Nitrous oxide surface fluxes in a low Arctic heath: Effects of experimental warming along a natural snowmelt gradient

Climate change is profound in the Arctic where increased snowfall during winter and warmer growing season temperatures may accelerate soil nitrogen (N) turnover and increase inorganic N availability. Nitrous oxide (N₂O) is a potent greenhouse gas formed by soil microbes and in the Arctic, the production is seen as limited mainly by low inorganic N availability. Hence, it can be hypothesized that climate change in the Arctic may increase total N₂O emissions, yet this topic remains understudied. We investigated the combined effects of variable snow depths and experimental warming on soil N cycling in a factorial field study established along a natural snowmelt gradient in a low Arctic heath ecosystem. The study assessed N₂O surface fluxes, gross N mineralization and nitrification rates, potential denitrification activity, and the pools of soil microbial, soil organic and soil inorganic N, carbon (C) and phosphorus (P) during two growing seasons. The net fluxes of N₂O averaged 1.7 μg N₂O–N m⁻² h⁻¹ (range −3.6 to 10.5 μg N₂O–N m⁻² h⁻¹), and generally increased from ambient (1 m) to moderate (2–3 m) snow depths. At the greatest snow depth (4 m) where snowmelt was profoundly later, N₂O fluxes decreased, likely caused by combined negative effects of low summer temperatures and high soil moisture. Positive correlations between N₂O and nitrate (NO₃⁻) and dissolved organic N (DON) suggested that the availability of N was the main controlling variable along the snowmelt gradient. The maximum N₂O fluxes were observed in the second half of August associated with high NO₃⁻ concentrations. The effect of growing season experimental warming on N₂O surface flux varied along the snowmelt gradient and with time. Generally, the experimental warming stimulated N₂O fluxes under conditions with increased concentrations of inorganic N. In contrast, warming reduced N₂O fluxes when inorganic N was low. Experimental warming had no clear effects on soil inorganic N. The study suggests that if increased winter precipitation leads to a deeper snow cover and a later snowmelt, total emissions of N₂O from low Arctic heath ecosystems may be enhanced in the future and, dependent on dissolved N availability, summer warming may stimulate or reduce total emissions.