Labile carbon limits late winter microbial activity near Arctic treeline

It is well established that soil microbial communities remain active during much of the Arctic winter, despite soil temperatures that are often well below −10°C 1 . Overwinter microbial activity has important effects on global carbon (C) budgets 2 , nutrient cycling and vegetation community composition 3 . Microbial respiration is highly temperature sensitive in frozen soils, as liquid water and solute availability decrease rapidly with declining temperature 4 . Thus, temperature is considered the ultimate control on overwinter soil microbial activity in the Arctic. Warmer winter soils are thought to yield greater microbial respiration of available C, greater overwinter CO 2 efflux and a flush of nutrients that could be available for plant uptake at thaw 3 . Rising air temperature, combined with changes in timing and/or depth of snowpack development, is leading to warmer Arctic winter soils 5 . Using observational and experimental approaches in the field and in the laboratory, we demonstrate that persistently warm winter soils can lead to labile C starvation of the microbial community and reduced respiration rates, despite the high C content of most arctic soils. If Arctic winter soil temperatures continue to rise, microbial C limitation will reduce cold season CO 2 emissions and alter soil nutrient cycling, if not countered by greater labile C inputs.