Abstract: Many factors, including land use, management history, soil type, climate, and soil landscape processes, affect the dynamics of soil organic carbon (SOC). The primary objective of this research was to determine the SOC level in a timberland of west-central Illinois and then to compare the level with the SOC levels in adjacent cropland and pastureland after 150 years of agricultural use. The SOC concentration of various soil layers, to a depth of 0.5 m, was measured. The timberland contained 68.3 Mg C ha−1 in 2009. Results suggested that the cropland landscape retained 59.7 Mg C ha−1 or 87% of the total SOC in the timberland on a volumetric basis after the last 150 years of cultivation and cropland use. Soil erosion and sediment transport resulted in some SOC being deposited in lower landscape positions but retained in landscape. The other 13% net loss of the SOC storage was either deposited in the water or released to the atmosphere. The last 90 years of pasture use increased the SOC of the site as a result of C loading in the form of manure, which resulted in increased SOC storage and offset all previous losses of SOC caused by cultivation, soil erosion, and aeration. The current pastureland had 34% more SOC than the timberland in 2009 and 53% more than the cropland. These results suggest that if west-central Illinois timberland soils were cleared and used for cropland, cultivation and subsequent soil erosion and transported C-rich sediment will result in SOC redistribution on landscape with greater SOC retention in lower landscape positions, with SOC being released to streams or to the atmosphere. However, if timberland is converted to pastureland, significant amounts of SOC and soil organic N (SON) would be stored in soil as humus. Conversion of timberland to cropland reduced SOC storage, and conversion to pastureland increased SOC storage and retention.