A model simulation of carbon dioxide and stable carbon isotope exchange between the atmosphere and terrestrial biosphere: Is the terrestrial d13C discrimination effect decreasing?

Terrestrial ecosystems exchange a huge amount of carbon with the atmosphere, causing seasonal, interannual, and even long-term variability in atmospheric CO2 concentration. They also affect stable carbon isotope composition of atmospheric CO2, because of photosynthetic discrimination against heavier stable carbon isotope (13C). In this study, atmosphere-biosphere exchange of CO2 was simulated with a process-based terrestrial carbon cycle model (Sim-CYCLE), including stable carbon isotopic features. The fractionation factor is affected by a couple of factors: photosynthetic pathway (C3/C4), stomatal gas diffusion, and recycling of respired CO2 within canopy. A long-term simulation from AD 1901 to 2000 suggested that average discrimination effect would be -16.5 per mill, including -1.8 per mil by canopy recycling. Apparently, C3 species had larger (more negative) discrimination effects than C4 species. It was remarkable that the discrimination factor became gradually small (less negative), from -16.65 per mill in 1900s to -16.40 per mill in 1990s. The decline of the discrimination effect was mainly attributable to stomatal response to elevated atmospheric CO2 concentration, reducing the ratio of intercellular (Ci) to ambient CO2 concentration (Ca). This stomatal response exceeded the competitive responses of C3/C4 composition and canopy recycling, both of which lead to larger discrimination effects. Importantly, the temporal change in the discrimination factor could induce an isotopic disequilibrium of the atmosphere-biosphere CO2 exchange, in addition to the disequilibrium induced by fossil fuel combustion (13C Suess effect). Including this factor, isotope-based analyses of the global carbon budget would be more or less amended.