Tropical- and subtropical-storm surges combined with sea level rise cause saltwater intrusions into low-lying coastal ecosystems along the southeastern coast of the United States, gradually converting freshwater forested wetland into saltmarsh. The transition zone between freshwater and saltwater ecosystems becomes a degraded forested wetland, where the combination of high levels of soil organic matter and elevated concentrations of halide ions creates a dynamic biogeochemical environment that may be a potential hotspot for halocarbon formation such as chloroform, methyl chloride, and methyl bromide. This study conducted field measurements at a transition zone in coastal South Carolina to quantify halocarbon exchange rates and laboratory soil incubations to determine the contributions of biotic versus abiotic processes. The degraded forested wetland showed significant chloroform emission rates (146 ± 129 nmol m–2 d–1). The degraded forested wetland remained a net sink for methyl chloride and a negligible source/sink for methyl bromide, unlike the saltmarsh which was a significant source for both. The laboratory incubations strongly suggest that methyl halide consumption in soils at the field site was biotic and that production of methyl halides and chloroform was largely abiotic and temperature-dependent, although additional experiments are required to rule out possible biotic production involving heat-resistant microbes. The results suggest that sea level rise and more frequent storm surges derived from global climate change, in the long term, may increase emissions of chloroform from coastal degraded forested wetlands and of methyl halides if salt marshes expand, with potential impacts for stratospheric ozone depletion.
The authors would like to thank the Natural Science Foundation (grants EAR-1529927 and EAR-1530375) for financial support. The authors are also grateful to Samuel Lin, Wenchen Liu for assistance on the access to the field, Jun-Jian Wang, Dennis Zellmann, David Miller and Hunter Robinson for the assistance on fieldwork, and Jerrold Acdan, Rory French, Bernard Koh, Anya Mikheicheva and Connor Shingai for assistance on laboratory analysis. The manuscript is greatly improved by incorporating the constructive comments and suggestions from three anonymous reviewers, who are hereby warmly appreciated. This work is technical contribution No. 6699 of the Clemson University Experiment Station.