Buried Potential? Evaluating Long-Term Sequestration 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 provides the first full-depth, site-wide assessment of 20 years of sediment organic carbon (OC) accumulation in a UK restored intertidal habitat (Paull Holme Strays), based on 3,987 sediment samples across saltmarsh and mudflat. We integrate depth-resolved sampling, OC fractionation, and elevation-stratified modelling to quantify stocks and characterise variability. Since breaching in 2003, the site has accumulated ~ 125,000 m³ of sediment, with 11–250 cm of elevation gain closely linked to initial elevation. OC declined non-linearly with depth (adj. R² = 0.62); labile OC was largely confined to an upper surface layers, while deeper layers were dominated by more persistent recalcitrant OC, consistent with breakpoint analysis. Elevation and depth were the primary correlates of OC variability, with bulk density and grain size acting as secondary influences. Annual OC accumulation averaged ~ 15 t C ha⁻¹ yr⁻¹, with roughly one third labile and two thirds recalcitrant, reflecting the combined effects of sediment volume and carbon concentration. These analyses disentangled near-surface turnover from deeper, persistent storage and clarified why rapidly accreting, low-elevation areas with lower OC concentrations, and OC-dense high-elevation areas with limited accretion can yield comparable annual accumulation. An empirically defined surface horizon, paired with elevation-stratified, fraction-resolved profiles, provides a scalable basis for consistent comparison and strengthens transparent, standardised blue-carbon assessment.