Sources of organic carbon in the surface sediments of the mangrove reserve in the Beilun River Estuary

Mangrove forests are critical ecosystems for carbon sequestration, effectively capturing atmospheric carbon and permanently storing soil organic carbon (SOC) in coastal wetlands. Recent restoration and afforestation initiatives have been undertaken to mitigate the rising atmospheric CO₂ concentrations along tropical ocean coasts worldwide. The expansion of mangrove forests has garnered significant attention for elucidating their contributions to the permanent storage of carbon in both surface and subsoil layers. In this study, we investigated a recently expanding mangrove forest in the Beilun River Estuary of Guangxi Province, focusing on the abundance and sources of organic carbon (OC) in surface sediments. We also analyzed sediment grain size distribution, SOC, total nitrogen (TN) content, and stable carbon isotope composition (δ¹³C). Additionally, we determined the molar ratio of SOC to nitrogen (C/N). The SOC content ranged from 0.25% to 6.58%, the C/N ratio varied from 9.85 to 17.73, δ¹³C values spanned from −29.58‰ to −24.93‰, and δ¹⁵N values ranged from 3.10‰ to 7.36‰. A strong correlation was observed between SOC and TN content. However, particle size did not significantly influence these relationships. In terms of SOC sources in the sediments, contributions from mangrove, terrestrial, and marine sources accounted for 37.1% to 99.5%, 0.5% to 47.4%, and 0% to 17.6%, respectively. The findings of this study provide essential insights for evaluating and managing carbon sinks within the mangrove ecosystems of the Beibu Gulf region, contributing to more effective carbon management strategies in coastal areas.