Carbon and nitrogen-based gas fluxes in subarctic ecosystems under climate warming and increased cloudiness†

Climate warming is projected to be particularly pronounced in the northern high latitudes coupled with reduced light availability due to increased cloudiness. The changing climate may alter the fluxes of greenhouse gases (GHGs) and atmospherically reactive trace gases, which can drive important climate feedbacks. We investigated the individual and combined effects of warming and increased cloudiness on methane (CH₄), carbon dioxide (CO₂), nitrous oxide (N₂O), nitric oxide (NO), nitrous acid (HONO) and biogenic volatile organic compound (BVOC) fluxes in mesocosms from two tundra and one palsa mire ecosystems kept under strict environmental control in climate chambers. We also examined whether and how prevailing soil physiochemical properties and plant species composition affected the fluxes. In control conditions, all sites were net sinks of CH₄ and CO₂ during both growing seasons except for the palsa site which was a net source of CO₂ in the second growing season. Warming enhanced CH₄ uptake, mostly observed in the palsa site, and turned the palsa site from a sink to a source of CO₂ in the first growing season and increased the CO₂ source strength in the second growing season. Warming increased BVOC emissions while increased cloudiness mostly decreased the emissions. The combined treatment of warming and increased cloudiness decreased CH₄ uptake, mostly observed in the palsa site, and BVOC emissions. Fluxes of CO₂ were linked to availability of soil carbon and organic matter, litter input, soil pH and bulk density, and cover of mosses. Low emissions of N₂O, NO, and HONO could mainly be explained by limited availability of mineral nitrogen. Warming-enhanced CH₄ uptake and BVOC emissions will provide a negative feedback to climate while enhanced CO₂ release from palsa mires will exacerbate global warming. Under combined warming and increased cloudiness, subarctic ecosystems may shift from sinks to sources of CH₄, providing a positive feedback to climate. Prevailing soil physiochemical properties and vegetation composition will play a significant role in controlling the fluxes, hence contributing to the overall climate change effects and feedback.