Organic carbon allocation and stability in mangrove surficial sediment

Abstract Mangroves are coastal vegetated ecosystems with a remarkable carbon sequestration capacity that is vital for mitigating global climate change. The sediment organic carbon (SOC) in mangroves originates from both autochthonous plants and allochthonous inputs. However, the primary factor governing the allocation and stability of SOC in mangroves remains under debate. Here, characterization of carbon content and multiple stable isotopes in a marine mangrove revealed that SOC was enriched in surface layers in the tidally driven groundwater recharge zone, and its content decreased progressively with depth. The composition of SOC sources remained consistent within the upper 30 cm but varied considerably across tidal zones. Contributions of three potential sources were quantified using bulk carbon and nitrogen isotope analysis combined with a Bayesian isotope mixing model. The results showed that the contribution of mangrove species diminished from 71.7% ± 9.3% in the recharge zone to 4.0% ± 5.5% at the tidal flat, whereas that of marine microalgae increased from 25.8% ± 11.0% to 89.8% ± 14.3%. Spartina alterniflora was identified as a minor contributor. Compound‐specific carbon isotope analysis of individual amino acids (AAs) showed that mangrove‐derived essential AAs (EAAs) dominate the sedimentary EAA pool. Water extraction experiments, together with the characterization of seawater and porewater, showed that SOC in the recharge zone likely released soluble fractions that underwent mineralization in porewater and exchanged dissolved inorganic carbon with surface waters driven by tidal pumping. Overall, our findings suggest that tidally induced zonation is the overarching factor governing both the allocation and stability of SOC in mangrove ecosystems.