Wetlands support diverse plant and animal communities, and perform essential ecosystem services including improving water quality, storing floodwater, helping recharge groundwater aquifers, and recycling nutrients. Wetlands are also important components of the global carbon cycle because they can store large amounts of carbon in their soils and function as both sources and sinks of greenhouse gases such as carbon dioxide and methane.

Wetlands are very sensitive to disturbance, and the effects of global change on these ecosystems are already evident. For example, wetlands in coastal areas are experiencing stress due to sea level rise and more frequent flooding. These changes in hydrology can cause saltwater intrusion wherein saline water moves into ecosystems that have historically been freshwater. This elevated salinity, or "salinization," can stress wetland plants and, eventually, lead to changes in plant community composition and productivity. In addition, increases in salinity can affect the composition and activity of soil microbial communities and alter the types of chemical reactions that take place in soil. The goal of this project was to study the response of this entire plant-microbe-soil system to salinization, with specific focus on how saltwater intrusion into tidal freshwater wetlands can affect greenhouse gas emissions and carbon storage potential.

This research focused on tidal marshes of the Pamunkey/York River system in Virginia, a major tributary of the Chesapeake Bay, and used three approaches: (1) an experimental manipulation wherein we irrigated a freshwater wetland with saline water to mimic saltwater intrusion, (2) a soil transplant experiment where freshwater marsh soils were transplanted to a salt marsh, and (3) an observational study of several wetlands along a riverine salinity gradient. The first two approaches allowed us to study short-term effects of salinization (weeks to years); the latter helped us understand longer term (years to decades) effects.

Some components of the wetland microbial community responded very quickly (within 1 week) to increasing salinity, most likely due to physiological stress.  This initial response was followed by a rather long transition period during which time microbial community composition changed to include more salt-tolerant species with unique functional profiles. For the plant community, coverage, size, and diversity all decreased with salinization. Together, these shifts in microbial and plant communities altered wetland carbon biogeochemistry.

Our findings indicate that rates of methane production are highly sensitive to increasing salinity, with lower rates of methane production at higher salinities. With the gradual and low magnitude of salinity increase in our experimental manipulation, we did not observe any salinity-related changes in soil carbon dioxide production. We did, however, measure a decrease in ecosystem carbon dioxide emissions to the atmosphere, likely due to decreased respiration by the salt-stressed plant community. In contrast, transplanting soils from a freshwater to salt marsh caused a doubling of soil carbon dioxide production that was likely related to the immediate and large change in salinity. Although carbon dioxide and methane emissions to the atmosphere were reduced by saltwater intrusion, there was a similar reduction in plant productivity such that net ecosystem production, a proxy for soil carbon storage, did not change with salinization. This implies that salinization may not have an appreciable effect on the ability of tidal freshwater marshes to build soil and keep pace with rising sea levels.

Our project generated multiple peer-reviewed publications and conference presentations that contribute to ongoing efforts to understand wetland responses to saltwater intrusion and sea level rise. Our findings are also of value to global change ecologists who are developing better ways to incorporate microbial communities into ecosystem models of carbon cycling.

Beyond its contribution to scientific understanding, our project had a broad impact by providing training for 15 undergraduates, 5 graduate students, 3 postdoctoral associates, and 1 high school environmental science teacher.  We are especially proud of our engagement of undergraduates, of which 75% were female and 50% were first generation college students. These participants gained field and laboratory experience in wetlands ecology, microbiology, and molecular genetics, and acquired advanced skills in data analysis and scientific communication. Nearly all of these participants contributed to our public outreach efforts, which focused primarily on public education. We developed and taught a field-based workshop about environmental microbiology, wetland science, and carbon cycling to K-5 students. We also led workshops with two local high schools and delivered presentations to multiple community groups.

Last Modified: 03/22/2019
Modified by: Rima B Franklin