Technical Abstract: Widespread cheatgrass (Bromus tectorum L) invasion represents a major shift in species dominance that may alter ecosystem processes across much of the western U.S. To investigate differences following such conversion, soil morphology and organic matter under cheatgrass-dominated and native shrub-steppe vegetation were compared by standard soil analysis procedures at seven paired sites in Idaho and Utah. Results suggest that, following conversion to cheatgrass dominance, increased porosity and labile organic inputs enhance microbial decomposition in near-surface horizons beneath cheatgrass compared to adjacent soils under native vegetation. Together with smaller inputs of less decomposable soil organic matter (SOM) from cheatgrass than native vegetation, enhanced decomposition could result in depletion of long-term SOM, leading to impoverished sites difficult to restore to native perennial vegetation.