Over the last decades, arsenic (As) contamination of soil-plant-water systems has become a major concern for Bangladesh and West Bengal, India. The geochemistry of the sediment depositional environment which subsequently, on sediment burial, give rise to elevated As in Holocene groundwater’s of Southeast Asia, may provide clues to unravelling the mechanistic basis and spatial heterogeneity of this phenomenon. The Sundarbans mangrove ecosystem, Bangladesh, is a modern analogue, and indeed forms a continuum with, the Holocene sediments of concern and thus studying As cycling in surface Sundarbans sediments. Similarly, rice paddy fields in many regions of the Bengal Basin form a continuum with Holocene sediments. Sediment cores were collected from a wide range of locations within the Sundarbans to study surface spatial, as well as down the profile (~1 m), As distribution and it’s association with other geochemical parameters. Pore and surface water, and Diffusive Gradients in Thin films (DGT) were sampled from 4 different interlocked sub-habitats over 0.29 km2: major river bank, main tributary, forest floor and minor tributary. Further experiments were conducted to observe As dynamics in mangrove surface sediments on application of mangrove detritus. Finally 3 interlinked experiments (field manuring, soil batch culture, greenhouse growth trials) were conducted to assess the effect of farm-yard manure (FYM) and rice straw, at a field application rate practised in Bangladesh (5 t/ha), on As mobilization in soil and subsequent assimilation by rice. As concentration in mangrove sediment down the profile was found to be more associated with elevated Fe and Mn than with organic matter (OM), with significant spatial variations among the locations. Sediment particle size was an important factor determining As retention and mobilization, which is also true for deeper Bengal Bay sediments. Proximity to mangrove vegetation and to water bodies was found to have significant effect on As dynamics. Porewaters from coarse textured, low OM riverbank sediment were high in As, but with only a small pool of As for resupply from the solid phase, showing similarities with grey aquifer sediments compared to fine textured and high OM content forest floor sediments. The As column dynamics study showed that As release into porewater was strongly associated with Fe release, indicating the strong association of the 2 elements, with OM playing a major role in their dissolution. The desorption studies also showed OM driving As mobilization within short time. Due to strong redox cycling very little evidence of As methylation was observed in biologically active mangrove porewaters. But we found 10-fold increase in dimethylarsinic acid (DMA) production compared to a non-amended control using the same sediments treated with mangrove detritus in the laboratory. OM amendments lead to considerable mobilization of As into both soil porewaters and standing surface waters in rice paddy. In a greenhouse rice cultivation experiment, flooding initially caused greatly enhanced As mobilization in porewater (< 24 d), but the effects of flooding on As mobilization decreased during later rice growth, particularly at grain fill. However, OM amendment did not cause significant As accumulation in grain and straw compared to control. It was noted in field trials and greenhouse studies that OM fertilization greatly enhanced As mobility to surface waters, which may have major implications for fate of As in paddy agronomic ecosystems.