Soil organic carbon content and reactivity at the pioneer saltmarsh-mudflat ecotone

The Intergovernmental Panel on Climate Change (IPCC) currently recognizes vegetated coastal systems, such as saltmarshes, as actionable blue carbon habitats. While not yet officially acknowledged by the IPCC, recent research has highlighted the important role of adjacent mudflat habitats in long-term carbon storage, leading to their growing recognition as “emerging” blue carbon systems. Currently, most research on soil organic carbon (OC) dynamics in the intertidal zone focuses on either saltmarshes or mudflats, while overlooking transitional ecotonal zones. While OC dynamics in saltmarshes have been the main focus of the research community, significantly less attention has been given to tidal mudflats.

We collected soil cores from two different estuaries in England, along transects spanning the pioneer saltmarsh–mudflat ecotone to compare changes in OC storage, accumulation rates, and organic matter (OM) reactivity across these habitats. OC accumulation rates were calculated using ²¹⁰Pb and ¹³⁷Cs dating techniques. OM thermal reactivity, reflecting the susceptibility of soil OM to decomposition largely influenced by the balance of labile and recalcitrant OM pools, was assessed using the Carbon Reactivity Index (CRI) via thermogravimetric analysis.

Our results reveal that OC stocks, OC accumulation rates, and CRI values are similar across the pioneer saltmarsh–mudflat ecotone. In comparison to other saltmarsh systems across Great Britain, OC stocks were lower in both the saltmarsh and mudflat zones, yet OC accumulation rates were similar to published saltmarsh data from other regions in the UK. The CRI values indicate that OC stored in pioneer saltmarsh soils are more reactive, and therefore potentially at higher risk of remineralization than adjacent mudflat soils.

Our results suggest pioneer saltmarshes and mudflats, while exhibiting similar OC stocks and OC burial rates, do however exhibit marked changes of OM reactivity. The dynamic nature of these intertidal systems likely increases mineralization of high-reactivity OM across the ecotone, leading to the observation that intertidal mudflats are stores of relatively more stable OM compared to the adjacent pioneer saltmarsh. Our research contributes to the overall understanding of OC storage in saltmarsh pioneer zones and mudflats. We aim to help managers prioritize areas based on both their total OC stocks and their potential for OC remineralization, focusing efforts on protecting high-degradation risk zones.