Continental-Scale Distributions of Vegetation Stable Carbon Isotope Ratios

All terrestrial higher plants preferentially assimilate CO 2 relative to CO 2 during photosynthesis as a result of physical and biological isotope fractionation processes, with much larger overall C fractionation by plants using the C 3 photosynthetic pathway (approximate range of 14–27‰) compared to plants using the C 4 pathway (approximate range of 3–6‰). Fractionation against CO 2 in both C 3 and C 4 plants includes kinetic and equilibrium isotope effects during the processes of gaseous diffusion of CO 2 along concentration gradients from the atmosphere to the sub-stomatal intercellular spaces, subsequent hydration and diffusion to the site of carboxylation in the chloroplast, and ultimately CO 2 fixation by the Rubisco enzyme (Farquhar et al. 1989). The key differences between C 3 and C 4 plants in photosynthetic C fractionation are described in detail elsewhere (Farquhar 1983; Farquhar et al. 1982, 1989), but the lower fractionation by C 4 plants is primarily the result of a negative isotope fractionation during hydration of CO 2 at equilibrium, combined with a reduced effective fractionation by Rubisco, as the bundle sheath cells approximate a closed compartment. As a result of these differences, C 4 plants have considerably more enriched dC values compared to C 3 plants; for example, a survey of grasses by Cerling et al. (1997) reported mean C 3 and C 4 plant dC values of −26.7‰ and −12.2‰, respectively.