Technical Abstract: Reforestation or afforestation of marginal agricultural lands offers opportunities to sequester soil organic carbon (SOC) and improve the quality of degraded soils. The objectives of this study were to identify the extent and distribution of marginally productive cropland that was originally under forest cover in the state of Iowa and to quantify the changes in SOC and relevant soil properties following tree planting. A GIS analysis was used to identify marginal cropland based on a productivity index and vulnerability to erosion. Based on these criteria, 1.05 million ha (7.2%) of the state’s land area was identified as marginal for crop production. Four locations were identified to provide a representative sample of marginal lands from which soil samples were collected from recent (< 35 yr-old) forest plantations and adjacent crop fields. Soil samples were analyzed for SOC, total nitrogen (TN), pH, cation exchange capacity (CEC), Mehlich 3 extractable nutrients, and particle size. At all locations, significantly greater concentrations of SOC were observed in 0-30 cm depth of the forested soils when compared to tilled crop fields. The forested soils had 30.0+/-5.1% (mean+/-standard error) more SOC than the tilled cropland. The overall average annual change in SOC following tree planting was 0.20+/-0.01 Mg C m-2 yr-1 with no significant difference between coniferous or deciduous species. Changes were observed in soil bulk density, pH, CEC, and nutrient content but strong correlations with SOC content were only observed for bulk density and Mehlich 3 Ca. Further study should be engaged to quantity the effects on hydrology and soil biology associated with the increase in SOC.