Soils store at least twice as much carbon (C) as plant biomass 1 , and, each year, soil microbial respiration releases ~60 Pg of C to the atmosphere as carbon dioxide (CO 2 ) 2 .In the short term, soil microbial respiration increases exponentially with temperature 3 , and thus models predict that warming-induced increases in CO 2 release from soils could represent an important positive feedback to 21 st century climate change 4 . However, the magnitude of this feedback remains uncertain, not least because the adaptation of soil microbial communities to changing temperatures has the potential to either substantially decrease ('compensatory adaptation' 5-7 ) or substantially increase ('enhancing adaptation' 8,9 ) warming-induced C losses. By collecting contrasting soils along a climatic gradient from the Arctic to the Amazon, we undertook the first global analysis of the role microbial thermal adaptation plays in controlling rates of CO 2 release from soils. Here we show that, enhancing adaptation was between three and ten times more common than compensatory adaptation. Furthermore, the strongest enhancing responses were observed in soils with high C contents and from cold climates; enhancing thermal adaptation increased the temperature sensitivity of respiration in these soils by a factor of 1.4. This suggests that the substantial stores of C present in organic and high-latitude soils may be more vulnerable to climate warming than currently predicted. Text:Short-term experiments have demonstrated that the rate of microbial respiration in soil increases exponentially with temperature, and this general relationship has been used in parameterising soil C and Earth system models 4,10 . However, plant physiologists have demonstrated that short-term measurements are inadequate for representing the dynamic response of plant respiration to changes in temperature. In plants, thermal acclimation, defined as the "subsequent adjustment in the rate of respiration to compensate for an initial change in temperature" 11 greatly reduces the impact of temperature change on plant respiration in the medium-to long-term, with major consequences for modelling C-cycle feedbacks to climate change 12 . In soil there is growing evidence of the potential for a similar compensatory effect through microbial adaptation to temperature 13 ('compensatory adaptation': defined here to include the potential for physiological acclimation, adaptation within populations, and changes in microbial community size and structure). However, it is unclear if microbial community-level responses should always be compensatory. In fact, responses that enhance the direct and instantaneous effect of temperature changes on soil respiration ('enhancing adaptation') have also been observed 8,9,14 . To date there has been no large-scale evaluation of the role of microbial adaptation in controlling the temperature sensitivity of soil respiration. This lack of understanding adds considerable uncertainty to predictions of the magnitude and direction of carbon-cycle feedb...

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