Technical Abstract: Elevated atmospheric carbon dioxide and ozone concentrations often have counteracting influences on many C3 crops depending on the concentration and sensitivity of the crop to the gases. Effects of the gas treatments on plant growth likely extend to soil microbial and decomposition processes, although changes in belowground growth, microbiology and C dynamics have been little studied. The objective of this experiment was to determine the separate and combined effects of elevated carbon dioxide and ozone on growth, yield and soil C dynamics in a soybean-wheat no-till system. Plants were treated with either ambient or elevated carbon dioxide (550 µmol mol-1) in combination with charcoal-filtered (CF) air or CF air plus ozone (1.4 x ambient ozone) using open-top field chambers. Results showed that elevated carbon dioxide stimulated soybean and wheat biomass production at harvest by 25 to 30%, while seed yield was increased 10 to 25%. Soybean residue biomass and yield were suppressed by added ozone by 11 to 27%. Wheat residue biomass and yield were not significantly affected by added ozone. In the combined gas treatment, soybean residue biomass and yield were restored to control or higher levels. After two years, surface litter was accumulating in the elevated carbon dioxide treatments and decomposing more slowly in the added ozone treatment. Soil respiration and soil C levels were unchanged by the gas treatments although delta 13C values of the coarse organic matter fraction decreased at elevated carbon dioxide. The effects of ozone on plant growth and yield will likely be attenuated by elevated levels of carbon dioxide, and vice-versa. Plant responses to these gases in a conservation-till system were similar to previous studies conducted in conventional tillage system, although soil surface residues were slow to be incorporated into the soil, especially at elevated carbon dioxide.