ATTEMPTS to model the global carbon cycle, and anthropogenic modifications to carbon flow between the atmospheric, oceanic and terrestrial carbon reservoirs, commonly rely on values assumed for the 13C/12C ratio and 'bomb-spike' 14C signature of carbon in each reservoir1,2. A large proportion of the carbon in the terrestrial biosphere resides in the soil organic carbon (SOC) pool3, most of which is derived from plants that assimilate carbon via the C3 photosynthetic pathway4. Here we report measurements of the 13C and 14C signatures of particulate organic carbon from surface soils of C3 biomes from a global distribution of low-altitude, non-water-stressed locations. We find that there is currently a latitudinal gradient in the signature, with low-latitude soils being relatively depleted in 13C. The 14C signatures indicate that today's gradient is due to a latitudinal gradient in the residence time of the soil organic carbon, coupled with anthropogenic modifications to the 13C/12C ratio of atmospheric CO2 (for example by fossil-fuel burning5). The long residence times (tens of years) of particulate organic carbon from high-latitude soils provide empirical evidence that if fluxes of carbon from vegetation to the soil increase, these soils have the capacity to act as a carbon sink on decadal timescales.