Abstract: Global change is presenting ecosystems with novel environmental stressors, which impact biotic communities and biogeochemical cycling. Coastal freshwater wetlands provide important ecosystem services such as carbon sequestration, yet are threatened by increasing inundation with marine salts from saltwater intrusion and sea level rise. It is estimated that nearly 10% of forested wetlands in the North American coastal plain were lost between 1996 and 2016, with most forest transitioning to shrubland. Despite the scale and importance of these tree- to shrub-dominated community shifts, how plant and microbial community composition in these vulnerable systems changes following saltwater exposure—and the relationship of those shifts to edaphic changes—remains unresolved. In this study we sought to address 1) how plant and microbial community assemblages change with increasing marine salt exposure and 2) whether community shifts follow signals of salinization or other soil characteristics impacted by salt exposure. We established three transects representing space-for-time gradients of salt water exposure in the Albemarle-Pamlico Peninsula of North Carolina, USA. Along each transect, we surveyed plant community composition and collected soil cores from approximately 10 plots. From soil cores, we sequencing fungal (ITS2) and bacterial (16S) community DNA and analyzed soil properties, including salt ion concentration, pH, and C:N ratio. We found that soil characteristics varied predictably across our space-for-time gradients, with both ion concentration and pH being positively correlated with and C:N ratio being negatively correlated with saltwater exposure potential. Shifts in pH and C:N ratio were positively correlated with changes in salt ion concentration. In fitting soil chemistry variables to a non-metric multidimensional scaling ordination of both plant and microbial communities, we found that all three soil characteristics were significant predictors of understory plant community composition and fungal and bacterial species assemblages. We identified several “indicator” taxa, or those which contributed the most to patterns across plots. These findings elucidate the potential fine scale community effects of climate change-induced saltwater intrusion and sea level rise on plants and microbes in coastal wetlands. They provide a critical foundation for future work investigating the mechanisms of change and identifying possible mitigation strategies.
