The long-term Net Ecosystem Exchange of a remote Siberian high arctic site

The arctic is warming at double the average global rate. This raises the concern that permafrost is beginning to thaw and could release large amounts of stored carbon, parts of which can be centuries old. If the total carbon release exceeds the carbon uptake the net ecosystem exchange (NEE) shifts from carbon sink to source, amplifying global warming. Here we present long-term eddy covariance (EC) data of a tundra ecosystem in northeast Siberia, showing the current NEE and its drivers in one of the most remote and coldest EC sites of the northern hemisphere. During the growing season the site is an overall carbon sink. The start of the carbon uptake quickly follows snowmelt and total growing season uptake is positively correlated with an earlier timing of the carbon uptake. While snowfall and the timing of snowmelt is highly variable no discernible trend can be seen in long-term data. In general, increased temperatures yield higher net carbon uptake during the growing season, although this effect levels off at roughly 20°C, likely due to the steadily decreasing solar radiation throughout the growing season. Because of the remoteness and extremely low temperatures, no winter measurements exist. However, machine learning gap filling suggests the site is a small net carbon source during most of the winter. This is in accordance with some of the recent findings at other sites and potentially offsets large parts of the growing season uptake. Thus, while the growing season initially might see increased terrestrial carbon uptake at higher regional temperatures, constraining yearly budgets with winter measurements is indispensable to get a full picture of changes in the total carbon budget of arctic tundra sites in Siberia.