Abundant crab burrows in carbon-rich, muddy salt marsh soils act as preferential water flow conduits, potentially enhancing carbon transport across the soil-water interface. With increasing recognition of blue carbon systems (salt marshes, mangroves, and seagrass) as hotspots of soil carbon sequestration, it is important to understand drivers of soil carbon cycling and fluxes. We conducted field observations and flow modeling to assess how crab burrows drive carbon exchange over time scales of minutes to weeks in an intertidal marsh in South Carolina. Results showed that continuous advective porewater exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients. The concentrations of dissolved inorganic (DIC) and organic (DOC) carbon in crab burrow porewater differ with that in the surrounding soil matrix, implying a diffusive C flux in the low-permeability marsh soil. Gas-phase concentrations of CO 2 in $ 300 crab burrows were approximately six times greater than ambient air. The estimated total C export rate via porewater exchange (1.0 AE 0.7 g C m −2 d −1) was much greater than via passive diffusion transport (6.7 AE 2 mg C m −2 d −1) and gas-phase CO 2 release (0.93 mg C m −2 d −1). The burrow-related carbon export was comparable to the regional salt marsh DIC export, groundwater-derived DIC export, and the net primary production previously estimated using ecosystem-scale approaches. These insights reveal how crab burrows modify blue carbon sequestration in salt marshes and contribute to coastal carbon budgets.

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Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon ( 222 Rn) mass balance and sediment carbon burial rates using lead ( 210 Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ 13 C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m −2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m −2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m −2 of carbon dioxide (CO 2 ) per day. The fluxes exceed the average surface water CO 2 emissions (0.6 ± 0.2 g m −2 day −1 ) and the average sediment carbon burial rates (0.17 ± 0.09 g m −2 day −1 ). The δ 13 C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.