Rising sea levels, reduced precipitation in watersheds, and global increases in water consumption may result in widespread saltwater intrusion into tidal freshwater wetlands (TFWs). The movement of saline water into these historically freshwater ecosystems is likely to impact organic carbon (OC) cycling, including plant productivity, the decomposition of roots and litter, and the metabolism of soil microbial communities. Because organic matter accumulation is important in driving vertical accretion in TFWs, changes in OC storage have implications for the ability of these wetlands to track rising sea levels and will ultimately play a role in determining the long-term stability and persistence of these valuable coastal ecosystems. The overall goal of this project is to understand how saltwater intrusion will impact the fate of OC in TFWs. This goal will be addressed by collecting soils from a matrix of TFWs, to cover a range of soil types, plant communities, and potential responses to saltwater intrusion. The fate of exchangeable OC will be studied using a series of concentration, kinetic, and characterization measurements. The whole soil OC pool will be studied in incubations designed to determine total effects of saltwater intrusion on soil decomposition and the underlying biogeochemical pathways. Mesocosm experiments, using a C4 plant/C3 soil design, will allow the integrated system-level effects to saltwater intrusion to be determined, while also partitioning autotrophic and heterotrophic responses. Field measurements of soil CO2 will assess in situ effects on OC sources and ages.

The intellectual merit lies in a better understanding of how soil C pools in TFWs respond to saltwater intrusion and will provide insight into the mechanisms driving those responses. Since not all TFW soils respond similarly to saltwater intrusion, it is necessary to develop a framework that will explain the results of previous research and provide a tool to allow future saltwater intrusion effects to be confidently predicted across a range of sites. This framework will be developed through a series of experiments that incrementally include larger portions of the entire wetland soil+plant carbon pools and related biogeochemical transformations: 1) Assessing the amount, age, and metabolic fate of OC desorbed from TFW soils, 2) Determining the responses and mechanisms by which saltwater intrusion affects the mineralization of the whole soil OC pool, 3) Integrating research on DOC desorption, whole soil processes, and plant responses to assess the overall effects on wetland plant-soil systems and 4) Examining the influence of salinity regime, depth and season on the sources/ages of soil CO2. Thus, this research has implications for understanding and responding to the effects of climate change on TFWs toward the objective of maintaining the goods and services supplied by coastal marshes.

The broader impacts include a commitment to enhancing learning and understanding in students from the high school to graduate and postdoctoral level, and ensuring data availability/access to all parties. High school students will gain an understanding of the linkages between ecosystems and human activities through a partnership between VCU and Church Hill Academy (CHA), a Richmond-area institution with the specific goal of engaging minority students and developing leaders who will engage fellow students. Funding from NSF will allow this program to be enhanced by field excursions and lectures with a specific focus on fresh and saltwater marshes. In addition, CHA students will have the opportunity to develop group and independent research projects that may be presented at the local Atlantic Estuarine Research Society meeting and Virginia Junior Academy of Science. This project also will support undergraduate students for internships that will include independent research projects and exposure to multiple areas of ecosystem science. In recruiting students for these internships, historically black colleges and universities in South Carolina, an EPSCoR state, will be targeted. Additionally, a Masters and postdoctoral fellow will be supported by this grant, will gain experience with issues and methods in aquatic and wetland biogeochemistry, and will be trained in radiocarbon analysis at LLNL. All data generated by this project will be associated with standardized metadata and will be archived at the publicly-accessible 'Data Archives' website of the Baruch Marine Field Laboratory, where robust data management structures are in place.