Carbon budgeting in plant-soil mesocosms under elevated CO2: locally missing carbon?

Studies have suggested that more carbon is fixed due to a large increase in photosynthesis in plant-soil systems exposed to elevated CO2 than could su bsequently be found in plant biomass and soils - the locally missing carbon phenomenon. To further understand this phenomenon, an experiment was carri ed out using EcoCELLs which are open-flow, mass-balance systems at the meso cosm scale. Naturally occurring C-13 tracers were also used to separately m easure plant-derived carbon and soil-derived carbon. The experiment include d two EcoCELLs, one under ambient atmospheric CO2 and the other under eleva ted CO2 (ambient plus 350 mu L L-1). By matching carbon fluxes with carbon pools, the issue of locally missing carbon was investigated. Flux-based net primary production (NPPf) was similar to pool-based primary production (NP Pp) under ambient CO2, and the discrepancy between the two carbon budgets ( 12 g C m(-2), or 4% of NPPf) was less than measurement errors. Therefore, v irtually all carbon entering the system under ambient CO2 was accounted for at the end of the experiment. Under elevated CO2, however, the amount of N PPf was much higher than NPPp, resulting in missing carbon of approximately 80 g C m(-2) or 19% of NPPf which was much higher than measurement errors. This was additional to the 96% increase in rhizosphere respiration and the 50% increase in root growth, two important components of locally missing c arbon. The mystery of locally missing carbon under elevated CO2 remains to be further investigated. Volatile organic carbon, carbon loss due to root w ashing, and measurement errors are discussed as some of the potential contr ibuting factors.