Winter dynamics of the greenhouse gas exchange in a natural bog

Abstract: Northern peatlands are important in the context of the climate change. Since they comprise huge amounts of soil organic carbon and are found where the most significant greenhouse warming is predicted, they have the potential to exert major feedback effects on the global warming. Studies of the greenhouse gas exchange in northern peatlands have mainly focused on the growing season when the soil is thawed. But recently, attention has been drawn to the importance of winter processes in the estimations of global greenhouse gas budgets. In this study, fluxes of CO2, CH4 and N2O in a natural, nutrient-poor peatland (ombrotrophic bog) in southern Sweden were measured from February to April using a closed chamber technique. Mean fluxes during this time for CO2 and CH4 were 68.6 ± 9.9 (mean ± SEM) and 0.70 ± 0.06 mg m-2 h-1, respectively. Fluxes were significantly higher when the soil was thawed as compared to when it was frozen. Fluxes of CO2 and CH4 also correlated strongly with temperature, and revealed high temperature sensitivities around 0 °C. Fluxes of N2O showed an interesting trend. During the time when soil was frozen, a net uptake (-0.013 ± 0.003 mg m-2 h-1) was detected. Matching the moment in time when soil thawed, the mire turned from being a sink to a source (0.020 ± 0.003 mg m-2 h-1) of atmospheric N2O. This delicate development can be ascribed to the theory that during the time when soil was frozen, N2O was the sole electron acceptor available for denitrifiers. When the soil thawed, nutrients derived from frost-killed soil organisms became available for the surviving microbial population, which allowed N2O production. More research is needed to quantify the N2O exchange in different mire sub-types, and to evaluate whether northern peatlands should be included in global N2O budgets. This study concludes that winter processes are important, and should be taken into consideration in global greenhouse gas budgets. Temperature and freeze-thaw cycles are important factors regulating winter dynamics. The temperature sensitivity of the mire ecosystem is high at these low temperatures, which indicates that northern peatlands can act to enhance the climate change.