Mounting evidence suggests that microbial symbionts can play a crucial role in promoting plant invasion. Still, ecologists lack a complete understanding of how microbially-mediated invasion mechanisms operate across different abiotic and biotic contexts. In this dissertation, I characterize the microbial communities associated with and invasive grass, Phragmites australis, and native marsh species in coastal Louisiana, and examine how saltwater intrusion may differentially alter microbially-mediated interactions among these species. In a field survey, I found that P. australis associates with root and soil fungal communities with higher richness, diversity, and pathogen abundances compared to native species; however, I found no evidence of compositional changes or pathogen spillover in native species in close proximity to the invasion front. Additionally, in a greenhouse mesocosm experiment, I found that salinity-induced changes in microbial communities may enhance P. australis invasion in freshwater wetlands, but the magnitude of these effects depended on plant community context. Finally, using a plant-soil feedback experiment, I found that salinity and microbes adapted to salinity synergistically promote native coexistence in native communities, but may facilitate invasive dominance in invaded communities. This dissertation demonstrates that outcomes of plant-microbe interactions may shift across heterogenous abiotic and biotic landscapes. Furthermore, results presented here may help in predicting how saltwater intrusion will influence microbially-mediated invasion dynamics as global change progresses.