Winter icing and summer warming increase CO2 emissions while warming offsets icing-induced litter decomposition declines in Arctic tundra

Aims As the Arctic warms, evidence shows that warmer summers substantially affect ecosystem functioning, including carbon cycling. Yet, the most rapid warming occurs in winter, with more frequent mid-winter warming events raising air temperatures above 0 °C. During these warm spells, rain-on-snow events often generate extensive, long-lasting ice layers (icing) at the snow-ground interface, potentially altering tundra ecosystems beyond the duration of the events. We quantified the effects of winter icing, alone and combined with summer warming, on tundra CO 2 -fluxes and litter decomposition potential with the Tea Bag Index. Methods Over consecutive years, we manipulated winter icing and summer warming in a full-factorial experiment in mesic tundra in high-Arctic Svalbard. Icing was induced by a single watering each winter to form a basal ice layer persisting until spring snowmelt, while open-top chambers increased summer temperatures. Results Icing and warming each shifted the ecosystem from a near-neutral summer carbon balance (–0.1 µmol m −2 s −1 ) to a net source, with their combination causing the largest change, increasing CO 2 emissions by 2.1 µmol m −2 s −1 . Decomposition responses were smaller in magnitude, yet detectable: icing reduced summer and winter litter mass loss (3–9% difference relative to unmanipulated tundra), whereas warming had weaker effects but offset icing impacts, resulting in decomposition comparable to unmanipulated tundra. Conclusion Winter icing and summer warming jointly influence tundra CO 2 -fluxes and decomposition, pointing to enhanced carbon losses. While elevated winter temperatures are known to directly affect Arctic ecosystem functioning, their indirect effects via rain-on-snow-induced icing may be similarly important.