Evaluating the long-term fate of carbon in a restored intertidal habitat

Restoration of intertidal habitats is increasingly promoted as a nature-based solution for climate mitigation, yet uncertainties remain around their carbon dynamics. This study presents a full-depth, site-wide assessment of 20 years of sediment organic carbon (OC) accumulation in a UK restored intertidal habitat, based on 3987 sediment samples across saltmarsh and mudflat. We integrate depth-resolved sampling, OC fractionation, and elevation-stratified modelling to quantify stocks and variability. Since breaching, the site has accumulated ~125,000 m³ of sediment, with elevation gains closely linked to initial elevation. OC declined non-linearly with depth, with labile organic matter (OM) concentrated near surface layers, and deeper sediments dominated by more persistent recalcitrant OM. Annual OC accumulation averaged ~15 t C ha⁻¹ yr⁻¹, reflecting the combined effects of sediment volume and carbon concentration. We show that standard global carbon density defaults substantially underestimate site-specific stocks and demonstrate that an elevation-stratified, fraction-resolved approach provides a more accurate and scalable basis for standardised blue carbon assessment.