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Tidal salt marsh ecosystems store copious amounts of carbon (C) within sediments. In order to predict how these C stores may be affected by environmental change, it is critical to assess current CO₂ and CH₄ production and efflux from these ecosystems. Production and efflux of these greenhouse gases (GHGs) are governed by coupled geochemical, hydrological, physical and biological processes in sediments that are sensitive to local conditions, which can result in large spatial and temporal heterogeneity of GHGs dynamics within the ecosystem. To understand how the drivers of GHGs dynamics vary across salt marsh ecosystems, we coupled solid-phase geochemistry to measurements of porewater chemistry (to ∼1 m), CO₂ and CH₄ production in sediments and efflux to the atmosphere in a temperate tidal salt marsh for over one year to capture seasonal patterns within two vegetation zones of the marsh …