Measuring and Modeling Methane Emissions at the Coastal Terrestrial-Aquatic Interface

Authors

Genevieve Noyce¹ (noyceg@si.edu), Alia Al-Haj¹*, Teri O’Meara², Roy Rich¹

Institutions

¹Smithsonian Environmental Research Center, Edgewater, MD; ²Oak Ridge National Laboratory, Oak Ridge, TN

Abstract

Incorporating the coastal terrestrial-aquatic interface (TAI) into Earth systems models requires a better mechanistic understanding of the biogeochemical processes that regulate greenhouse gas emissions, including methane (CH₄). In 2021, researchers started a model-experiment (ModEx) project to address some of these knowledge gaps that includes automated measurements of CH₄ emissions in a coastal wetland, a mesocosm warming × salinity × flooding experiment, and updates to the TAI configuration of PFLOTRAN.

The network of 12 automated chambers in Smithsonian’s Global Change Research Wetland has been operating continuously since April 2021, with soil heating added in February 2022. The high frequency of these flux measurements mean that researchers can look at responses to drivers that are both cyclical (diurnal, tidal) or stochastic (heat waves, storm events). Researchers found that CH₄ emissions are higher from warmer, wetter soils, as expected, but also that emissions are consistently higher at night than during the day, regardless of environmental conditions.

Using these field data, researchers were able to update the PFLOTRAN configuration to incorporate periodic hydrological cycling, including diurnal cycles. While previous simulations did not alter plant-mediated transport to account for respiration at night, researchers were able to use the empirical data to incorporate the simultaneous effects of day/night and tides, allowing the model to match the dynamic the team captured in the autochamber measurements. Using this updated model structure, researchers found that the indirect effects of environmental stress on vegetation can produce greater shifts in biogeochemical processing capacity than the direct effects on biogeochemical processes.

The mesocosm experiment was designed to understand some of these underlying mechanisms, by including pots with and without vegetation at two sites of differing salinity. As expected, vegetated pots from the freshwater marsh released about 10 times more CH₄ than vegetated pots at the brackish marsh and about 10 to 100 times more CH₄ than bare pots at both sites. Surprisingly, sea level rise and warming increased CH₄ emissions from the brackish site but had minimal to no impact on CH₄ emissions from the freshwater site. These relationships are either a function of carbon limitation at the freshwater marsh or of mechanistic differences in CH₄ production and consumption between the sites, both of which researchers will investigate.