Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH₄ and CO₂, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH₄ and CO₂ to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH₄ production, oxic conditions during period 2 did not result in a lower potential CH₄ production rate during period 3, although production was delayed 1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH₄, oxic conditions during period 2 resulted in zero production of CH₄ during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH₄ production, the oxic period had a negligible effect on anoxic CO₂ production during period 3, in surface as well as deep peat. In both surface and deep peat, CO₂ production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO₂ production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.

Recent investigations indicate that winter carbon emission from peatlands and low temperature carbon mineralisation processes conld have the potential to affect annual carbon budgets. Special emphasis has been put on periods of freeze-thaw events during spring and autumn. The aim of this study was to investigate the impact of freeze-thaw cycles on peat carbon mineralisation. This was addressed by following production rates and total amounts of CO,- and CH4-formation in surface (0-5 cm) and deep (45-50 cm) peat samples incubated at 4°C and comparing controls with replicate samples subjected to three consecutive freeze-thaw cyclesprior to incubation. Accumulation of fermentation products (H2, VF A and ethanol) were measured in order to gain further insight to the biogeochemical processes and transformation pathways involved. We conclude that freeze-thaw cycles affect both short and long term formation and exchange of C02 by altering the availability and amount of peat carbon substrates. Freeze-thaw events also resulted in an inhibition of methanogenesis with a concomitant accumulation in H2 and butyrate. We conclude that freezethaw cycling events can be of large importance for the carbon budgets of northern peatland ecosystems, although they are of a limited temporal duration.

The first subarctic wetland CO2 and CH4 flux measurements were made at Stordalen in the beginning of the 1970s in connection with the IBP study. A return to this area in 1994-95 offered a unique opportunity to study possible interdecadal changes in northern wetland CO2 and CH4 emissions. Measurements of CO2 and CH4 fluxes were carried out in similar habitats as those investigated in 1974. The mire distribution of wet minerotrophic areas relative to the elevated ombrotrophic areas had changed dramatically over the twenty years. There were no significant differences between the CH4-flux in 1974, 1994, and 1995. However, the CO2 fluxes were significantly higher in 1995 than in 1974. Since differences in climatic conditions gave no cause for such a change it suggests a possible increase in decomposition rate to be due to other factors. We suggest changes in vegetation composition, altered mineralization pathways and disintegration of permafrost as causes for the interdecadal increase in decomposition rates.

Vascular plant functions as controlling mechanisms of methane emissions were investigated at two contrasting habitat types at a subarctic peatland ecosystem in northern Sweden. One of the habitats was ombrotrophic (vegetation dominated by Eriophorum vaginatum and Carex rotundata), while the other was minerotrophic (vegetation dominated by Eriophorum angustifolium). Through shading manipulations we successfully reduced the gross photosynthetic rates of the vascular plant communities. At the ombrotrophic site a 25% reduction in gross photosynthesis lead to a concomitant 20% reduction in methane emission rates, indicating a strong substrate-based coupling between the vascular plant community and the methanogenic populations. At the minerotrophic site, methane emission rates were unaffected, although plant photosynthesis was reduced by almost 50%. However, the methane emission rates at the minerotrophic site were significantly correlated with the number of vascular plants. We conclude that at the minerotrophic site the vegetation influences methane emission rates by facilitating methane transportation between the soil and the atmosphere, while at the ombrotrophic site the relationship between the vascular plant community and methane emissions is mediated by substrate-based interactions regulated by plant photosynthetic activity. Copyright 2002 by the American Geophysical Union.

Globally, wetlands are at estimates ranging 115-237 Tg C4/yr1 the largest single source of the greenhouse gas CH4 to the atmosphere. Important feedback mechanisms on climate change arising from changing exchanges of C02 between the terrestrial biosphere and the atmosphere have recently been identified2. A related question is how will possible changes in the CH4 emissions from wetlands affect the further development of the greenhouse effect? Here we show using comparable methods in a wide range of wetlands ranging from Greenland to Siberia that regardless the dependency on soil moisture, plant productivity and other factors, temperature is the strongest control and predictor of CH4 emissions across both temporal and large spatial scales. Furthermore, we show that CH4 flux variations not explained by temperature can beattributed to differences in microbial substrate availability (expressed as the organic acid concentration in peat water). Combined, soil temperature and organic acid concentrations explains 99% of the variation in CH4 fluxes between the different sites. The temperature sensitivity of the CH4 emissions shown suggests a strong feedback mechanism on climatechange that should valid incorporation in developments of global circulation models.