Increased ecosystem carbon accumulation despite unimodal soil organic carbon patterns with stand age across afforested mangroves in Bangladesh

Mangroves are crucial coastal ecosystems that safeguard shorelines and store large amounts of carbon. This study assessed the dynamics of vegetation carbon (VC), soil organic carbon (SOC), and ecosystem carbon (EC) densities across five stand ages (12–44 years) of Sonneratia apetala plantations on six coastal islands in Bangladesh. Results revealed pronounced age-dependent shifts: VC (247 t ha -1 ) and EC (461 t ha -1 ) densities were highest in the oldest stand (44 years). In contrast, SOC density showed a unimodal pattern with stand age, rising from 105 t ha -1 in the 12-year-old stand to a peak of 195 t ha -1 in the 26-year-old stand before declining in the older stands. The rate of VC density increase was greatest (13.17 t ha -1 yr -1 ) in the 18-year-old stand. However, the accumulation rates of SOC density (6.39 17 t ha -1 yr -1 ) and EC density (19.79 t ha -1 yr -1 ) peaked in the 26-year-old stand. The initial rise in SOC density (0-60 cm depth) reflected high litter and root activity in the middle-aged stands, while the later decline in SOC density was likely a result of greater allocation of belowground carbon to the lower soil horizons in the older stands with deep roots. These findings reveal age-driven carbon allocation shifts and underscore the potential of S. apetala afforestation in enhancing the long-term blue carbon storage in coastal zones. This study highlights the need for more widespread afforestation in S. apetala to enhance carbon storage, and further investigations of SOC dynamics in deeper soil layers to fully assess the carbon sequestration potential of S. apetala plantations, especially in the older stands.