Dear Rebecca,

Respired CO2 and CO2 from the breakdown of organic matter rapidly
equilibrate with soil water. The diffusion of CO2 out of the soil
produces a slight depletion because the light isotopic species diffuse
faster. So, my opinion is, the original oxygene isotopic composition of
CO2 is not accessible.
You may want to have a look at Tans' paper in Tellus 50B, (1998)
p163-178: "Oxygen isotopic equilibrium between carbon dioxide and water
in soils."

Regards,

Thorsten Agemar.