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.