Salinity, mineralogy, porosity, and hydrodynamics as drivers of carbon burial in urban mangroves from a megacity

Mangrove ecosystems are an important blue carbon store, but, globally, exhibit great variation in soil carbon stocks. Unravelling the conditions controlling carbon stock is, therefore, critical for assessing current and future carbon budgets. Mangrove soil biogeochemical cycles can strongly influence carbon storage capacities. We thus investigated carbon sequestration, and the environmental parameters shaping variability in biogeochemical cycling and carbon storage. We examined sediment samples at four mangrove sites along an estuarine to marine gradient in Hong Kong, a megacity. Our results showed that organic matter in Hong Kong mangroves is sourced principally from autochthonous mangrove plants. Total nitrogen was higher in the freshwater-influenced sites and supplied from different sources. Marine-influenced sites had larger sulfur fractionations, reflecting higher marine sourced sulfate concentrations, and indicating a relatively open sulfate system. We estimated an average organic carbon stock of 115 ± 8 Mg C ha-1 in the upper 100 cm soil layer placing Hong Kong mangroves in the lower end of the global spectrum of the soil carbon stock. Carbon accumulation was found to be driven by a combination of higher total organic matter inputs, soil fluxes, and porosity. Mai Po, despite its highest mass-specific soil organic carbon contents, had the lowest integrated soil organic carbon storage (77 ± 3 Mg C ha-1), mainly because of a lower sediment density and higher tidal pumping leading to a decrease in carbon retention. Total organic matter input, sediment characteristics, and hydrodynamics were the main factors influencing soil organic carbon storage. Overall, our results suggest that (1) while multiple parameters can enhance soil organic carbon content and increase carbon storage capacities, (2) hydrodynamics and sediment characteristics can increase the potential for leakage of carbon, and (3) high carbon content does not always equal high carbon sequestration and stock.