A latitudinal gradient in carbon turnover times in forest soils

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.