The impact of land use and land cover changes on mangrove carbon sinks and methane fluxes in Qi’ao Island, China

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: mudflat, 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 (2.13 ± 1.70 t CO2 hm-2 a-1). By contrast, the exotic S. apetala exhibited a carbon sink 3-4 times stronger (60.55 ± 5.83 t CO2 hm-2 a-1) than the native K. obovata (16.79 ± 1.98 t CO2 hm-2 a-1). Abandoned fish ponds (7.73 ± 1.26 t CO2 hm–2 a–1) retained a moderate carbon sink due to abundant aquatic vegetation, whereas deforestation resulted in a carbon source (–1.09 ± 0.93 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.