A demonstration of the compositional stability of saline-hosted sedimentary carbon in a coastal wetland

Tidal wetlands sequester around ~4.8-87.2 Tg of carbon per year¹ and represent approximately 25% of the global carbon sink, as a result of high inputs of organic matter and slow below-ground decomposition. The rate of carbon storage is hypothesised to be linked to the composition, particularly the relative oxidation, of soil organic carbon and the relationship with the sediment mineral matrix. The complex and dynamic biogeochemical processes governing tidal wetland carbon are not yet fully understood and therefore, the aim of this research was to understand how the composition of organic material varies across a tidal wetland and how this in turn is related to source, mineralogy and salinity.

Topsoil (0-20cm) and subsoil (20-40 cm) samples were collected from three marshes along a transect representing a salinity, inundation and elevation gradient and were analysed to assess the variations of SOC source, prevalence and composition, as well as the influence of salinity and sediment mineral composition on SOC stabilization.

SOC in the saline environments was highly oxidised with a low oxidation potential and thus had a particularly stable composition. In contrast, the carbon in the freshwater marsh was significantly less oxidised and thus demonstrated a greater potential for CH₄ and CO₂ emission. Carbon isotope (δ¹³C) and C:N analysis revealed that the SOC in all sites was produced in-situ by C3 plants, but highlighted differences between the photosynthetic pathway of the vegetation in each marsh. The freshwater marsh carbon was also more δ¹³C depleted, indicative of methane-consuming organisms and we hypothesise this variation in production pathway links to oxidation state.  

The compositional stability of SOC affected overall concentrations with the highest concentration of SOC in the first 20 cm of sediment in the high saltmarsh (between 0.35% and 5.34% w/w) and the lowest concentration of SOC in the lower 20-40 cm of sediment of the freshwater marsh (0.4% - 2.7% w/w). SOC prevalence is also positively associated with Fe-Al clay minerals, which was the dominant sediment type in the saltmarshes, whereas the freshwater marsh was predominantly siliceous sediment.

We conclude that the relative stability and concentration of SOC is greatest in the saltmarshes compared to the freshwater marsh. This aligns with emerging theory that mineral association is an important pathway of SOC stabilisation, and that salinity may exhibit a positive effect on cation bridging between organic material and mineral surfaces.

Acknowledgements: This project received funding from the National Environmental Isotope Facility (# 2656.0424).

References: Mcleod, E., Chmura, G. L., Bouillon, S., Salm, R., Björk, M., Duarte, C. M., Lovelock, C. E., Schlesinger, W. H., and Silliman, B. R.: A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2, Frontiers in Ecology and the Environment, 9, 552–560, https://doi.org/10.1890/110004, 2011.