A Model of the Spatiotemporal Dynamics of Soil Carbon Following Coastal Wetland Loss Applied to a Louisiana Salt Marsh in the Mississippi River Deltaic Plain.

The potential for carbon sequestration in coastal wetlands is high due to protection of carbon (C) in flooded soils. However, excessive flooding can result in the conversion of the vegetated wetland to open water. This transition results in the loss of wetland habitat in addition to the potential loss of soil carbon. Thus, in areas experiencing rapid wetland submergence, such as the Mississippi River Delta, coastal wetlands could become a significant source of carbon emissions if land loss is not mitigated. To accurately assess the capacity of wetlands to store (or emit) carbon in dynamic environments, it is critical to understand the fate of soil carbon following the transition from vegetated wetland to open water. We developed a simple soil carbon model representing soil depths to 1 m using the data collected from a Louisiana coastal salt marsh in the Mississippi River Deltaic Plain to predict soil carbon density and stock following the transition from a vegetated salt marsh to an open water pond. While immediate effects of ponding on the distribution of carbon within the 1‐m soil profile were apparent, there were no effects of ponding on the overall, integrated, carbon stocks 14 years, following wetland submergence. Rather, the model predicts that soil carbon losses in the first meter will be realized over long periods of time (∼200 years) due to changes in the source of carbon (biomass vs. mineral sediment) with minimal losses through mineralization. Plain Language Summary: In nature, many processes, while dynamic, are thought to settle to a stable state. For example, coastal wetlands are dynamic systems, continually changing in response to environmental conditions, such as sea‐level rise, including submergence of the wetland. We use the model to ask what happens to the steady state of soil carbon if the wetland is lost to submergence. We use parameters that specify environmental conditions to assess how the steady state changes, how long the system takes to reach the new steady state, and the fate of the carbon as it changes, how much is buried, and how much lost to the atmosphere. The answer to these questions has important implications for understanding the contribution of coastal wetlands to the global carbon cycle as wetlands are lost to rising sea levels. Key Points: We use a dynamic model of wetland soil processes to understand the fate of soil carbon upon land loss by submergenceThe key parameters controlling the changing patterns of soil carbon are related to how humification and mineralization vary with depthFitting the model to a Louisiana coastal salt marsh suggests that the timescale of changes after submergence is long, about 200 years [ABSTRACT FROM AUTHOR]