Elevated atmospheric CO₂ may lead to increased C sequestration in terrestrial ecosystems if the C is stored in long-lived pools such as organic matter and soil minerals.  Above- and belowground primary production were stimulated by elevated CO₂ in an open-top chamber (OTC) experiment on a semi-arid grassland ecosystem in northeastern Colorado. We evaluated responses of soil organic and inorganic C after five years of experimental treatment by excavating soil monoliths to 75-cm depth. Soil organic matter (SOM) was partitioned into water extractable, particulate and mineral associated portions, and organic and inorganic C content and isotopic composition were determined. We found no change in total SOM, but water soluble organic C was higher under the elevated CO₂ treatment. The storage rate of new organic C, estimated from stable C isotopes, averaged 30 g C m^-2 y^-1 in ambient chambers and 75 g C m^-2 y^-1 in elevated CO₂ chambers.  Inorganic C (IC) content increased substantially below 40-cm depth under elevated CO₂, amounting to a doubling of the inorganic C content after 5 years. IC storage rate averaged 27 g C m^-2 y^-1 in ambient and 52 g C m^-2 y^-1 in elevated chambers, demonstrating the importance of this C pool in semi-arid systems. Total soil N content decreased by 70 g m^-2 in the top 60-cm after 5 years of elevated CO₂, suggesting “mining” of soil N to support increased plant growth. Soil electrical conductivity and pH also changed in response to the 5-year OTC experiment. Elevated CO₂ thus affected the total soil environment, in addition to doubling C sequestration rates.