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

Laetitia Allais; Benoit Thibodeau; Nicole S Khan; Sean A Crowe; Stefano Cannicci; Christelle Not

doi:10.1016/j.scitotenv.2023.168955

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

Sci Total Environ. 2024 Feb 20:912:168955. doi: 10.1016/j.scitotenv.2023.168955. Epub 2023 Dec 4.

Authors

Laetitia Allais¹, Benoit Thibodeau², Nicole S Khan³, Sean A Crowe⁴, Stefano Cannicci⁵, Christelle Not³

Affiliations

¹ Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region; Swire Institute of Marine Science, University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region. Electronic address: u3006445@connect.hku.hk.
² Earth and Environmental Sciences Programme & School of Life Sciences, Chinese University of Hong Kong, Hong Kong Special Administrative Region. Electronic address: benoit.thibodeau@cuhk.edu.hk.
³ Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region; Swire Institute of Marine Science, University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region.
⁴ Department of Microbiology & Immunology, Life Sciences Institute, University of British Columbia, Vancouver, BC, Canada; Department of Earth, Ocean, Atmospheric Sciences, University of British Columbia, BC, Canada.
⁵ Department of Biology, University of Florence, Firenze, Italy.

PMID: 38056642
DOI: 10.1016/j.scitotenv.2023.168955

Abstract

Mangrove ecosystems are an important blue carbon store but exhibit considerable variation in soil carbon stocks globally. Unravelling the conditions controlling carbon stock is 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 in sediment samples from 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 at 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. Notably, despite having the highest mass-specific soil organic carbon contents, Mai Po had the lowest integrated soil organic carbon storage (77 ± 3 Mg C ha^-1). This was primarily due to 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.

Keywords: Biogeochemical cycles; Blue carbon; Carbon sequestration; Environmental gradient; Isotope; Wetland.