Global Biogeochemical Cycles

A global calculation of the δ13C of soil respired carbon: Implications for the biospheric uptake of anthropogenic CO2


  • P. Ciais,

  • P. Friedlingstein,

  • D. S. Schimel,

  • P. P. Tans


The continuing emissions of fossil CO2 depleted in 13C have been causing a gradual decrease in atmospheric δ13C by roughly 1.4‰ since preindustrial times. The progressive penetration of this perturbation into the land biota causes the soil organic matter to be enriched in 13C with respect to recently formed plant material. This effect which we call the “biotic isotope disequilibrium” is important when it comes to deducing the terrestrial carbon fluxes by using δ13C in atmospheric CO2. We have estimated the geographical distribution of the isotopic disequilibrium, which is primarily influenced by the turnover of carbon in the various ecosystems, from the output of two biospheric models, (SLAVE and CENTURY). The disequilibrium is estimated to shift up the δ13C of atmospheric CO2 by the same amount as a net sink of 0.6 Gt C yr−1 in the land biota. This “fake” terrestrial sink due to the isotopic disequilibrium is distributed mainly in northern midlatitudes (0.2 Gt C yr−1) and tropical forests (0.3 Gt C yr−1).