Coastal saltmarshes provide a range of ecosystem services, including critical roles in global C storage and cycling. Tidal restrictions from coastal development have considerable impacts on biogeochemical processes in saltmarshes, particularly due to increased freshwater input leading to potential changes in greenhouse gas flux and carbon breakdown. To better understand the drivers and variability of carbon cycling after a bund wall installation in saltmarsh sediments, intact soil cores were collected from a saltmarsh in Melbourne, Australia and incubated in laboratory microcosms to simulate tidal restriction through freshwater addition. Throughout the 30-day incubation, greenhouse gas flux (CO2 and CH4) and litter decomposition were measured, and soil cores were segmented and analyzed for pH, redox, particle size distribution, and organic carbon content. No significant differences were observed in CO2 and CH4 flux between the treatment (freshwater) and control (saltwater) cores. It is possible the experimental design did not allow for total flushing of the cores into freshwater, thus not resulting in the shift from sulfate reduction to methanogenesis and associated CH4 emissions, as hypothesized. Freshwater addition to the soil cores increased decay rate compared to the reference saltwater addition cores. This study suggests restriction through bund wall installation accelerates short-term decomposition of organic matter, possibly enhancing soil organic carbon (SOC) loss compared to a saltmarsh with regular tidal flow.
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