The 'overflow tap' theory: linking GPP to forest soil carbon dynamics and the mycorrhizal component

Quantifying soil organic carbon (SOC) dynamics accurately is crucial to underpin better predictions of future climate change feedbacks within the atmosphere-vegetation-soil system. Measuring the components of ecosystem carbon fluxes has become a central point of the research focus during the last decade, not least because of the large SOC stocks, potentially vulnerable to climate change. However, our basic understanding of the composition and environmental responses of the soil CO2 efflux is still under debate and limited by the available field methodologies. For example, only recently did we separate successfully root (R), mycorrhizal fungal (F) and soil animal/microbial (H) respiration based on a mesh-bag/collar methodology and described their unique environmental responses. Yet it might be these differences which are crucial for understanding C-cycle feedbacks and observed limitations in plant biomass increase under elevated carbon dioxide (e.g. FACE) studies. It is becoming clear that these flux components and their environmental responses must be incorporated in models that link but also treat the heterotrophic and autotrophic fluxes separately. However, owing to a scarcity of experimental data, separation of fluxes and environmental drivers has been ignored in current models. We are now in a position to parameterize realistic soil C turnover models that include both, decomposition and plant-derived fluxes. Such models will allow (1) a direct comparison of model output to field data for all flux components, (2) include the potential to link plant C allocation to the rhizosphere with increased decomposition activity through soil C priming, and (3) to explore the potential of plant biomass C sequestration limitations under increased C assimilation. These mechanisms are fundamental in describing the stability of future SOC stocks due to elevated temperatures and carbon dioxide, altering SOC decomposition directly and indirectly through changes in plant productivity. The work presented here focuses on three critical areas: