Intraspecific variation in indirect plant–soil feedbacks influences a wetland plant invasion

Plant–soil feedbacks (PSFs) influence plant competition via direct interactions with pathogens and mutualists or indirectly via apparent competition/mutualisms (i.e., spillover to co-occurring plants) and soil legacy effects. It is currently unknown how intraspecific variation in PSFs interacts with the environment (e.g., nutrient availability) to influence competition between native and invasive plants. We conducted a fully crossed multi-factor greenhouse experiment to determine the effects of Phragmites australis rhizosphere soil biota, interspecific competition, and nutrient availability on biomass of replicate populations from one native and two invasive lineages of common reed (P. australis) and a single lineage of native smooth cordgrass (Spartina alterniflora). Harmful soil biota consistently dominated PSFs involving all three P. australis lineages, reducing biomass by 10%. Indirect PSFs (i.e., soil biota spillover) from the two invasive P. australis lineages reduced S. alterniflora biomass by 7%, whereas PSFs from the native P. australis lineage increased S. alterniflora biomass by 6%. Interestingly, interspecific competition and PSFs interacted to weaken their respective impacts on S. alterniflora, whereas they exerted synergistic negative effects on P. australis. Phragmites australis soil biota decreased S. alterniflora biomass when grown alone (i.e., a soil legacy), but increased S. alterniflora biomass when grown with P. australis, suggesting that P. australis recruits harmful generalist soil biota or facilitates S. alterniflora via spillover (i.e., apparent mutualism). Soil biota also reduced interspecific competition impacts on S. alterniflora, although it remained competitively inferior to P. australis across all treatments. Competitive interactions and responses to nutrients did not differ among P. australis lineages, indicating that interspecific competition and nutrient deposition may not be key drivers of P. australis invasion in North America. Although soil biota, interspecific competition, and nutrient availability appear to have no direct impact on the success of invasive P. australis lineages in North America, intraspecific lineage variation in indirect spillover and soil legacies from P. australis occur and may have important implications for co-occurring native species and restoration of invaded habitats. Our study integrates multiple factors linked to plant invasions, highlighting that indirect interactions are likely commonplace in influencing plant community dynamics and invasion success and impacts.