Coastal Wetland Carbon Processes in a Warmer Climate

Coastal wetlands are global hotspots of carbon storage, and locations where carbon and nitrogen cycles have a disproportionately large impact on land, water, and air in comparison to the area they occupy. The mechanisms by which coastal wetlands respond to elevated CO2 and warming are similar to other terrestrial ecosystems, but coastal wetlands are also unique because biological and physical responses to disturbance are strongly mediated by rates of sea level rise. High rates of carbon sequestration in these systems are the result of complex feedbacks between vegetation and the physical environment. Plants are 'ecosystem engineers' because they build soil elevation, and control aerobic and anaerobic microbial decomposition of organic matter. Such complex feedbacks are largely absent in present day global-scale forecast models. We will investigate the complex interactions between plants, soils, and climate that influence carbon cycling using an existing experiment that began in 2016, the Salt Marsh Accretion Response to Temperature eXperiment (SMARTX). SMARTX is a field experiment located at the Global Change Research Wetland, a 23 hectare brackish tidal marsh on the western shore of Chesapeake Bay in Maryland. The site is operated by the Smithsonian Environmental Research Center with support from the Department of Energy (the site's original sponsor), National Science Foundation, United States Geologic Survey, and National Oceanic and Atmospheric Administration for over three decades. Several other experiments located at the site will be leveraged to extend the data provided by SMARTX.

SMARTX is a whole-system experiment deployed in two plant communities, with warming from the plant canopy to 1.5 m depth. Warming treatments are applied as a gradient from ambient to +5.1°C. Elevated carbon dioxide is crossed at the temperature extremes in one of the plant communities. To date, we observed that warming and elevated carbon dioxide change shoot and root growth in ways that are most easily explained by a warming-induced increase in microbial decomposition leading to higher soil nitrogen availability. Warming also increased emissions of the greenhouse gas methane in SMARTX.

We will install a new system of soil sensors that continuously measure soil water content and salinity, which we hypothesize are controlling the plant growth response to warming. A new technique will be used to quantify soil organic matter decomposition rates and nitrogen availability to test hypotheses that explain changes in root versus shoot growth across the treatments. A new system of soil oxygen sensors will be installed to test hypotheses about the influence of root growth on methane production and emissions. Finally, stable isotope techniques will be used to gain insights on the mechanisms by which methane is produced, consumed, and emitted, and their response to our treatments. Data from SMARTX will be used to refine our ecosystem-scale marsh carbon model, and to modify an Earth system model to enable the first-ever representation of tidal wetland dynamics at a global scale. The result will be an improved ability to forecast the stability of coastal wetlands and the carbon they contain in the 21st century.