Haydn J.D. Thomas, Isla H. Myers-Smith , Toke T. Høye, Matteo Petit Bon, Jonas J Lembrechts , Eleanor R. Walker, Katrin Björnsdóttir, Isabel C Barrio, Ingibjörg Svala Jónsdóttir, Susanna E. Venn, Juha Mikael Alatalo , Jennifer Baltzer, Cory Wallace, Daniel Ackerman, Laura Gough, Janet S. Prevéy, Christian Rixen, Michele Carbognani, Alessandro Petraglia, Casper T. Christiansen, David W. Inouye, Jane E. Ogilvie, Mario Trouillier, Martin Wilmking, Rachel Treharne, Sandra Angers-Blondin, Christine Urbanowicz, Jonathan von Oppen, Sonja Wipf, Paul A. Smith, Satoshi N. Suzuki, Ryo O. Suzuki, Anna-Maria Virkkala, Miska Luoto, Svetlana Serikova, Anne D. Bjorkman, Daan Blok, Elise C. Gallois, Judith M. Sarneel

Considerable uncertainty exists regarding the strength, direction and relative importance of the drivers of decomposition in the tundra biome, partly due to a lack of coordinated decomposition field studies in this remote environment. Here, we analysed 3717 incubations of two uniform litter types, green and rooibos tea, buried at 330 circum-Arctic and alpine sites to quantify the effects of temperature, moisture and litter quality on decomposition. We found a surprisingly linear positive relationship between decomposition and soil temperature across all sites, counter to theory and previous model estimates. Litter mass loss was greater at wetter sites, even where soils reached almost full water saturation. However, litter quality was the strongest driver of litter mass loss across the tundra biome, explaining six times more variation in summer decomposition than soil temperature. Our results indicate that climate warming will directly increase decomposition across tundra environments. However, the indirect effects of climate change on vegetation communities, and thus plant litter inputs and quality, could have a more profound impact than direct effects on the balance of this globally important carbon store.