Divergent carbon allocation and GHG dynamics in urban mangroves: contrasting ecological restoration with natural conservation
Qinchang Chen¹ ,  Xiaoxuan Gu^1, 2*, Luzhen Chen^{1, *
1} State Key Laboratory of Marine Environmental Science, Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, College of the Environment and Ecology, Xiamen University, China
² Department of Ocean Science, The Hong Kong University of Science and Technology, Hong Kong, China
* Corresponding author: Luzhen Chen, email: luzhenchen@xmu.edu.cn
Abstract: Mangroves fringing coastal megacities mitigate urban warming and provide essential ecosystem services as critical nature-based climate solutions. However, quantifying their carbon budgets remains challenging due to spatial heterogeneity and existing data gaps. This study assesses carbon stocks and greenhouse gas (GHG) fluxes across mangrove communities under contrasting management methods in the highly urbanized Shenzhen Bay, China. Our results reveal a total ecosystem carbon stock of 122,276 Mg C across 490 ha of Futian and Mai Po mangroves. Notably, a divergent carbon allocation strategy was observed, where the restoration-dominated Futian mangroves exhibited significantly higher biomass carbon density (111.08 Mg C ha^-1), while the conservation-dominated Mai Po mangroves stored substantially more carbon in soils (185.27 Mg C ha^-1). Although soils acted as a methane (CH₄) source, strong canopy CO₂ uptake ensured the ecosystem functioned as a net C sink. The assessment of carbon dioxide removal (CDR) potential indicates a net uptake of 8,312 Mg CO_2-eq yr^-1. These findings highlight a strategic trade-off in urban wetland management, where rapid biomass gains from planting fast-growing species must be weighed against the need to conserve mature stands for long-term soil carbon storage. This study provides high resolution calibration data for urban blue carbon inventories, supporting carbon neutrality pathways and ecological restoration assessment in the Greater Bay Area.

Keywords: Mangrove; Blue carbon; Carbon stock; Carbon sequestration; Greenhouse gas.