Closing the Winter Gap—Year‐Round Measurements of Soil CO₂ Emission Sources in Arctic Tundra

Non‐growing season CO 2 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 CO 2 emissions from a graminoid tundra based on year‐round in situ measurements of the radiocarbon content of soil CO 2 (Δ 14 CO 2 ) and of bulk soil C (Δ 14 C), microbial activity, and temperature. Combining these data with land‐atmosphere CO 2 exchange allows estimates of the proportion and mean age of microbial CO 2 emissions year‐round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Δ 14 CO 2 = 74 ± 4.7‰, 37% ± 3.4% microbial, mean ± se) to increasingly older soil carbon in fall and winter (March Δ 14 CO 2 = 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.