Conventional tillage (CT) with high N rates and irrigation is used more frequently than no-till (NT) for growing continuous corn (Zea mays L.) in the central Great Plains of the United States. The objective of this study was to evaluate soil organic C (SOC) stocks throughout the soil profi le as well as the potential for maintaining or sequestering SOC within the soil profile (0– 120 cm) under irrigated, continuous corn as affected by NT and CT and three N rates. Isotopic δ13C techniques provided information about the fate of C added to soil by corn (C4–C) and of residual C3–C from cool-season plants grown before this study. Relative contributions of C4–C and C3–C to SOC stocks after 8 yr were determined. Retention of C4–C from corn was measured under NT and CT. Nitrogen fertilization slowed losses of C3–C and improved retention of C4–C. No-till was superior to CT in maintaining SOC. Deep soil sampling to 120 cm and the use of stable C isotope techniques allowed evaluation of changes in SOC stocks during the 8-yr period.

Change in SOC under NT vs. CT resulted from greater loss of C3–C stocks under CT throughout the soil profi le. Irrigated corn has a low potential to sequester SOC in the central Great Plains, especially under CT. The results of this study indicate that stability of the soil organic matter and its perceived “recalcitrance” is altered by environmental and biological controls.