[1] Global wetlands are, at estimate ranging 115 -237 Tg CH 4 /yr, the largest single atmospheric source of the greenhouse gas methane (CH 4 ). We present a dataset on CH 4 flux rates totaling 12 measurement years at sites from Greenland, Iceland, Scandinavia and Siberia. We find that temperature and microbial substrate availability (expressed as the organic acid concentration in peat water) combined explain almost 100% of the variations in mean annual CH 4 emissions. The temperature sensitivity of the CH 4 emissions shown suggests a feedback mechanism on climate change that could validate incorporation in further developments of global circulation models.

Abstract. This article provides an overview of the effects of land-use on the fluxes of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) 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 CO 2 and CH 4 depending on the weather conditions. Drainage for agriculture has turned peatlands to significant sources of GHGs (mainly N 2 O and CO 2 ). Annual mean GHG balances including net CH 4 , N 2 O and CO 2 emissions are 2260, 2280 and 3140 g CO 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, N 2 O and CO 2 emissions remain high. The mean net GHG emissions in abandoned and afforested agricultural peatlands have been 1580 and 500 g CO 2 eq. m −2 , respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 g CO 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 g CO 2 eq. m −2 ). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 g CO 2 eq. m −2 , mostly from high CH 4 emissions). Peatland forests, originally drained for forestry, may act as Correspondence to: M. Maljanen (marja.maljanen@uef.fi) net sinks (mean −780 g CO 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 g CO 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 CO 2 , CH 4 and N 2 O (mean annual emission 240 g CO 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.

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Managed northern peatlands are an important source of the strong greenhouse gas nitrous oxide (N 2 O). However, N 2 O emissions from these managed peatlands display a high spatial variability, and processes governing N 2 O production and emissions from these ecosystems are still not well understood. To constrain the factors regulating N 2 O emissions from managed northern peat soils, we determined a wide set of soil physical and chemical properties of peatlands with different management histories spread across Finland, Sweden and Iceland. We included eleven peatland sites with available in situ N 2 O flux data, and complemented our analyses with detailed measurements of soil nitrogen (N) cycling processes such as N 2 O production, gross N mineralization and gross nitrification and, in addition, soil microbial biomass. This study included drained peatlands with different land-use types and management intensities, comprising forested, cultivated or only drained peatlands and afforested or abandoned agricultural peatlands. All selected peatland sites displayed a low soil carbon to nitrogen (C/N) ratio of 15-27, traditionally used to predict high N 2 O emissions. Despite the narrow C/ N range, the N 2 O emissions at our sites varied greatly within and between land-use groups, ranging from 0.03 to 2.38 g N m −2 y −1. Thus, our findings provide valuable insights into the regulatory factors underlying the variability in N 2 O emissions and show that a low C/N ratio in managed peatlands cannot be used to predict high N 2 O emissions. Instead, our results demonstrate that higher N 2 O emissions are linked to higher peat phosphorus (P) and copper (Cu) content, suggesting that low P and Cu concentrations can limit N 2 O production in peat even with sufficient N availability. While known factors such as soil moisture, oxygen content and the degree of peat humification partially explained the variability in N 2 O emissions, this study directly links soil P and Cu availability to N 2 O production processes. The availability of P and especially Cu seemed to promote nitrification activities, thereby increasing N 2 O production. Our study highlights the link between N 2 O emissions and soil P and Cu availability and the strong coupling of the soil N and P cycles in peatlands, which is to date severely understudied.