Plants can fix co2 without light, but it requires an enzyme, ATP, and NADPH produced in the light dependant stages of photosynthesis..
From what I've read, you need high levels of CO2 in the air (not roots) to increase the productivity of the Calvin-Benson cycle, but the increase is most drastic right near the co2 toxicity level..
These same light independant co2 fixing pathways also take up o2.. Its called photorespiration, and its not that great.. Different sugars are formed, and these create toxic compounds that can actually inhibit photosynthesis..
I wonder if the co2 toxicity is related to the o2 though.. I wonder if you could supass the limits in an inert co2/argon environment etc..
A bit of a tangent I guess, I just wanted to point out that even the Calvin-Benson cycle occurred in the leaves where the ATP etc is..