First extensive manual N₂O flux measurements reveal a light-dependent N₂O sink in a thawing permafrost peatland

Nitrous oxide (N₂O) is one of the most important greenhouse gases (GHG) with a global warming potential about 298 times stronger than carbon dioxide (CO₂) over a period of 100 years. While most N₂O emissions are released from natural ecosystems (60%), research has focussed largely on nutrient-rich agricultural soils, leading to a lack of understanding of nutrient-poor (sub-) Arctic ecosystems. Recent findings indicated significant N₂O emissions from organic-rich Arctic soils, resulting in a bias towards high emitting sites and particularly poor knowledge on N₂O consumption. As a result, the contribution of N₂O fluxes from the (sub-) Arctic regions to the global budget remains highly uncertain. Recent advances in portable gas analysers have improved our ability to measure low in-situ N₂O fluxes. To study the impact of environmental drivers (e.g. soil moisture, temperature, and photosynthetically active radiation) on N₂O fluxes using a portable N₂O analyser, we conducted chamber-based field measurements across a thaw gradient (palsa to bog to fen) in a sub-Arctic permafrost peatland in northern Sweden (Stordalen mire, Abisko), covering May to September. Conducting light (transparent) and dark (opaque) measurements, we found that soils in the Stordalen mire show a light-dependency, emitting N₂O in light conditions with a median of 0.56 µg m^-2 h^-1 (n = 480), and consuming N₂O in dark conditions with a median of -1.36 µg m^-2 h^-1 (n = 478). Since these changes can happen very rapidly, potential drivers of this dependency could be different active microbial communities, or vegetation impacts through photosynthesis. These results suggest that measurements with both transparent and opaque chambers are crucial for future N₂O flux studies to accurately estimate the N₂O budget. Generally, we measured low N₂O fluxes with a median flux of 0.02 µg m^-2 h^-1, of which all flux rates were above the minimal detectable flux. However, we also found one hot spot which continuously emitted high N₂O fluxes, with a maximum of 159.43 µg m^-2 h^-1 compared to 4.38 µg m^-2 h^-1 for all other plots. These are novel findings, suggesting that complex N₂O dynamics occur in nutrient-poor sites and further investigations are needed to understand the processes underlying the N₂O fluxes.