Plant surplus carbon underlies belowground carbon fluxes

I propose that patterns of belowground carbon flux observed under various environmental conditions can be largely explained by plant production of ‘surplus carbon’. Under common environmental conditions such as moderate deficiencies of water, nitrogen or phosphorus, high light, low temperatures, or elevated atmospheric carbon dioxide concentrations, plant leaf cells produce more photo-assimilates than they are able to use for primary metabolism, and so have surplus fixed carbon. Accumulation of surplus carbohydrates can damage leaf cells and so must be either transformed to other compounds or removed from the leaf. Active carbohydrate sinks are essential for the transport and removal of surplus C. Moderate deficiencies of N or P do not interfere with phloem loading, so much of the surplus C can be transported through the phloem, eventually reaching the roots. Active sinks for surplus carbon in roots include phosphorylated and non-phosphorylated respiration, conversion to starch, transfer to mycorrhizal fungi, or carboxylation to malate which is exuded or taken up by bacteria both inside and outside the root. These active sinks prevent metabolite accumulation and feedback inhibition of photosynthesis. The foundational benefit of belowground C fluxes and transfers to root-associated organisms may be assisting with the removal of surplus fixed carbon.