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The objective of this 20-year tillage study was to quantify the amount and rates of soil organic carbon (SOC) storage and retention as a result of a conversion to no-till (NT) or chisel plow (CP) tillage systems using the comparison method with moldboard plow (MP) tillage SOC data as baseline. The NT and CP plots did store and retain 8.4 Mg C ha−1 and 0.6 Mg C ha−1 more SOC in the soil than MP. That SOC amount was retained in the soil and not decomposed and re-emitted to the atmosphere as a result of cultivation or in the transported sediment moved off of the plots. However, no SOC sequestration occurred in the NT, CP, and MP plots because the SOC level of the plot area was higher at the start of the experiment than at the end of the study. Pretreatment SOC baseline of the plot area was used to determine NT, CP, and MP SOC sequestration or loss. The NT plots actually lost a total of −6.8 Mg C ha−1, the CP lost −15.1 Mg C ha−1, and the MP lost −15.2 Mg C ha−1 during the 20-year study. Soil erosion and transport of SOC rich sediment off of the sloping plots contributed substantially (29%-39%) to these SOC losses. These findings suggest that determination of a pretreatment SOC baseline is essential in all tillage comparison studies to determine the amount and rate of SOC sequestration or loss.