Crop biomass has been proposed as a source stock for bioethanol production. Levels of crop residue removal must be determined to prevent degradation of soil physical and chemical properties resulting from soil organic carbon (SOC) loss. Carbon inputs from crop residues and an estimate of inputs from roots and rhizodeposition (C return) were calculated and compared with changes in SOC after seven cropping seasons at Akron, CO. Tillage treatments included a chisel plow (CP) and a no-till (NT) 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 C return varied from 25 Mg ha -1 for the delayed irrigation, crop rotation plots to 63 Mg ha -1 for the fully irrigated, continuous corn plots. 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 4.6 Mg ha -1 yr -1 C return is needed to maintain SOC levels for NT cropping systems and an average of 7.4 Mg ha -1 yr -1 C return 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. Residue removal for off-farm use should consider only amounts that can be harvested without decreasing SOC levels.
