Saltwater intrusion due to global change is expected to have a detrimental effect on the biogeochemistry of tidal freshwater wetlands (TFW). Salinization can alter the role these ecosystems in the global carbon cycle by causing shifts in microbial metabolism that alter greenhouse gas emissions and increase carbon mineralization rates. Microorganisms are critical decomposers in wetland ecosystems, however our understanding of how wetland microbial community dynamics will respond to saltwater intrusion is limited. To address this knowledge gap and gain insight into how microbial communities respond to increased salinity we performed an in situ soil transplant experiment. The short term (1 week, 1 month, and 3 months) and long term (1 year) responses of prokaryotes and fungi were assessed by measuring functional gene abundances and sequencing phylogenetic markers. Coupled with these microbial community analyses, we also assessed the functional response by comparing rates CO2/CH4production. By examining changes in microbial dynamics, we hope to elucidate how saltwater intrusion will impact the overall carbon mineralization in TFW.
Saltwater exposure had an immediate effect on potential rates of carbon mineralization; overall, CO2 production doubled and CH4 production decreased by ~20-fold. These changes persisted over time and suggest a shift in the terminal step of organic matter degradation from methanogenesis to sulfate reduction. Conversely, changes in the prokaryotic community composition developed gradually with duration of salinity exposure. However, even after a year of exposure, the transplanted communities still resembled the freshwater origin more than the saltwater host communities. There was no clear pattern in the fungal composition with salinity. Multiple regression revealed sulfate concentration as well as bacterial, fungal and archaeal abundances to be strong predictors of soil respiration. Taken together, results from this study indicate that the response of tidal freshwater wetlands to salinization is driven by complex interactions of microbially-mediated processes that are dependent on the duration of exposure. Impacts of saltwater intrusion of wetland biogeochemistry are likely to manifest rapidly despite slower shifts in microbial community composition. However, the gradual changes in microbial community structure suggests that previously freshwater wetlands may not experience an equilibration of ecosystem function until long after initial saltwater intrusion.