Invasive Phragmites australis Reshapes Soil Biogeochemistry and Microbial Community Structure in a Northeastern U.S. Salt Marsh

Background: Coastal salt marshes of the northeastern United States have experienced major compositional shifts due to invasive species. While previous research has shown that these invasions impact microbial composition and nutrient dynamics, a significant knowledge gap remains regarding taxonomic and biochemical changes. This study investigated the effects of invasive Phragmites australis on soil chemistry and microbial community structure in the Quinnipiac River salt marsh in southern Connecticut, comparing invaded zones to areas dominated by the native foundation species, Sporobolus alterniflorus (formerly Spartina alterniflora ). We employed 16S rRNA sequencing to characterize microbial taxa and measured soil organic carbon (SOC), total nitrogen (TN), carbon-to-nitrogen (C:N) ratio, as well as environmental variables. Results: Our findings reveal that although mean TN and SOC stocks did not differ significantly between native and non-native dominated areas, there were spatial and temporal distinctions in soil chemistry and microbial assemblages. Sporobolus soils had a significantly greater C:N ratio, which may favor slower organic matter decomposition leading to greater capacity for carbon sequestration. In contrast, Phragmites soils, with higher pH and bulk density, could support more copiotrophic microbial taxa. Microbial community structure showed a significant separation between the two vegetation types at all taxonomic levels, with plant type explaining between 9.1 and 13.5% of the community variation. While Sporobolus soils had greater diversity at broader taxonomic levels, Phragmites soils supported more diversity from Family to Genus levels. Conclusions: These results underscore the role of vegetation in shaping microbial ecology and soil function, providing crucial insights for the management and restoration of salt marsh ecosystems.