Salt marsh, mangrove, and seagrass ecosystems are colloquially termed ‘blue carbon’ ecosystems because of their effectiveness at storing organic carbon in their soils. The productive tidal wetland and seagrass plants that make up blue carbon ecosystems turn carbon dioxide (CO2) from the atmosphere into new biomass and trap sediment and organic particles washed in by tides. Some fraction of the organic carbon becomes buried in soils and can remain there for minutes to thousands of years. The length of time that organic carbon persists in soils reflects the many variables – oxygen levels, mineral composition, pore water chemistry, etc. – that affect whether microbes can access and decompose buried carbon.
Long-term preservation of soil carbon is important for removing CO2 from the atmosphere and ensuring future sustainability of blue carbon ecosystems. These ecosystems are unique in their ability to bioengineer their elevation within the tidal frame. Rising sea levels trigger shifts in plant community composition and productivity with the net result that blue carbon ecosystems gain vertical elevation at a rate that, on average, has matched changes in sea level. Whether or not blue carbon ecosystems continue to keep pace with accelerated sea-level rise depends in a large part on the fate of the soil carbon. Understanding controls on decomposition is therefore important for improving both global carbon budgets and managing blue carbon ecosystems.
Despite decades of research, we still lack a full understanding of the factors controlling soil organic carbon accumulation and loss. Accumulation is often attributed to the molecular complexity of structural plant compounds (e.g., lignin) and low soil oxygen levels that slow decomposition. Yet, there is enormous variability in soil carbon stocks and decay rates across blue carbon ecosystems that is not directly correlated to plant lignin production rates.
Over lunch during a recent Coastal and Estuarine Research Federation meeting, we discussed potential mechanisms contributing to spatial heterogeneity in soil carbon stocks and challenges in developing future predictions, particularly under a rapidly changing climate and along developed coastlines. How will the importance of blue carbon storage change at a global scale in the future? What is the potential role of these ecosystems in affecting atmospheric CO2 concentrations? What will be the fate of ancient carbon eroded from wetlands and seagrass beds? What kinds of management and restoration practices will promote carbon storage and ecosystem sustainability?
Over the next year and a half we combed the coastal, marine, and terrestrial literature and discussed the key mechanisms affecting microbial access to buried carbon and how their effectiveness may change over spatial and temporal scales and between ecosystem types. We considered how global change and anthropogenic disturbances would affect the fate of recently-deposited and ancient soil carbon. Synthesizing this information in a way that could lead to new mechanistic understanding and more refined predictive models was more challenging than we imagined. We decided that a conceptual framework, such as provided in this article, would be useful for evaluating how the relative importance of different decomposition mechanisms vary within and between ecosystems and in response to disturbances. This conceptual framework forms the basis of our manuscript, and we hope that our paper will provide a platform from which blue carbon researchers can build an integrated understanding of the role coastal wetlands will play in future carbon storage.