Technical Abstract: Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate (Rydb.) Boivin) ecosystems. Sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased carbon (C) storage on the landscape; however, little is currently known about the distribution of C on these landscapes, or the role that nitrogen (N) plays in carbon retention. As part of a Joint Fire Sciences funded project called the Sagebrush Treatment Evaluation Project (SageSTEP), we quantified the relationship between tree cover and soil C and N in expansion woodlands at 13 sites in Utah, Oregon, Idaho, California, and Nevada, USA. Each site contained three core plots, and within each core plot we sampled three sub-plots which represent a gradient of woodland expansion into sagebrush systems. One hundred and thirty soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm, or until bedrock or a restrictive layer was encountered. Soil, coarse fragments, and roots were analyzed for organic C and N, and data was expressed on a mass per unit area basis. Woodland expansion had significant affects on the total mass of root C and N, and influenced the vertical distribution of soil organic C. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store significant amounts of belowground C, and must be weighed against the risk of increased wildfire and exotic grass invasion.