Sea level rise and saltwater intrusion affect soil microbial communities and carbon biogeochemistry in tidal freshwater wetlands

Background/Question/Methods

One of the more certain effects of global climate change is the continued rise in sea level, which can lead to more frequent flooding and saltwater intrusion into tidal freshwater wetlands; even modest changes can alter plant community composition, water chemistry, and soil redox status. Most prior studies of these phenomena have focused on biogeochemistry and ecosystem function, while relatively little research has considered the soil microbial communities that govern these responses.  The goal of our research is to link a metagenomics-based characterization of soil microbial communities with process-level measurements of important ecosystem carbon transformations, and to examine their collective responses to environmental change.  This research primarily focuses on an on-going in situ field manipulation at a pristine freshwater wetland in the Pamunkey River (Virginia, USA). A solar-powered automated pumping system dispenses either brackish or fresh river water onto experimental plots at low tide during the growing season to simulate increased salinity and inundation due to sea level rise.  We are also conducting a seasonal survey of five sites along the existing salinity gradient in the river, which serves as a space-for-time substitution for future saltwater intrusion and a contextual reference for our manipulative studies.

Results/Conclusions

Throughout the field manipulation, we monitored soil biogeochemistry, process rates, and ecosystem gas exchange (carbon dioxide and methane flux). These measurements were linked with a comprehensive study of the diversity, composition, and functional potential of the soil microbial community, as well as an analysis of expression levels for key functional genes associated with anaerobic carbon mineralization (methanogenesis, sulfate reduction, and iron reduction). We found that the contribution of different microbial processes to carbon transformations changed over both temporal (field manipulation) and spatial (transect survey) scales.  These changes could be correlated with functional gene expression/abundance as the metabolically active microbial community undergoes a succession with changing salinity and redox conditions. This research provides insight into the complex interactions between vegetation, microbial communities, biogeochemical transformations, and ecosystem processes, and is essential for the development of more holistic conceptual models of wetland response to long-term changes in environmental conditions.