Invasive grasses threaten native grassland ecosystems by altering soil processes and outcompeting native species, yet the mechanisms driving their competitive advantage remain unclear. Understanding how soil microbiomes and nutrient availability influence competition is essential for improving restoration strategies. We conducted three studies to separate density effects, soil microbiome dependence, and direct interspecific competition to better understand how these factors may impact changes to grassland ecosystems. Across the studies we compared the growth and competitive interactions of little bluestem [Schizachyrium scoparium (Michx.) Nash], a native North American C₄ grass, and yellow bluestem [also King Ranch bluestem; Bothriochloa ischaemum (L.) Keng var. songarica], an introduced C₄ grass from Afro-Eurasia, in three greenhouse experiments. In an evaluation of intraspecific competition across a range of densities (1 to 16 plants per pot), S. scoparium reached peak biomass at intermediate density (13.9 g per pot with 8 plants), while B. ischaemum achieved a much higher mass (22.7 g per pot) at the same density, indicating greater tolerance to crowding. Soil sterilization reduced biomass by 95% for S. scoparium and 89% for B. ischaemum, confirming the dependence of these species on soil microbiota. In mixtures, B. ischaemum produced more biomass than S. scoparium at all proportions. The relative competitive intensity (RCI)—a measure of the effect of competition relative to growth in isolation—was significantly greater for S. scoparium than for B. ischaemum, indicating stronger suppression of the native species. Differences in root morphology, with B. ischaemum exhibiting finer roots (0.35 ± 0.04 mm) than S. scoparium (0.63 ± 0.30 mm), may enhance nutrient foraging efficiency and contribute to the former’s competitive dominance. These results suggest that B. ischaemum’s success is driven by traits that promote resource acquisition and tolerance to crowding in low-nutrient soils.