Technical Abstract: Elevated atmospheric CO2 can affect both the quantity and quality of plant tissues, which will impact the cycling and storage of carbon within plant/soil systems and the rate of CO2 release back to the atmosphere. Research is needed to more accurately quantify the effects of elevated CO2 on soil CO2 efflux in order to predict the potential of terrestrial ecosystems to store carbon in soil. Effects of elevated atmospheric CO2 on soil CO2 efflux were examined in a long-term study comparing row crops managed as either a conventional (grain sorghum and soybean were rotated each year using conventional tillage practices and winter fallow) or a conservation (grain sorghum-soybean rotation, with three winter cover crops - wheat, crimson clover, and sunn hemp - grown using "no-till" practices) tillage system. Plants were exposed to either 365 ppm (ambient) or 725 ppm (elevated) levels of atmospheric CO2 using open top field chambers. Soil CO2 efflux, over a full two-year cropping cycle, was increased by both elevated atmospheric CO2 and by conservation management; in both cases soil CO2 efflux was increased by approximately 50 % due to these treatments. These increases were due, primarily, to increased biomass inputs from these treatments. Despite higher levels of soil CO2 efflux, these systems may still increase soil carbon storage and help mitigate the rising atmospheric CO2 concentration. The total amount of carbon fixed by plants, either due to elevated atmospheric CO2 and/or use of no-till management with cover crops (as in our conservation system) can far exceed that released back to the atmosphere as soil CO2 efflux. [GRACEnet Publication]