Bioturbation Amplifies Greenhouse Gas Emissions from Coastal Wetlands: Insights from a 3D Reactive Transport Model

Benthic bioturbation in coastal wetlands may substantially alter greenhouse gas (GHG) emissions by reshaping scalar transport and redox conditions, yet its net effect and mechanistic pathways remain poorly constrained. We develop a three-dimensional LES--Darcy reactive-transport framework that couples overlying flow, burrow-driven ventilation, and porewater biogeochemical reactions to quantify CO₂, CH₄, and N₂O exchange from crab-burrowed sediments. We designed an ensemble of simulations spanning a broad range of hydrodynamic forcing, surface topography, and bioirrigation conditions, including contrasts in burrow depth and ventilation strength. Time-series results show a consistent response sequence: CO₂ fluxes are elevated at the onset of ventilation and relax toward a quasi-steady level that remains above the flat-sediment baseline; CH₄ fluxes are generally enhanced, with the strongest amplification early in the simulations when flushing can export reduced gases faster than they are oxidized; and N₂O exhibits a pronounced transient pulse as the oxic--anoxic structure reorganizes around the burrow. At quasi-steady state, CO₂ and CH₄ fluxes are enhanced by up to ~12-fold and ~3-fold relative to undisturbed sediments. Across scenarios, burrow depth and bioirrigation intensity emerge as the dominant, synergistic controls on multi-gas fluxes, whereas external hydrodynamic forcing and mound-scale relief exert secondary, context-dependent effects. These results provide a process-based foundation for incorporating fauna-driven ventilation into blue-carbon budgets and wetland restoration planning by linking burrow-scale transport--reaction dynamics to ecosystem-scale GHG emissions.