Technical Abstract: Crop biomass has been proposed as a source stock for bioethanol production. Acceptable levels of crop residue removal must be determined to prevent loss of soil organic carbon (SOC) and the degradation of soil physical and chemical properties resulting from SOC loss. Crop residue inputs and changes in SOC were evaluated for seven cropping seasons within an irrigation-tillage-crop rotation study at Akron, Colorado. Tillage treatments included a chisel plow treatment and a no-till treatment. A crop rotation alternating grasses and broadleaf crops was compared with continuous corn (Zea mays L.). Irrigation treatments included water application to meet evapotranspiration demand or application only during the reproductive stage of each crop. Total biomass production varied from 30 Mg ha-1 for the delayed irrigation, crop rotation plots to 74 Mg ha-1 for the fully irrigated, continuous corn plots during the seven cropping seasons. The change in SOC in the surface 30 cm of soil varied from -0.8 Mg SOC ha-1 for the rotation plots to a gain of 2.8 Mg ha-1 for the continuous corn plots after 7 yr. Correlating crop residue input with change in SOC showed that about 5.9 Mg ha-1 yr-1 crop residue addition is needed to maintain SOC levels for no-till cropping systems and an average of 8.7 Mg ha-1 yr-1 crop residue addition is needed to maintain SOC levels under chisel tillage. Continuous corn was the only system that consistently provided sufficient crop residue to maintain SOC levels. The quantity of crop residue available for off-farm use is much less than the total production of residue. Thus, feasibility studies for bio-energy production using crop residues should account for the lower amounts of residue that can be harvested without decreasing SOC levels.