EOMF-14. Using Observed Carbon Residence Times to Improve Simulation of Total CO2 and 13CO2 Land Carbon Fluxes

Abstract
Land sequesters up to half of emitted carbon but land carbon cycle responses to climate variability remain uncertain and difficult to predict. Present-day land carbon cycle models include a variety of ecosystem processes linking moisture and climate to carbon pool dynamics and land-atmosphere biospheric exchanges. In this study, we explore these links using the Simple Biosphere Model version 4.2 (SiB4). SiB4 uses prognostic phenology to assimilate and allocate carbon during photosynthesis. Carbon is then transferred and stored in live and dead pools after accounting for growth and maintenance respiration costs. The residence time of carbon in ecosystems is an indicator of model performance and is additionally linked to an ecosystem’s capacity to act as a carbon sink. Studies comparing modeled and observed carbon residence times show that ecosystem models tend to underestimate biospheric residence times, and this is also true of SiB4. To address this bias, we developed a range of empirical tuning factors for respiration fluxes across a variety of ecosystems. We then apply these model improvements to explore interannual and seasonal variability in the carbon cycle using carbon isotopes (13CO2). While atmospheric CO2 observations trace NEE, 13CO2 traces mechanistic plant stress responses during photosynthesis through stomatal conductance and water use efficiency. However, a significant impediment to using atmospheric 13CO2 in this way originates from our lack of knowledge of terrestrial isotopic disequilibrium flux. This is the land-to-atmosphere 13CO2 flux that derives from the long-term decrease in the 13C:12C ratio of atmospheric CO2, and is tied to the age of biospheric pools from which carbon is released. In most representations of the atmospheric 13CO2 budget, terrestrial disequilibrium is not large enough to explain the observed atmospheric 13CO2:12CO2 trend. Improving simulation of residence times in terrestrial biosphere models should therefore also improve estimates of terrestrial isotopic disequilibrium. This, in turn, should improve our ability to use atmospheric 13CO2 to trace plant water stress. We will present results showing the relationships between improved representation of carbon residence times in SiB4 and changes in isotopic disequilibrium.