Volatile halocarbons emissions through interaction of saltwater intrusion and terrestrial organic matter along a salinity gradient in coastal southeastern United States

Abstract:

Freshwater wetlands along the coast of southeastern United States can be subjected to inputs of halogens from seawater intrusion following long-term sea level rise. The interaction of halogens with organic-rich sediments can lead to the formation of organohalogens. In this study, we report field and laboratory emission rates of volatile halocarbons (methyl halides and chloroform) along a salinity gradient in coastal South Carolina, including freshwater forest, degraded oligohaline forest, and salt marsh. For chloroform, the oligohaline (intermediate) forest showed the largest mean emissions, compared to the freshwater forest and mesohaline saltmarsh. Soil cores were measured intact live, spread out live, intact dead, and spread out dead. Interestingly, the dead soil and live soil incubations showed no statistical difference in chloroform emissions, suggesting that their formation is predominately abiotic. For methyl chloride and methyl bromide, saltmarsh soils were sources while freshwater forest and degraded oligohaline forest soils were sinks. Sterilization of soils caused emissions rates to be higher, even converting sinks to sources, suggesting that live microorganisms and enzymes in the soils were sinks for the methyl halides, thereby masking the abiotic production rates. The simultaneous production and consumption of methyl halides in these soils is consistent with prior studies investigating the bidirectional fluxes of these compounds. Our study indicates that long-term sea level rise that turns freshwater forest wetlands to degraded forest wetlands or saltmarsh can significantly change the halocarbon biogeochemistry in southeastern United States.