Peatlands at a Threshold: Greenhouse Gas Dynamics in a Changing Climate

Sammanfattning: To be able to predict the strength of climate change and its consequences for mankind, it is of major importance to understand the contemporary greenhouse gas exchange of the terrestrial biosphere, as well as its response to changes in climate. To achieve such understanding both descriptive studies and manipulation experiments are required. Northern peatlands represent an important ecosystem type in this context, as they store large amounts of carbon (C) in their soils and can thus potentially affect the climate strongly. Greenhouse gas measurements using various methods in a temperate, ombrotrophic peatland (Fäjemyr) in southern Sweden constitute the backbone of this study. This particular ecosystem has been sequestrating atmospheric carbon dioxide (CO2) during thousands of years. During three years of continuous measurements of the land-atmosphere CO2 exchange, the bog acted on average as a small sink for atmospheric CO2 (-8.7±12.2 g C m-2 yr-1). However, during a year with an extended summer drought period, the bog switched into a source of CO2, which indicates that Fäjemyr is at a threshold in terms of C balance. The bog was also an emitter of meth-ane (CH4). The CH4 source strength was 3.1±1.5 g C m-2 yr-1 based on two years of measurements. This indicates that if summer drought conditions become more abundant in the future, the C sink functioning of Fäjemyr may cease. Fäjemyr is situated in an area subjected to relatively high levels of nitrogen (N) deposition. In a fertilization experiment on Fäjemyr, it was found that phosphorus (P) was limiting gross primary production (GPP). As a contrast it was found that N was limiting GPP and ecosystem respiration (Reco) in a north Swedish ombrotrophic peatland, subjected to low N deposition. No significant fertilization effects were found for the CH4 exchange in either site; however high nitrous oxide (N2O) emissions were detected in N fertilized plots. Higher temperatures induced by climate change will lead to enhanced soil mineralization rates and thus increased nutrient availability. The long-term effects of increased nutrient availability on the greenhouse gas exchange will likely be modified by changes in vegetation composition. Drivers of variation in the greenhouse gas exchange in peatlands are scale-dependent. Within four peatland sites, it was found that temperature was the most explanatory factor for GPP and Reco using half-monthly averages. Between twelve wetland sites, seasonally and annually averaged Reco correlated with several variables such as growing season period, temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit, whereas GPP and the net CO2 exchange (NEE) showed significant relationships with leaf area index and pH. These results indicate that vegetation properties play a fundamental role, both for differences in contemporary peatland C exchange as well as for a potentially dissimilar response between peatlands to a changing climate.