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[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.

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A tropical ombrotrophic peatland ecosystem is one of the largest terrestrial carbon stores. Flux rates of carbon dioxide (CO 2 ) and methane (CH 4 ) were studied at various peat water table depths in a mixed-type peat swamp forest floor in Central Kalimantan, Indonesia. Temporary gas fluxes on microtopographically differing hummock and hollow peat surfaces were combined with peat water table data to produce annual cumulative flux estimates. Hummocks formed mainly from living and dead tree roots and decaying debris maintained a relatively steady CO 2 emission rate regardless of the water table position in peat. In nearly vegetation-free hollows, CO 2 emission rates were progressively smaller as the water table rose towards the peat surface. Methane emissions from the peat surface remained small and were detected only in watersaturated peat. By applying long-term peat water table data, annual gas emissions from the peat swamp forest floor were estimated to be 3493 AE 316 g CO 2 m À2 and less than 1.36 AE 0.57 g CH 4 m À2 . On the basis of the carbon emitted, CO 2 is clearly a more important greenhouse gas than CH 4 . CO 2 emissions from peat are the highest during the dry season, when the oxic peat layer is at its thickest because of water table lowering.

We show that sediment respiration is one of the key factors contributing to the high CO 2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (o100 km 2 ) and 32 lakes with the highest total phosphorus (P tot ) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO 2 concentrations plotted against depth and time demonstrate strong CO 2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O 2 departure from the saturation and CO 2 departure from the saturation was strong in the entire data set (r 2 5 0.79, n 5 2 740, Po0.0001). CO 2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO 2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake-area-weighted annual CO 2 evasion based on our 177 randomly selected lakes and all Finnish lakes 4100 km 2 (Rantakari & Kortelainen, 2005) was 42 g C m À2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO 2 evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.

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Abstract. Closed (non-steady state) chambers are widely used for quantifying carbon dioxide (CO 2 ) fluxes between soils or low-stature canopies and the atmosphere. It is well recognised that covering a soil or vegetation by a closed chamber inherently disturbs the natural CO 2 fluxes by altering the concentration gradients between the soil, the vegetation and the overlying air. Thus, the driving factors of CO 2 fluxes are not constant during the closed chamber experiment, and no linear increase or decrease of CO 2 concentration over time within the chamber headspace can be expected. Nevertheless, linear regression has been applied for calculating CO 2 fluxes in many recent, partly influential, studies. This approach has been justified by keeping the closure time short and assuming the concentration change over time to be in the linear range. Here, we test if the application of linear regression is really appropriate for estimating CO 2 fluxes using closed chambers over short closure times and if the application of nonlinear regression is necessary. We developed a nonlinear exponential regression model from diffusion and photosynthesis theory. This exponential model was tested with four different datasets of CO 2 flux measurements (total number: 1764) conducted at three peatlands sites in Finland and a tundra site in Siberia. Thorough analyses of residuals demonstrated that linear regression was frequently not appropriate for the determination of CO 2 fluxes by closed-chamber methods, even if closure times were kept short. The developed exponential model was well suited for nonlinear regression of the concentration over time c(t) evoCorrespondence to: L. Kutzbach (kutzbach@uni-greifswald.de) lution in the chamber headspace and estimation of the initial CO 2 fluxes at closure time for the majority of experiments. However, a rather large percentage of the exponential regression functions showed curvatures not consistent with the theoretical model which is considered to be caused by violations of the underlying model assumptions. Especially the effects of turbulence and pressure disturbances by the chamber deployment are suspected to have caused unexplainable curvatures. CO 2 flux estimates by linear regression can be as low as 40% of the flux estimates of exponential regression for closure times of only two minutes. The degree of underestimation increased with increasing CO 2 flux strength and was dependent on soil and vegetation conditions which can disturb not only the quantitative but also the qualitative evaluation of CO 2 flux dynamics. The underestimation effect by linear regression was observed to be different for CO 2 uptake and release situations which can lead to stronger bias in the daily, seasonal and annual CO 2 balances than in the individual fluxes. To avoid serious bias of CO 2 flux estimates based on closed chamber experiments, we suggest further tests using published datasets and recommend the use of nonlinear regression models for future closed chamber studies.

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.