Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils
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The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO2) and methane (CH4)], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO2 and, surprisingly, CH4 increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following salt-water intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m−2, respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m−2, respectively), and to losses in soil organic C content (0 and 44.1 mol C m−2, respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.
KeywordsTidal freshwater marshes Carbon Organic matter mineralization Sulfate reduction Methanogenesis Carbon dioxide Methane Delaware River
We thank P. Costello, A. Foskett, O. Gibb, P. Kiry, D. Lammey, C. McLaughlin, T. Prša, J. Quinn, D. Russo, M. Santini, K. Scott, A. Smith, R. Thomas, and P. Weibel, for assistance in the field and laboratory. We are grateful to S. Joye and two anonymous reviewers for their comments on the manuscript. This research was supported by EPA-STAR grant RD 83222202 and the Department of Biology at Villanova University. This is contribution #1605 from the University of South Carolina’s Belle W. Baruch Institute for Marine and Coastal Sciences.
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