Black Carbon Contributes Substantially to Allochthonous Carbon Storage in Deltaic Vegetated Coastal Habitats

Vegetated coastal habitats (VCHs) rank among the most intense carbon sinks in natural ecosystems, playing an important role in the global carbon cycle. A significant part of the organic carbon (OC) they store may be allochthonous OC that has been sequestered elsewhere. Yet, the compositions of allochthonous OC are largely unknown. Here, we present concentrations and carbon isotopic (¹³C and ¹⁴C) compositions and accumulation rates of carbon in the VCHs from major temperate-subtropical deltas of China: Yellow, Yangtze, and Pearl river deltas. We find that black carbon (BC) amounts to 9-25% of OC across sites. Temperate VCHs exhibit lower BC contents but higher BC contributions than subtropical VCHs. This seemingly counterintuitive result can be explained by increased accumulation of long-term, stable, allochthonous OC in temperate VCHs. BC in temperate VCHs contains 1.5-2 times more fossil BC than that in subtropical VCHs in the 1 m depth soil, which is likely influenced by atmospheric input and the aging effect. We estimate an accumulation rate of BC in China's VCHs of 33.1 ± 14.5 g m^-2 year^-1, acting as a hotspot for BC burial. These results point to a substantial and hitherto unquantified contribution of BC components to blue carbon storage, as well as the VCHs to global BC storage. Preservation of this old-aged, stable OC implies an important ecosystem service of the VCHs for climate change mitigation.