A disproportionately large amount of plastic accumulates in the sediment, which potentially contributes to the sediment carbon stock and alters the sediment biogeochemistry. In blue carbon ecosystems (BCEs), slow decomposition of organic matter due to hypoxic conditions limits the remineralisation process, but the extraneous input of labile organic matter may provide bioavailable substrate to reconvert organic carbon to carbon dioxide. We hypothesise that if plastics leach bioavailable organic matter into the sediments, the positive or negative ‘priming effects’ by microorganisms may impact the degradation of labile and recalcitrant organic matter, which will impact the blue carbon budget. To test this hypothesis, this study examines the leaching from both hydrocarbon and biobased plastics and their effects on natural SDOM turnover. We expose different types of plastics in the sediment through a microcosm setup and analyse the greenhouse gas emissions, dissolved organic carbon leaching and SDOM transformation at different time points. We also examine the physiochemical properties of plastics through scanning electron microscopy and thermogravimetry. The outcomes of this study reveal how the physicochemical properties of plastics modulate the process of degradation and leaching of plastics in the blue carbon sediment at different temporal scales.