CO₂ emissions from dredged sediment as a function of moisture, temperature, and oxygen

Abstract. Estuaries represent a crucial compartment in the global carbon cycle, with high rates of organic matter formation, burial and degradation. Sedimentary processes control the balance between long-term burial of carbon and CO₂ and CH₄ emissions upon OM degradation, for which estuaries are a global hotspot. The profound and globally intensifying perturbation of estuarine sediment by anthropogenic activities such as harbor dredging has a far-reaching but poorly understood impact on sedimentary carbon cycling processes in estuaries and by extension potentially on global carbon budgets. Hence, understanding carbon emissions from dredged sediments under varying environmental conditions is critical for assessing their environmental impact and informing large-scale sediment reuse strategies. This study investigates the effects of moisture, temperature, and oxygen availability on CO₂ emission rates from dredged sediments collected from the Port of Rotterdam, the largest port in Europe. Results are compared with soil CO₂ emissions from a global database of nearly 400 laboratory incubations under different conditions. Our sediment incubation showed that CO₂ emissions increased 1.5–8.1 times with higher moisture levels, 3.8–6.0 times with elevated temperatures, and 4.5–6.4 times with oxygen exposure. Applying machine-learning tools (XGBoost) to a global database of soils and sediment incubations suggested that moisture and temperature responses observed in our experiment are widespread in both soils and sediments. However, functions that described these responses differed significantly from those used in global biogeochemical carbon-cycle models, indicating a need to revisit these functions. Oxygen displayed a relatively stronger effect in sediments, likely due to better preservation of labile organic matter (OM) in anoxic conditions and its rapid oxidation upon re-exposure to oxygen. A model incorporating organic matter with different degradation rates showed that while labile OM fueled high initial rates of CO₂ emission, more recalcitrant OM was a much more abundant OM pool (> 80 %) that dominated cumulative CO₂ emissions on longer timescales. Overall, our experiment and meta-analysis on a global soil dataset suggest the importance of environmental controls on carbon emissions and that dredged sediments are an organic-rich, potent source of CO₂ upon oxidation after dredging, which should be considered in sediment management and reuse practices.