Process Controls of the Live Root Zone and Carbon Sequestration Capacity of the Sundarbans Mangrove Forest, Bangladesh

Abstract

The conservation of coastal wetland ecosystems, like mangrove forests and salt marshes, represents a critical strategy for mitigating atmospheric emissions and climate change in the 21st century. Yet the existence of these environments is threatened by human-induced disturbances, namely deforestation and accelerated sea-level rise. Coastal systems maintain surface elevation in response to sea-level rise through a combination of physical and biological processes both above and below the ground surface. The quantification and relative contribution of belowground process controls (e.g., seasonal water content, organic matter decomposition) on surface elevation change is largely unexplored but crucial for informing coastal ecosystem sustainability. To address this knowledge deficit, we integrated measurements of surface elevation change of the live root zone (0.5 to 1 m depth) with geotechnical data from co-located sediment cores in the Sundarbans mangrove forest (SMF) of southwest Bangladesh. Core data reveal that the primary belowground controls on surface elevation change include seasonal fluctuations in pore-water content and the relative abundance of fine-grained sediments capable of volumetric expansion and contraction. In contrast to many mangrove environments, the soils of the SMF contain little organic matter and are dominantly composed (>90%) of inorganic clastic sediments. The mineral-rich soil texture likely leads to less compaction-induced subsidence as compared to organic-rich substrates and facilitates surface equilibrium in response to sea level rise. Despite a relatively high soil bulk density, soil carbon (C) density of the SMF is very low owing to the dearth of preserved organic content. However, rates of C accumulation are balanced out by locally high accretion rates, rendering the SMF a greater sink of terrestrial C than the worldwide mangrove average. The findings of this study demonstrate that C accumulation in the SMF, and possibly other alluvial mangrove forests, is highly dependent on the continued delivery of sediment to the mangrove platform and associated settings.

Is soil organic carbon underestimated in the largest mangrove forest ecosystems? Evidence from the Bangladesh Sundarbans

“…and Avicennia sp. are resilient to disturbances leading to higher SOC than climax and seral species (Table A.1) and accumulate a large quantity of organic litter in the tidal channel close to the river or seafront (Sarker et al, 2016;Bomer et al, 2020a). The variability of SOC stocks among forest types followed a similar pattern to the global studies by Atwood et al (2017), except for Sonneratia sp.…”

“…The soil is mainly fine-grained, grey coloured, slightly calcareous, and mostly composed of silts to clayey silts (Allison et al, 2003;Bomer et al, 2020a). The subsurface sediment extends up to 6 m in depth in the landward direction and up to 4 m in depth in the seaward direction (Allison et al, 2003).…”

“…The median grain size ranges between 16-32 µm reflecting the medium silt range. The average dry bulk density (0.81 g cm -3 ) is higher in the Sundarbans in comparison to other mangroves in the world (Bomer et al, 2020a). The soil physical and chemical properties are varied from the eastern to the western part of Bangladesh Sundarbans, the eastern part is softer, more fertile and receives more fresh sediments than the western part (Siddiqi, 2001).…”

“…However, despite the historical reduction of sedimentation, the Sundarbans is itself still keeping pace with sea-level rise with the highest average surface elevation and vertical accretion rate (0.74 and 2.71 cm yr −1 ) compared to the worldwide average (Bomer et al, 2020a;Bomer et al, 2020b). This high sedimentation rate is the outcome of the massive flux of clastic sediments which attenuates the amount of organic carbon per unit area.…”

Remote sensing of fish-processing in the Sundarbans Reserve Forest, Bangladesh: an insight into the modern slavery-environment nexus in the coastal fringe

“…Moreover, the formation of sites 1-4 was identified in the Landsat 8 OLI data and corroborated by GE Pro (Table 2). Further investigation of camp age could be Table 1 Area of mangrove forest (ha) that the fish-processing camps covered across the SRF as measure in the high-resolution data available on Google Earth Pro (Provided by DigitalGlobe, Maxar Technologies, and CNES/Airbus), and the estimated volume of carbon which is no longer sequestered as a result of the removal of these mangroves for the camps-based on the volume of 240 g/m 2 /year by Bomer et al (2020). Not all years are available (no data marked -) due to irregular updates to the platform, and some camps only have partial coverage (marked *) achieved by applying a similar algorithm to the fish camps as has previously been undertaken in the brick-making sector (see Li et al 2019).…”