Blue Carbon Sequestration Potential of Soils in Degraded Mangroves of Sundarban, India: A Geochemical Approach

Mangrove ecosystems are important for global carbon sequestration, with their soils storing approximately 1.71 ± 0.17 Mg C ha−1 yr−1 of organic carbon. However, these ecosystems face challenge of degradation at a rate of nearly 42% as observed in the past decade in India. Moreover, degraded mangroves have become an integral part of the wetland ecosystems in Sundarbans. In this study, we investigate the geochemical interactions influencing soil carbon sequestration potential in degraded mangrove soils of the Indian Sundarbans. Soil samples were collected from three major estuaries: Bidyadhari, Hooghly and Matla up to 5 metre depths (at 1 m interval) with replications. Samples were analysed for total carbon (TC), soil organic carbon (SOC), major oxide compositions, trace element concentrations, and clay minerals. TC (0.67 - 0.94%) and SOC (0.58 - 0.73%) showed highest concentrations at 0 - 1 m soil depth due to greater organic inputs and declined with depth across all sites. Silicon dioxide (46% - 51%) and aluminium oxide (18% - 23%) were observed in higher concentrations across all sites which may contribute to clay-humus complexes with the potential to stabilize organic carbon. Trace elements showed stratified distributions influenced by redox cycling, tidal inputs, and anthropogenic inputs, particularly in Hooghly and Bidyadhari estuaries. Presence of Chromium (Cr), manganese (Mn) and zinc (Zn) indicated the potential to form insoluble oxides and hydroxides that could act as adsorption sites for organic carbon, thereby enhancing its stability. Secondary clay minerals like glauconite and argentopyrite indicated stable soil matrices that further supported carbon stabilization. This study emphasizes the geochemical characteristics of Sundarban degraded mangrove system in relation to carbon sequestration potential and factors influencing the soil carbon dynamics.