From abandoned fishponds to mature mangroves: the integrated CO2 balance of two contrasting coastal wetland landscapes

Mangrove restoration from abandoned fishponds is a promising natural climate solution, but its role in the landscape-scale carbon balance remains poorly understood. This is primarily attributed to 1) a lack of landscape-scale flux measurements that can integrate various carbon sink-source components from the tightly coupled land cover compositions and 2) the scarcity of comparisons between restored fishponds and benchmark mature mangroves within the same coastal region. Using paired eddy covariance towers approximately 73 km apart, we measured the landscape-scale CO 2 fluxes at two heterogeneous coastal wetlands in southern China: one dominated by mature mangroves (MANG) and the other by abandoned fishponds (POND) where mangrove seedlings had been planted five years ago. We found that both sites acted as annual CO 2 sources over 2023–2024, with a similar mean annual net CO 2 flux of 301 and 283 g C m −2 yr −1 for MANG and POND, respectively. However, their CO 2 sources weakened during a hot and wet year (2024), driven by contrasting mechanisms: gross primary production (GPP) at MANG increased due to the diffuse radiation fertilization effect, whereas heat stress primarily inhibited landscape respiration (RE) at POND. Footprint analysis showed that net CO 2 fluxes at MANG decreased markedly when the source area contribution of mangroves (f mangrove ) reached 90% due to contrasting responses of GPP and RE to f mangrove changes. At POND, all CO 2 fluxes were insensitive to land cover variations. Based on a machine learning model, our restoration scenario analysis demonstrated that achieving ∼50% f mangrove in the fishpond landscape can counterbalance its total CO 2 emissions. These restored fishponds were projected to further switch the coastal landscape to a robust CO 2 sink upon full restoration and maturation. Our study provided direct quantifications of landscape-scale CO 2 flux variations with coastal wetland management, highlighting the critical role of coastal wetlands in mitigating the co-located natural and anthropogenic CO 2 emissions.