Unlike their native counterparts, early-successional non-native plants often form dense, persistent populations on disturbed soils.  These disturbed, invaded soils may differ from undisturbed, uninvaded soils in ways that could explain this demographic success.  More specifically, historical disturbance or non-native plant growth may create soil legacies that increase non-native plant growth or decrease native plant growth.  It can be difficult, however, to separate the effects of disturbance from the effects of non-native plant growth on soils under field conditions because of the strong correlation between these two factors.  To distinguish these effects, soils were collected in a complete factorial design representing land-use legacies (disturbed or undisturbed soils) and plant-growth legacies (non-native or native cultivated soils) from 14 study sites in a shrub-steppe ecosystem.  The dominant non-native and native plant species were grown in these soils as a bioassay for legacy effects.  The microbial communities in these soils were analyzed to describe potential mechanisms through which legacies may affect plant growth.  Non-native cultivated soils demonstrated smaller microbial abundance, activity, and diversity than soils cultivated by native plants, suggesting that non-native plants suppressed the growth of microbial symbionts of the native plant.  Microbial abundance, activity, and diversity were also smaller in ex-arable than undisturbed soils, but this effect was small relative to the effect of plant-growth legacies.  The dominant native plant grew poorly in non-native cultivated soils, while no other plant growth measure differed between treatments.  The results suggest that non-natives are successful on soils with low microbial activity and these same plant species create soils with low microbial activity.  In summary, we found that disturbance confers a competitive advantage to non-natives by reducing microbial abundance and activity, and that non-natives perpetuate a competitive advantage by maintaining small microbial communities.