Abstract: Mangroves are critical blue carbon ecosystems, essential for coastal preservation and carbon sequestration. Despite widespread attention to mangrove deforestation and reforestation driven by land-use/land-cover changes (LULCCs), integrated assessments of CO₂ and CH₄ fluxes across diverse transitions under the same spatiotemporal context remain scarce, hindering advances in restoration planning and land-use emission modeling. This study investigated five LULCC types on Qi’ao Island, China: mudflats, native Kandelia obovata, exotic Sonneratia apetala, S. apetala deforestation areas, and abandoned fish ponds. Combining one year of in situ carbon flux measurements with remote sensing data (1980-2020), we assessed changes in carbon sink dynamics and CH₄ fluxes at both site and regional scales. At the site level, mudflats acted as weak carbon sinks (2.1 ±1.7 t CO₂ hm^-2 a^-1). By contrast, the exotic S. apetala exhibited a carbon sink 3-4 times stronger (60.6 ±5.8 t CO₂ hm^-2 a^-1) than the native K. obovata (16.8 ±2.0 t CO₂ hm^-2 a^-1). Abandoned fish ponds (7.7 ±1.3 t CO₂ hm^-2 a^-1) retained a moderate carbon sink due to abundant aquatic vegetation, whereas deforestation resulted in a carbon source (-1.1 ±0.9 t CO₂ hm^-2 a^-1). Notably, sites with higher CO2 uptake also emitted higher CH₄ emissions, which indicated a climate mitigation trade-off. CH₄ emissions offset ~12% of the climate benefit from CO₂ uptake. Regionally, LULCCs, driven largely by mangrove reforestation, shifted the region from a net carbon source to a net carbon sink around 1990. These findings highlight the impacts of LULCC in shaping carbon sinks, and they offer valuable insights for optimizing future restoration strategies with greater climate benefits.

Key words: land-use/land-cover changes, mangrove, deforestation, reforestation, climate change, fish pond