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Permafrost thawing and the age of carbon respired from arctic tundra.

Schuur, Edward*,1, Dutta, Koushik1, Vogel, Jason1, Crummer, Kathryn1, 1 University of Florida, Gainesville, FL, USA

ABSTRACT- Up to 450 Pg of soil carbon (C) has accumulated in high latitude ecosystems after the retreat of the last major ice sheets. This soil C has until now been largely protected from decomposition by cold temperature, waterlogging, and permafrost. Recent studies suggest that, due to climate warming, these ecosystems may no longer be accumulating C, and in some cases may be losing stored C to the atmosphere. We hypothesize that sustained transfers of C to the atmosphere that could cause a significant positive feedback to climate change must come from old C, which forms the bulk of the soil pool. We used radiocarbon measurements of carbon dioxide to detect the age of C respired from tussock tundra near Denali National Park, Alaska. At this tundra site, permafrost has been observed to warm and melt over the past several decades, causing the ground surface to subside as ice volume in the soil decreased. We established three sites within this area that differed in vegetation and surface topography. These sites represent differences in time since the onset of permafrost melting and/or in the magnitude of ecosystem change. We made radiocarbon measurements of ecosystem respiration and of incubations of above and belowground plant biomass to determine the age of C respired from these sites. Ecosystem respiration radiocarbon values ranged from +58‰ to +114‰ and there was as much variation within a site as there was between sites. In the early season, respiration from belowground plant biomass had an average value of +83‰ and was significantly higher than the value of +67‰ respired from aboveground plant biomass and was higher than the current atmospheric value of +60‰. This suggests that early-season plant respiration was derived in part from a C storage pool within stems that was more than a year old. During the rest of the growing season, however, plant respiration radiocarbon was similar to atmospheric values. Most ecosystem respiration radiocarbon values were higher than values for plant respiration, indicating that the majority of decomposition of soil organic matter was derived from C fixed over the past several decades. In contrast, some individual plots had radiocarbon values below that of plant respiration, suggesting a larger contribution to respiration from organic matter that was more than 50 years old, likely as a result of deeper soil thaw.

Key words: carbon, isotopes, climate change, arctic

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