Closing the Winter Gap—Year‐Round Measurements of Soil CO2 Emission Sources in Arctic Tundra. Issue 6 (24th March 2022)

• Record Type: Journal Article
• Authors: Pedron, Shawn A.; Welker, J. M.; Euskirchen, E. S.; Klein, E. S.; Walker, J. C.; Xu, X.; Czimczik, C. I.
• Is Part Of: Geophysical research letters. Volume 49:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-24
• Subjects: carbon flux -- permafrost -- radiocarbon -- respiration -- winter -- vulnerability; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL097347
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Non‐growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year‐round in situ measurements of the radiocarbon content of soil CO2 (Δ 14 CO2 ) and of bulk soil C (Δ 14 C), microbial activity, and temperature. Combining these data with land‐atmosphere CO2 exchange allows estimates of the proportion and mean age of microbial CO2 emissions year‐round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Δ 14 CO2 = 74 ± 4.7‰, 37% ± 3.4% microbial, mean ± se) to increasingly older soil carbon in fall and winter (March Δ 14 CO2 = 22 ± 1.3‰, 47% ± 8% microbial). Thus, rising soil temperatures and emissions during fall and winter are depleting aged soil carbon pools in the active layer and thawing permafrost and further accelerating climate change. Plain Language Summary: The Arctic is warming and large quantities of organic matter in permafrost soils are at risk of becoming decomposed to carbon dioxide (CO2 ) by microbes. Measurements of soil CO2 emissions do not provide direct estimates of permafrost emissions, however, because CO2 is produced by microbes and plant roots. Using a new sampler, we collected soil CO2 in northern Alaska over 3–6 weeks for 2 years and analyzed its radiocarbon content to distinguish CO2 sources. To calculate CO2 budgets, we added observations of soil and microbial CO2 emissions and soil temperature. We find that microbes rely on fresh plant matter in summer but older organic matter from fall to spring; a process not captured by standard laboratory experiments. Thus, warming permafrost soils are not just more rapidly cycling fresh plant litter but losing organic matter that accumulated over decades to millennia. Key Points: Combining continuous soil CO2 flux with Δ 14 CO2 observations enables a systematic evaluation of carbon cycling in Arctic soils year‐round Outside the growing season, soil microorganisms rely on older, local soil carbon pools not captured in short‐term incubation experiments Permafrost warming and thaw is depleting soil carbon pools; fluxes of older carbon are greatest in summer but dominate emissions in winter