The temperature sensitivity of soil respiration will largely determine the effects of a warmer world on net carbon nux from soils to the atmosphere. CO2 flux from soils to the atmosphere is estimated to be 50-70 petagrams of carbon per year and makes up 20-38% of annual inputs of carbon (in the form of CO2) to the atmosphere from terrestrial and marine sources(1,2). Here we show that, for a mixed temperate forest, respiration by roots plus oxidation of rhizosphere carbon, which together produce a large portion of total effluxed soil CO2, is more temperature-sensitive than the respiration of bulk soil. We determine that the Q(10) value (the coefficient for the exponential relationship between soil respiration and temperature, multiplied by ten) is 4.6 for autotrophic root respiration plus rhizosphere decomposition, 2.5 for respiration by soil lacking roots and 3.5 for respiration by bulk soil. If plants in a higher-CO2 atmosphere increase their allocation of photosynthate to roots(3-6), these findings suggest that soil respiration should be more sensitive to elevated temperatures, thus limiting carbon sequestration by soils.