Modeling the effect of elevated CO2 on root exudation and ecosystem carbon storage in mature forests

Increased atmospheric carbon dioxide (CO₂) is known to enhance  leaf-level photosynthesis. Following the carbon fertilization hypothesis, the increased photosynthetic assimilation may lead to an increase in plant biomass, therefore representing a negative feedback mechanism to rising CO₂ emissions. The magnitude and limitations of this effect remain one of the major uncertainties in projecting the future influence of increasing atmospheric CO₂ on terrestrial biogeochemical cycles and climate change.

Forests contribute strongly to the contemporary terrestrial carbon (C) sink, however, the magnitude of the effect of elevated carbon dioxide (eCO₂) on these ecosystems is still not fully understood. While experiments have demonstrated that young forests show increased aboveground biomass production under eCO2, the evidence for the effect on mature forests is still ambiguous. In these ecosystems, enhanced translocation of additional assimilated C belowground instead of investing in aboveground structure may significantly reduce C accumulation due to enhanced photosynthesis. One key mechanism in this process is exudation of C via roots into soil, which can increase nutrient availability to plants but also leads to soil C losses. By using a terrestrial biosphere model (QUINCY), which comprises a representation of the coupled carbon-nitrogen-phosphorus cycles in terrestrial ecosystems, we simulate the effect of elevated CO₂ on exudation and its consequences for the C cycling and storage in mature forests. In comparison to other existing models we calculate exudation rates dynamic, based on plant carbon surplus and nutrient demand. We evaluate the effect using alternative representations of soil biogeochemical processes at the example of the EucFACE experiment in a mature Eucalyptus forest. We show that nutrient-stress induced increases in the exudation rate under eCO₂ partially explains higher soil respiration and therefore lower C accumulation in this forest ecosystem.