Technical Abstract: Sequestration and storage of carbon (C) by agricultural soils has been cited as one potential part of the solution to soil degradation and global climate change. However, C sequestration in soils is an extremely slow and dynamic process. As a result, the objective was to evaluate the effects of crop rotation and N fertilizer management on changes in soil organic C (SOC) levels throughout eighteen years in a long-term study in the Western Corn Belt. Seven cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three levels of N fertilizer were compared. Soil samples were taken in the spring in 1984, 1992, 1998 and 2002 to a depth of 30-cm in 0 to 7.5, 7.5 to 15, and 15 to 30-cm increments. No differences were obtained in 1984, but rotation significantly affected SOC levels in the 0 to 7.5-cm depth in 1992, 1998, and 2002 and SOC levels in the 7.5 to 15 and 15 to 30-cm depths in 1998 and 2002. Nitrogen fertilizer level also significantly affected SOC levels in the 0 to 7.5-cm depth in 1998 and 2002. More importantly, SOC changes in the surface 30-cm indicated significant losses were occurring in all monoculture and 2-yr cropping systems, but not in the 4-yr cropping systems after 18 years. The results also indicated that when tillage depth was increased after 8-yrs by simply using a tandem disk with larger diameter disks that it resulted in a loss of SOC in several of the cropping systems from that point on. Sometimes, apparent small changes in management can have significant long-term results, but long-term experiments are required so these changes can be measured under current climate conditions.