Carbon storage and accumulation across the United Kingdom’s saltmarsh habitat. 

Saltmarshes play a key role in the coastal carbon cycle through the capture and storage of organic carbon. Assessments of both organic carbon (OC) stocks and rates of OC accumulation are vital for quantifying saltmarsh contributions to climate-change mitigation and for guiding efforts to protect and restore coastal ecosystems. Current assessments of the magnitude of the store and rate of OC accumulating in UK saltmarshes are based on a small and spatially limited dataset. To address this knowledge gap, we collected sediment cores to quantify the OC stored in the soil and biomass of 26 saltmarshes and estimate OC accumulation rates for 22 saltmarshes distributed around the UK. Across the saltmarshes, the estimated average store is 11.55 ± 1.56 kg C m-2 with values ranging between 2.24 kg C m-2 and 40.51 kg C m-2. These saltmarshes accumulate OC at a rate of 110.88 ± 43.12 g C m-2 yr-1 with values ranging from 27.57 g C m-2 yr-1 to 343.68 g C m-2 yr-1. These highly variable OC stocks and accumulation rates are dependent on interlinked factors, including local geomorphology, organic carbon source, sediment type (mud vs sand), sediment supply, and relative sea-level history. By upscaling these estimates to all UK saltmarshes, it is calculated that these systems currently store 5.20 ± 0.65 Mt of OC and accumulate 46563 ± 4353 tonnes of OC annually. The low OC accumulation rates indicate that UK saltmarshes have relatively low additional Greenhouse Gas (GHG) abatement potential, but that they contain significant stores of OC within the ecosystem. This highlights the crucial need for the protection and restoration of existing OC stores within UK saltmarshes, providing climate benefits several times more significant than the annual accumulation of OC in these ecosystems.