Saltwater Intrusion Modifies Microbial Community Structure and Decreases Denitrification in Tidal Freshwater Marshes
Environmental changes can alter the interactions between biotic and abiotic ecosystem components in tidal wetlands and therefore impact important ecosystem functions. The objective of this study was to determine how saltwater intrusion affects wetland nutrient biogeochemistry, with a specific focus on the soil microbial communities and physicochemical parameters that control nitrate removal. Our work took place in a tidal freshwater marsh in South Carolina, USA, where a 3.5-year saltwater intrusion experiment increased porewater salinities from freshwater to oligohaline levels. We measured rates of denitrification, soil oxygen demand, and dissimilatory nitrate reduction to ammonium (DNRA) and used molecular genetic techniques to assess the abundance and community structure of soil microbes. In soils exposed to elevated salinities, rates of denitrification were reduced by about 70% due to changes in the soil physicochemical environment (higher salinity, higher carbon:nitrogen ratio) and shifts in the community composition of denitrifiers. Saltwater intrusion also affected the microbial community responsible for DNRA, increasing the abundance of genes associated with this process and shifting microbial community composition. Though rates of DNRA were below detection, the microbial community response may be a precursor to increased rates of DNRA with continued saltwater intrusion. Overall, saltwater intrusion reduces the ability of tidal freshwater marshes to convert reactive nitrogen to dinitrogen gas and therefore negatively affects their water quality functions. Continued study of the interrelationships between biotic communities, the abiotic environment, and biogeochemical transformations will lead to a better understanding of how the progressive replacement of tidal freshwater marshes with brackish analogues will affect the overall functioning of the coastal landscape.
Keywordsecosystem ecology global change nitrogen cycle salinization sea level rise wetland
We thank Rebecca Schwartz and Amanda Rotella for excellent field assistance, especially in maintaining the field salinity manipulation experiment and collecting the soil cores analyzed herein. Additionally, we offer sincere thanks to Wayne Gardner, Liana Nichols, Suzanne Thompson, Gabriella Balasa, and David Berrier for their invaluable roles in this project. We are also grateful to Bob Jewell and the staff of Brookgreen Gardens. Without their permission to access the site, this research would not have been possible. We also acknowledge the efforts of two anonymous reviewers, whose comments helped improve the manuscript. This research was primarily supported by grants to S.C.N. from the University of South Carolina, Office of Research and Health Sciences Research Funding Program, and the US Department of Energy’s Office of Science (BER) through the Coastal Center of the National Institute for Climatic Change Research at Tulane University (DOE Grant # DE-FC02-06ER64298). Additional support was provided by the National Science Foundation (NSF EAR-0815627 to M.F.P. and NSF DEB-1355059 to R.B.F. and S.C.N.). This is contribution #1868 from the University of South Carolina’s Belle W. Baruch Institute for Marine and Coastal Sciences.
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