Soil Carbon, Nitrogen, and Phosphorus Stoichiometry and Fractions in Blue Carbon Ecosystems: Implications for Carbon Accumulation in Allochthonous-Dominated Habitats

Blue carbon ecosystems (BCEs) including mangroves, saltmarshes, and seagrasses are highly efficient for organic carbon (OC) accumulation due to their unique ability to trap high rates of allochthonous substrates. It has been suggested that the magnitude of OC preservation is constrained by nitrogen (N) and phosphorus (P) limitation in response to climate and anthropogenic changes. However, little is known about the connection of soil OC with N-P and their forms in response to allochthonous inputs in BCEs. By analyzing soil OC, N, and P densities of BCEs from 797 sites globally, we find that, in China, where allochthonous OC provides 50- 75% of total OC, soil C/P and N/P ratios are 4-to 8-fold lower than their global means, and 23%, 29%, and 20% of buried OC, N, and P are oxidation-resistant fractions that linked with minerals. We estimate that the OC stocks in China should double over the next 40 years under high allochthonous inputs and elevated N/P ratio scenarios during BCE restoration. Allochthonous-dominated BCEs thus have the capacity to enhance refractory and mineral bound organic matter accumulation. Protection and restoration of such BCEs will provide long-term mitigating benefits against sea level rise and greenhouse gas emissions.