Quantification of Sedimentary Organic Carbon Storage and Turnover of Tidal Mangrove Stands in Southern China Based on Carbon Isotopic Measurements

Mangrove ecosystems are highly productive and play an important role in tropical and global coastal carbon (C) budgets. However, sedimentary organic carbon (SOC) storage and turnover in mangrove forests are still poorly understood. Based on C isotopic measurements of sediment cores of 2 mangrove stands in southern China, SOC density was 431.77 Mg ha−1 at site 1 (a Aegiceras corniculatum-dominated high tidal stand) and 243.65 Mg ha−1 in site 2 (a Bruguiera gymnorrhiza + Kandelia candel-dominated middle tidal stand). SOC δ13C values at both mangrove sites ranged from -29.4% to −26.0%. SOC δ13C was enriched with depth at 20–50 cm at site 1, which possibly resulted from preferential microbial decomposition. SOC δ13C at site 2 experienced frequent tidal flushing, and presented relatively stable values with depth. A bomb-14C-based SOC turnover model indicated that turnover times of SOC at 20–50 cm at site 1 were 4.44–26.04 yr. Modern C input from abundant roots might account for the very short SOC turnover times at these subsurface layers. As a result, our study suggested that tidal processes had a great influence on SOC storage and turnover in mangrove forests.