An ongoing cropping systems study was initiated in 1988 at the NDSU Carrington Research Extension Center to determine the effects of annual crop rotation, tillage, and nitrogen (N) level and source on crop yield. Published data from this study indicated no interaction between crop yield, N source or tillage. We analyzed 18 years of soil test data to determine effects of N source and tillage on soil fertility parameters. During the 18 years, N treatments were 0, 45 (1X) and 90 (2X) kg N ha^-1 applied annually as ammonium nitrate and a once every four year application of composted beef feedlot manure (CBM) to supply 45 kg N ha^-1 annually. Triple-super-phosphate was applied twice throughout the study to non-manured plots based on soil analysis. Tillage treatments were conventional (T), minimum (M), and no-till (NT). Fall soil samples were taken annually from each plot, sectioned into 0-15, 15-30, and 30-60 cm depths, and analyzed for nitrate-N, Olsen P, and soil organic matter (SOM). Soil test data were analyzed using SAS GLM procedures with significant differences expressed at the P<0.05 level. Significant interactions were observed between tillage and SOM level with NT>M>T indicating that tillage and SOM levels are inversely related. However, SOM levels in the 0-15 cm depth decreased in all treatments over time indicating a net soil carbon (C) loss. A significant interaction was also observed between N source and rate and SOM level with CBM=1X>2X. Nitrogen rate and source significantly affected nitrate-N test levels regardless of sampling depth with 2X>CBM=1X. Olsen P levels were also significantly different with CBM>1X>2X at the 0-15 cm depth. These results indicate that long-term N additions at these rates and cropping systems based on annual crops have a negative impact on SOM levels and C sequestration regardless of tillage or N source.