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{degrees}C1. Overwinter microbial activity has important effects on global carbon (C) budgets2, nutrient cycling and vegetation community composition3. Microbial respiration is highly temperature sensitive in frozen soils, as liquid water and solute availability decrease rapidly with declining temperature4. 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 CO2 efflux and a flush of nutrients that could be available for plant uptake at thaw3. Rising air temperature, combined with changes in timing and/or depth of snowpack development, is leading to warmer Arctic winter soils5. 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 CO2 emissions and alter soil nutrient cycling, if not countered by greater labile C inputs.