Ecological disturbance can affect carbon storage and stability and is a key consideration for managing lands to preserve or increase ecosystem carbon to ameliorate the greenhouse gas problem. Dryland soils are massive carbon reservoirs that are increasingly impacted by species invasions and altered fire regimes, including the exotic-grass-fire cycle in the extensive sagebrush steppe of North America. Direct measurement of total carbon in 1184 samples from landscapes of this region that differed in invasion and wildfire history revealed that their impacts depleted soil carbon by 42-49%, primarily in deep horizons, which could amount to 24.8-29.0 Tg carbon lost across the >500,000 ha affected annually. Disturbance effects on soil carbon stocks were not synergistic, suggesting that soil carbon was lowered to a base-level ‘floor’, beneath which further loss was unlikely. Restoration and maintenance of resilient dryland shrublands/rangelands could stabilize soil carbon at magnitudes that are relevant to the global carbon cycle.