Arctic Soils in a Warming Climate: Plot-scale Changes of CO2 Fluxes after Five Years of Experimental Warming.

Terrestrial arctic ecosystems store large amounts of carbon (C). With global warming, this C might be released into the atmosphere as CO2 by stimulation of soil microbial degradation. At the same time, CO2 uptake in plants is enhanced, which might, in parts, offset CO2 losses. Yet, the future balance of these two contrasting feedbacks remain uncertain. This study aimed to better understand changes of input and output CO2 fluxes in an arctic tussock tundra ecosystem in response to global warming, with a special focus on the contrast between two sub-ecosystem habitats, the tussocks and the space between tussocks. An experimental setup was used, where snow fences simulated winter warming by increasing snow depth, and open top chambers simulated summer warming. Daytime ecosystem respiration (ER), reflecting the outward CO2 flux, gross ecosystem production (GEP), reflecting the inward CO2 flux, and net ecosystem exchange (NEE), reflecting the net balance of both, were measured in the summer 2019 in the tussock and the intertussock habitat. In the tussock, both ER and GEP were as twice as high compared to the intertussock and increased with summer warming in a similar magnitude, resulting in an unchanged NEE. Fluxes in the intertussock were not altered with summer warming. Winter warming had no significant effects on ER and GEP in neither of the habitats. However, winter warming increased NEE and green biomass in the intertussock, indicating that in this habitat, plants benefit from warmer winter soil temperatures. Interaction effects of winter and summer warming underline the role of ecological processes outside the summer season, which are to date poorly understood. Contrasting responses of the two sub-ecosystem habitats highlight the challenges in predicting future C balances that are caused by small-scale spatial and temporal heterogeneity of C dynamics