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Cheatgrass (Bromus tectorum L.) is an invasive annual grass that increases wildfire frequency, degrades native ecosystems, and threatens agriculture across vast areas of the Western United States. This research examines how cheatgrass invasion may alter physical and biological properties of soils. Proliferation of very fine roots and high production of low-quality litter by cheatgrass increases porosity and near-surface microbial activity, which may enhance decomposition of soil organic matter (SOM) similar to cultivated systems. This may enlarge active SOM pools (mineral and microbial biomass C and N) at the expense of slow pools and humus. To test this hypothesis, soil properties beneath long-term cheatgrass-invaded areas were compared with carefully matched soils under shrub canopies and grass-covered interspaces at seven undisturbed Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) plant communities in northern Utah and southeastern Idaho. Soils under cheatgrass had (1) higher porosity in surface horizons, (2) higher concentrations of mineral N throughout the soil profiles at the time of sampling, and (3) a larger proportion of mineralizable C and N in total SOM of surface horizons than soil under native shrub-steppe plant communities. These results support our hypothesis and suggest that long-term cheatgrass invasion may alter ecological stability and resilience by depleting slow and passive SOM pools. This research will contribute to improved understanding of fundamental ecosystem processes required for successful ecological restoration.