Significance The Arctic is warming rapidly, causing permafrost soils to thaw. Vast stocks of nitrogen (>67 billion tons) in the permafrost, accumulated thousands of years ago, could now become available for decomposition, leading to the release of nitrous oxide (N 2 O) to the atmosphere. N 2 O is a strong greenhouse gas, almost 300 times more powerful than CO 2 for warming the climate. Although carbon dynamics in the Arctic are well studied, the fact that Arctic soils store enormous amounts of nitrogen has received little attention so far. We report that the Arctic may become a substantial source of N 2 O when the permafrost thaws, and that N 2 O emissions could occur from surfaces covering almost one-fourth of the entire Arctic.