Climate warming accelerates CO2-release from subsurface soil organic carbon in a subarctic peatland

Summary by Ellen Dorrepaal

Northern peatlands store about one-third of the total global soil organic carbon pool in only 3% of the earth’s land surface1. Considerable increases in temperature have been recorded at high latitudes over the past decades and continued warming is projected for the coming century2. Release of the large carbon stores in northern peatlands, through temperature-enhanced decay rates, might form a major, positive feedback to our global climate. It has been suggested, however, that climate warming may only affect
© Govert de Groot, Arctic Peoples Alert
the respiration of plants, fresh litter and young soil organic matter, while older, more recalcitrant peat might be hardly sensitive to warming3. Differences in the responsiveness of short-term and longer-term C-cycles to climate change under realistic field conditions have received little attention so far, and the long-term effects on carbon release from peatlands therefore remain uncertain. Our aims were therefore:
(1) to investigate the responses of ecosystem respiration of a sub-arctic peatland to long-term climate change, and (2) to identify the sources of increased respired CO2 (plants versus shallow, young peat versus deeper, older peat). We used open-top chambers to subject a subarctic peatland (Abisko, north Sweden) to realistic climate-change scenarios, consisting of warming in summer and/or spring, and/or winter snow addition.
(1) Experimental climate warming in spring and/or summer strongly enhanced ecosystem respiration rates within their respective seasons of application (May or June-September), and these effects were sustained for at least 8 years. Winter snow addition, however, did not affect ecosystem respiration rates during the subsequent spring and summer seasons4.
(2) To find out whether climate warming only enhances the fast turn-over of recently fixated carbon in plants, the decay of freshly senesced, superficial organic material or whether even older, subsurface peat layers respond, we used two different approaches. Removal of the above ground vegetation showed that the respiration of peat without living vegetation or roots was as responsive to summer warming as respiration in the presence of vegetation, indicating that climate warming may stimulate both plant-related respiration (recently fixated carbon) and microbial breakdown of the peat. In addition to this we tracked the origin of the respired carbon by comparing the δ13C isotopic signatures of the emitted CO2 with that of different peat layers. Laboratory analyses and incubations of peat layers down to 50 cm showed that δ13C values of bulk and respired carbon consistently increase along our peat profile (up to 1.7‰). Subsequent field measurements showed clear increases in delta 13C signatures of respired CO2 upon warming, indicating that the decay of subsurface peat layers is strongly enhanced by climate warming4.
Taken together, our data demonstrate a high sensitivity of the respiration of both young and old, long-term peatland carbon stocks to climate warming, which – because of the large size of peatland carbon stocks – may feedback to the atmospheric carbon balance and climate at a global scale.

Literature Cited
1: Gorham, E. Ecol. Appl. 1, 182-195 (1991).
2: ACIA. Arctic Climate Impact Assessment (Cambridge University Press, Cambridge, UK, 2005).
3: Christensen, T. R. et al. Appl. Soil Ecology 11, 127-134 (1999).
4: Dorrepaal, E. et al. Nature 460, 616-619 (2009).

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Willem Barentsz Poolinstituut

Bundeling van kennis, onderzoek en onderwijs over de Noord- en de Zuidpool

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