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

Summary Studies have suggested that more carbon is fixed due to a large increase in photosynthesis in plant–soil systems exposed to elevated CO 2 than could subsequently be found in plant biomass and soils –‐ the locally missing carbon phenomenon. To further understand this phenomenon, an experiment was carried out using EcoCELLs which are open‐flow, mass‐balance systems at the mesocosm scale. Naturally occurring 13 C tracers were also used to separately measure plant‐derived carbon and soil‐derived carbon. The experiment included two EcoCELLs, one under ambient atmospheric CO 2 and the other under elevated CO 2 (ambient plus 350 μL L − 1 ). By matching carbon fluxes with carbon pools, the issue of locally missing carbon was investigated. Flux‐based net primary production ( NPP f ) was similar to pool‐based primary production ( NPP p ) under ambient CO 2 , and the discrepancy between the two carbon budgets (12 g C m − 2 , or 4% of NPP f ) was less than measurement errors. Therefore, virtually all carbon entering the system under ambient CO 2 was accounted for at the end of the experiment. Under elevated CO 2 , however, the amount of NPP f was much higher than NPP p , resulting in missing carbon of approximately 80 g C m − 2 or 19% of NPP f 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 carbon. The mystery of locally missing carbon under elevated CO 2 remains to be further investigated. Volatile organic carbon, carbon loss due to root washing, and measurement errors are discussed as some of the potential contributing factors.