New research by NCCOS scientists provides data to help fill a gap in our understanding of the fate of coastal wetland sediment carbon following disturbance events.

Stored organic carbon in Earth’s living vegetated biomass and preserved organic matter is enormous compared to the growth rate of atmospheric CO2. Among vegetated ecosystems, coastal wetlands, including salt marshes, have received attention for their “blue carbon” sediments that contain disproportionately large carbon stocks compared to their relative surface area. Organic matter (OM) formed by salt marshes can exist for millennia in sediments that are meters deep and are protected in place from photo-oxidation and aerobic microbial breakdown. This “blue carbon” in wetland sediments is a sink for atmospheric CO2 and a potential source of greenhouse gases (GHGs) if coastal habitats are lost.

Figure 1. Conceptual diagram depicting salt marsh carbon liberation and redistribution vectors. Vectors 1-2 illustrate processes that release sediment carbon from intact marshes. Vectors 3-6 show physical processes that move sediment to locations where decomposition can occur. Diagram by Quin Walker. NOAA NCCOS.

A missing gap in the role of coastal habitats in the global carbon cycle is understanding the fate of wetland sediment carbon following disturbance events, such as erosion. Disturbance events can release OM to an oxygenated environment where decomposition can more readily occur (Figure 1). Quantifying the range and temperature sensitivity of decomposition rates of this buried OM addresses a major knowledge gap for advancing blue carbon science.

The study investigated what proportion of salt marsh belowground organic carbon will re-mineralize to CO2 or CH4 after an erosion event. Salt marsh erosion under oxygenated conditions was simulated by incubating sediment organic matter from two depths in oxygenated chambers at two different temperatures and directly measuring CO2 and CH4 production. These results were used to estimate the amount of belowground carbon released annually via shoreline erosion in the contiguous US.

Study results showed that temperature rather than organic content and carbon/nitrogen ratios played the larger role in regulating salt marsh sediment organic matter decomposition; the warmer treatments of both sediment treatment depths decomposed faster than the lower temperature treatments. Other factors, such as water content, reduction/oxidation (anaerobic/aerobic) zonation, and plant-microbe interactions are also important factors that determine the decomposition of wetland OM.

The temperature sensitivity of salt marsh OM decomposition has implications in scaling up results to landscape or regional climate models, Temperature would have a positive feedback on CO2-induced warming since the deeper sediment that acts as a carbon reservoir prior to erosion becomes an increasing source of GHGs as it experiences warmer temperatures post-erosion.

To estimate the decomposition rates throughout the coastal US required extrapolation of the study results. Extrapolated decomposition rates are not an exact linear function of salt marsh shoreline due to regionally different temperature regimes. For example, North Carolina, South Carolina, and the Gulf Coast of Florida erode less C each year than Maryland, but each has higher total carbon emissions from decomposition since their coastal waters are warmer. Louisiana, with the longest marsh shoreline tallied here, erodes almost seven times more carbon annually than Virginia with the next longest marsh shoreline, but the warmer temperatures in Louisiana account for almost 10 times the carbon emissions (Figure 2).

Because few experimental data exist on the range of belowground organic carbon conversion to GHGs, this study provides empirical data to refine the range of potential emissions from the post-disturbance decomposition of salt marsh sediment carbon. These data will provide context for estimates of the effect of global warming on marsh carbon cycles.

Citation: McTigue, Nathan D., Quentin A. Walker and Carolyn A. Currin. 2021. Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition. Frontiers in Marine Science. https://doi.org/10.3389/fmars.2021.661442

Figure 2. Map of the Conterminous United States coast. Each station denoted by a circle is a NOAA CO-OPS temperature station colored by the amount of temperature-dependent decomposition (% loss per year) expected from eroded salt marsh. Percentages in each state are averages. Map by Quin Walker, NOAA NCCOS.