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Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. This change is particularly important given the large amounts of carbon stored in the yedoma permafrost soils that become vulnerable to thaw and release under warming. Data from this region pertaining to year-round carbon, water, and energy fluxes are scarce, particularly in sensitive ecotonal ecosystems near latitudinal treeline, as well as those already impacted by permafrost thaw. Here we investigated the interannual and seasonal carbon dioxide, water, and energy dynamics at an ecotonal forested site and a disturbed thermokarst-impacted site. The ecotonal site was approximately neutral in terms of CO2 uptake/release, while the disturbed site was either a source or neutral. Our data suggest that high rates of plant productivity during the growing season at the disturbed site may, in part, counterbalance higher rates of respiration during the cold season compared to the ecotonal site. We also found that the ecotonal site was sensitive to the timing of the freezeup of the soil active layer in fall, releasing more CO2 when freezeup occurred later. Both sites showed a negative water balance, although the ecotonal site appeared more sensitive to dry conditions. Water use efficiency at the ecotonal site was lower during warmer summers. Overall, these Siberian measurements indicate ecosystem sensitivity to warmer conditions during the fall and to drier conditions during the growing season and provide a better understanding of ecosystem response to climate in a part of the circumpolar Arctic where current knowledge is weakest. Plain Language Summary As Siberia warms, the frozen soils known as permafrost start to thaw, causing an irregular terrain of pits and mounds called thermokarst. Large amounts of carbon in Siberian soils have been locked away in permafrost for thousands of years, becoming vulnerable to release under thaw and thermokarst formation. This will potentially result in large amounts of additional greenhouse gases in the atmosphere, amplifying climate warming. We examined carbon dioxide (CO2) fluxes over multiple years at two sites in northeastern Siberia, an ecotonal site that lies at the transition between the boreal forest and tundra biomes, and a site with thermokarst. We found that the ecotonal site is carbon neutral, consuming the same amount of CO2 as it takes up from the atmosphere. However, this site releases greater amounts of CO2 in years when soil freeze occurred later, which is expected to become common in the future. The thermokarst site released significantly more CO2, but it was also marked by greater plant growth, thereby off-setting some of the CO2 lost. Due, in part, to a lack of data, models represent terrestrial ecosystem carbon dynamics in Siberia poorly and do not take into changes in carbon cycling that occur with thermokarst formation.

来源平台：JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES