Simulating Coastal Wetland Chemistry to Improve Methane Modeling
Coastal wetlands store large amounts of carbon in their sediments, but can also release methane, a potent greenhouse gas, as organic matter decomposes. This study used a new model that incorporated interactions between methane cycling and carbon, sulfur, and nitrogen cycles to predict methane fluxes from wetlands covering a range of salinity levels in the Mississippi Delta.
Current land models omit coastal processes, particularly the influence of saltwater, in their simulations of organic matter decomposition and methane production from wetlands. As a result, they may overestimate methane production from saline coastal wetlands. The new model developed for this study allows more accurate simulations of methane production across complex coastal landscapes.
Coastal wetlands can accumulate large amounts of carbon, commonly referred to as “blue carbon,” due to high vegetation productivity and waterlogged sediments. Organic matter under saturated conditions can also be decomposed into methane, which is a potent greenhouse gas. Surface methane emissions are sensitive to multiple factors that influence methane production, consumption, and transport from the subsurface to the surface. This study presents a new model that simulates key subsurface chemical reactions, including nitrification, denitrification, sulfate reduction, methane production, and methane consumption. The model was applied to wetland sites in the Mississippi Delta representing a range of water levels and salinities. Surface methane fluxes decreased from freshwater marsh to salt marsh sites due to a combination of lower methane production and higher subsurface methane consumption in wetlands with stronger saltwater influences. Rates of methane emission over short time scales depended on episodic gas bubbling events, which were sensitive to subsurface chemical interactions and drove high temporal variability in surface flux rates. The model was developed using the reactive transport simulator PFLOTRAN, and represents a step toward directly simulating redox biogeochemistry within the Energy Exascale Earth System Model (E3SM) Land Model (ELM).