An irrigated monoculture corn, monoculture soybean, and soybean-corn cropping systems study was initiated in 1991 on a uniform site in the Platte Valley near Shelton, Nebraska. The objective was to determine the long-term effects of these cropping systems on soil organic carbon levels. Four corn hybrids were tested in both the monoculture and rotation systems in combination with five N fertilizer rates. A similar approach was utilized for soybean in the monoculture and rotation systems. Prior to initiation of the study, the experimental area was grid sampled and various soil properties including soil organic carbon levels were determined. In 2005, soil samples were taken from all 3 cropping systems to a depth of 30 cm. Soil organic carbon results as affected by cropping system and N fertilizer rates after 14 years will be presented.