewetted) indicate that such sites are on average a net sink (190 gCO(2) eq. m(-2)). The mean emissions from drained peatlands presented here do not include emissions from ditches which form a part of the drainage network and can contribute significantly to the total GHG budget. Peat soils submerged under water reservoirs have acted as sources of CO2, CH4 and N2O (mean annual emission 240 gCO(2) eq. m(-2)). However, we cannot yet predict accurately the overall greenhouse gas fluxes of organic soils based on the site characteristics and land-use practices alone because the data on many land-use options and our understanding of the biogeochemical cycling associated with the gas fluxes are limited.