Plant-soil feedbacks (PSFs) can influence competitive interactions between plant species via direct interactions with pathogens and mutualists or indirectly via apparent competition/mutualism (spillover) and soil legacy effects. Presently, little is known about how the environment (e.g., nutrient availability) modifies PSFs to influence conspecific native and invasive plant species. In a greenhouse, we conducted a fully crossed multi-factor experiment to determine the effects of soil biota, interspecific competition, and nutrient availability on clonal growth (i.e., biomass) of three different source populations from one native and two invasive lineages of (Phragmites australis) and native smooth cordgrass (Spartina alterniflora). Harmful soil biota appears to consistently dominate PSFs involving all three P. australis lineages, reducing biomass by 10%, regardless of nutrient availability or the presence of S. alterniflora as a competitor. In contrast, spillover of soil biota derived from the rhizosphere of the two invasive lineages reduced S. alterniflora biomass by 7%, whereas soil biota from the native lineage increased biomass by 6%. Moreover, P. australis soil biota negatively affected S. alterniflora biomass when grown alone, suggesting generalist pathogens predominate soil biota from the P. australis rhizosphere, resulting in a soil legacy that indirectly impacts native species. However, when grown with P. australis, the effects of P. australis soil biota on S. alterniflora were surprisingly positive, indicating that beneficial soil biota may spillover (i.e., apparent mutualism) from P. australis to S. alterniflora, or that pathogenic soil biota simply prefer to interact with P. australis. Live P. australis soil biota also reduced negative impacts of interspecific competition on the biomass of S. alterniflora, which was competitively inferior to P. australis across all treatments. However, competitive interactions and the response to nutrients did not differ among P. australis lineages, indicating interspecific competition and nitrogen deposition may not be key drivers of P. australis invasion in North America. Taken together, these results suggest that while soil biota, interspecific competition, and nutrient availability appear to have no direct impact on invasion success of P. australis in North America, indirect soil legacies and spillover from P. australis may have important implications for co-occurring native species and restoration of sites invaded by P. australis.