Multiple drivers for carbon stocks and fluxes in different types of mangroves.

Mangroves are highly efficient in sequestering carbon from the atmosphere and can accumulate carbon in sediments for millennials. However, The fate of mangrove carbon has not been well constrained due to the lack of data on different pools of sediment carbon sinks and sources. This study examined the variation of carbon stocks and fluxes at the water-sediment-air interface in both estuarine mangroves (natural: Mai Po, restored: Gei Wai) and oceanic mangroves (Ting Kok). There are divergent patterns in biogeochemical variables at the sediment-water-air interface, likely due to significant variation within sites. Total sediment carbon stocks (TCs) ranked in the order of restored estuarine mangroves (392.5 ± 8.8 Mg ha), natural estuarine mangroves affected by aquaculture (315.2 ± 21.4 Mg ha) and oceanic mangroves (229.1 ± 32.3 Mg ha). Sediment organic carbon stocks (SOC) and inorganic carbon stocks (SIC) accounted for 84.1-90.2 % and 9.8-15.9 % of TC, respectively. The highest sediment-air CO and CH fluxes occurred in restored and natural estuarine mangroves affected by aquaculture, respectively. The isotope of CO fluxes (δC-CO) indicates higher contributions from the degradation of mangrove-derived organic carbon in restored (-25.94 ‰ ± 3.37 ‰) and natural estuarine mangroves affected by aquaculture (-25.54 ‰ ± 0.96 ‰) than in oceanic mangroves (-21.55 ‰ ± 1.36 ‰). The isotope of CH fluxes (δC-CH) indicates CH production dominated by acetate fermentation in restored estuarine mangroves but dominated by the reduction of CO for other sites. Future studies should better constrain the fate of mangrove carbon by considering local drivers.