5th May 2014.
An analysis of 19,000 methane flux measurements from 71 sites has revealed new detail on controls driving methane emissions from wetlands. The new paper is by a group of international researchers and is published in Global Change Biology. The study examined bogs, swamps, poor fens and rich fens across a range of regions (subarctic to subtropical) in the northern hemisphere. The investigation also considered sites that had been drained (either solely by drainage or by peat harvesting) and those that had been wetted (either through experimental flooding or permafrost thaw in thermokarst collapse scars).
The analysis yielded numerous interesting findings. For instance, large methane fluxes (above 500 mg CH4 m-2 d-1) were observed more frequently in bogs and poor fens when compared to swamps and rich fens. The effect of wetland type on methane flux changed with region; emissions from bogs and rich fens decreased in the higher latitudes, whilst poor fens in the subarctic region displayed large fluxes. When results for all wetland types were averaged according to region, it was found that temperate sites emitted the largest amounts of methane; a mean flux of 125.4 mg CH4 m-2 d-1.
Examinations of the controls on methane flux revealed that vascular plants played a significant role in contributing to emissions. Large fluxes were associated with sampling locations rich in sedges and grasses, whilst locations with trees or where vascular plants were absent showed low fluxes. This result is consistent with the frequent observation that vascular plants with aerenchymatous tissue can transport methane from the soil directly into the atmosphere. Furthermore, vascular plants can contribute labile carbon compounds into the soil, thus stimulating methanogenesis.
As would be expected, both temperature and water table played a part in controlling methane fluxes. For bogs and poor fens it was found that mean water table was a significant predictor of flux, with fluxes increasing as the water table came closer to the peat surface. Regarding temperature, for bogs and swamps a significant positive relationship was found between methane flux and mean annual temperature. However, the authors stress that these relationships are not straightforward; for instance, flooding can suppress emissions due to reduced diffusion of methane through the water.
Finally, the analysis suggested that wetland drainage lead to a reduction in methane emissions. Additionally, drainage was found to alter the relationship between methane flux, soil temperature and water table. In both drained bogs and fens, neither short-term warming nor wetting resulted in a high flux, though this response was observed for pristine sites.
Lead author, Merritt Turetsky of the University of Guelph, said: “Our analyses show that northern fens, such as those created when permafrost thaws, can have emissions comparable to warm sites in the tropics, despite their cold temperatures. That’s very important when it comes to scaling methane release at a global scale.”
Reference: Turetsky, M.R., et al. 2014. A synthesis of methane emissions from 71 northern, temperate, and subtropical wetlands. Global Change Biology, DOI: 10.1111/gcb.12580.
Photo: Bog – St-Daniel sector – Frontenac National Park (Québec, Canada) – July 2008, by Boréal.