Land use change is an important driver of terrestrial carbon cycling in the United States and quantifying the effects of afforestation and deforestation on soil carbon is important for national and international assessments. We developed matrices representing area transitions over time between forest, agricultural, and urban land uses and incorporated the results in a model to estimate changes in soil and forest floor carbon stocks. Historical estimates of land use transitions were based on governmental forest and agricultural inventories. Future land use changes were based on projections of historical data and projections from the USDA Forest Service 2003 RPA timber assessment base run. Estimates of soil carbon stocks were derived primarily from the STATSGO database. The effects of specific land use changes on soil and forest floor carbon fluxes were based on data from the literature. For the conterminous US as a whole, there was net loss of carbon from 1900 until the early 1980s, and net gain (sequestration) of carbon from that time until the present. In the future, the model predicts continued net sequestration of carbon until the 2030s, with little net change after that time. With two exceptions (Northeast, Pacific Coast), regions within the US have similar dynamics of net carbon change over time, but different magnitudes. The pattern in the Northeast region is very different from all other regions, with a large, steady rate of carbon sequestration from 1900 to the present, and predicted continued sequestration through the 2030s. The cumulative effect of land use change in the Northeast is greater than any other region, and is greater than the effect of all regions combined. This effect occurs not because the rate of change is greater in the Northeast, but because there is always carbon sequestration rather than a pattern of net carbon loss for many decades, followed by a pattern of carbon sequestration for most other regions. These results reflect the predominant trend toward afforestation of marginal agricultural land that took place through most of the Northeast region during the 20th century. The Pacific Coast has the opposite pattern as does the Northeast – a steady loss of carbon from 1900 to 2050. The results of this model are intended to improve the U.S. Greenhouse Gas Inventory, which is produced annually to meet reporting requirements under the United Nations Framework Convention on Climate Change. From 1990 to 2004, afforestation caused sequestration of 231 Tg C, of which 81 Tg C was in the soil and 150 Tg C was in the forest floor. During this same period, deforestation caused emission of 175 Tg C, of which 50 Tg C was in the soil and 125 Tg C was in the forest floor. However, the net effect of land use change on carbon mass in soil and forest floor from 1990 to 2004 was about 25-fold smaller than the net change in carbon stocks in trees on all forestland during this time period. Thus land use change effects for the overall carbon budget for this period are dominated by changes in tree carbon stocks.