Allochthonous particulate organic carbon in blue carbon ecosystems: Implications for accounting, stability, and policy.

Blue carbon ecosystems (BCEs) store a significant amount of organic carbon (OC) in their soils, including OC sequestered by other ecosystems and transported into these ecosystems, referred to as allochthonous OC. This study reviews the state-of-the-art knowledge on the abundance and sources (including still unexplored ones) of allochthonous particulate organic carbon (POC) and reviews the ways allochthonous POC is managed under the current blue carbon policy frameworks for carbon benefits accounting in blue carbon restoration and conservation projects. Based on a review of 102 studies, we find that 56 ± 25% of the soil organic carbon (SOC) deposits in BCEs are allochthonous POC, most commonly identified as originating from terrestrial ecosystems, seston and macroalgae. Whether allochthonous POC should be included in blue carbon accounting is a matter of debate among the scientific community, due to the risks of overestimating carbon benefits and double-counting. Consequently, it is often excluded from voluntary carbon markets, but its inclusion can be justified given the role of BCEs in stabilising OC that may otherwise be remineralised. Based on existing knowledge, a deeper understanding of the stability and behaviour of different types of allochthonous POC under varying environmental conditions is needed to assess whether and to what extent these fractions can be counted toward carbon benefits.