Department of Earth System Science

The residence times of carbon in plants, litter, and soils are required for partitioning land and ocean sinks using measurements of atmospheric δ¹³C and also for estimating terrestrial carbon storage in response to net primary production (NPP) stimulation by elevated levels of atmospheric CO₂. While ¹³C-based calculations of the land sink decline with increasing estimates of terrestrial carbon residence times (through the fossil fuel-induced isotopic disequilibrium term in equations describing the global atmospheric budgets of ¹³CO₂ and CO₂), estimates of land sinks based on CO₂ fertilization of plant growth are directly proportional to carbon residence times. Here we used a single model of terrestrial carbon turnover, the Carnegie–Ames–Stanford Approach (CASA) biogeochemical model, to simultaneously estimate 1984–1990 terrestrial carbon storage using both approaches. Our goal was to identify the fraction of the ¹³CO₂-based land sink attributable to CO₂fertilization. Uptake from CO₂ fertilization was calculated using a β factor of 0.46 to describe the response of NPP to increasing concentrations of atmospheric CO₂ from 1765 to 1990. Given commonly used parameters in the ¹³C-based sink calculation and assuming a deforestation flux of 0.8 Pg C/y, CO₂ fertilization accounts for 54% of the missing terrestrial carbon sink from 1984 to 1990. CO₂ fertilization can account for all of the missing terrestrial sink only when the terrestrial mean residence time (MRT) and the land isodisequilibrium forcing are greater than many recent estimates.