Background and aims <p>Soil biota can promote or hinder plant coexistence and alien plant invasion, depending on the relationship between soil-biota effects and plants’ competitive abilities. In the presence of competitors, soil-biota effects on a plant comprise soil-legacy effects from prior plants and overall soil-biota-mediated indirect effects (SMIEs) of current competitors. However, no study has yet disentangled the two effects, and their relationships with competitive abilities remain unclear.</p> Methods <p>We grew 14 native and 14 alien species in sterilized or non-sterilized conspecific soils, alone or with a native (<i>Pterocypsela indica</i>) or alien (<i>Solidago canadensis</i>) competitor. We separated conspecific soil-legacy effects from heterospecific SMIEs, and examined their relationships with intrinsic competitive ability (estimated when grown in sterilized soil).</p> Results <p>We show that the effect of resource-mediated competition exceeded overall soil-biota effect on focal plants’ growth. Notably, heterospecific SMIEs, not conspecific soil-legacy effects, were negatively correlated with focal plants’ intrinsic competitive abilities, resulting in stronger negative overall soil-biota effects for highly competitive species. Moreover, native focal plants experienced negative and neutral soil effects, primarily driven by variation in heterospecific SMIEs, in the presence of invasive and native competitor, respectively.</p> Conclusions <p>Our study provides the first exploration of the relationships among soil-legacy effects of conspecific plants, and resource-mediated competition and SMIEs by heterospecific competitor. Our findings suggest that soil biota, via SMIEs of competitors, could reduce the advantage of having high intrinsic competitive abilities, with potential consequences for plant invasions and species coexistence.</p>