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 C-13/C-12 ratio and ’bomb-spike’ C-14 Signature of carbon in each reservoir(1,2). A large proportion of the carbon in the terrestrial biosphere resides in the soil organic carbon (SOC) pool(3), most of which is derived from plants that assimilate carbon via the C-3 photosynthetic pathway(4). Here we report measurements of the C-13 and C-14 signatures of particulate organic carbon from surface soils of C-3 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 C-13. The C-14 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 C-13/C-12 ratio of atmospheric CO2 (for example by fossil-fuel burning(5)). The long residence times (tens of gears) 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.