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

Abstract

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 g CO₂ 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 g CO₂ eq. m⁻², respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 g CO₂ 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 g CO₂ eq. m⁻²). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 g CO₂ eq. m⁻², mostly from high CH₄ emissions). Peatland forests, originally drained for forestry, may act as net sinks (mean −780 g CO₂ 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 g CO₂ 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 g CO₂ 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.