Arctic soils hold large carbon (C) stocks vulnerable to rapid decomposition, because the Arctic is warming two to three times faster than the global mean, especially during winter. Microbes remain active in frozen soils1,2, but controls on metabolism and decomposition below 0 ˚C are poorly understood. To address this knowledge gap, we incubated soil microcosms between -6 and 10 °C with different lengths of cellulose polymers and measured microbial respiration, biomass, and activities of endo- and exo-acting enzymes that decompose cellulose and hemicellulose. Low temperatures disproportionately inhibited endo-enzymes that cleave long-chain polysaccharides (endo-cellulase and -hemicellulase), with an abrupt threshold in endo-cellulase activities between -2 and -6 °C. Thus, endo-cellulase activities are expected to increase with winter warming, dramatically increasing CO2 emissions by increasing substrate availability to microbes. This threshold reveals a biochemical tipping point for Arctic soil C losses essential to predicting future Arctic greenhouse gas emissions with winter warming.