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

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 terrestial 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).