Saltwater reduces CO 2 and CH 4 production in organic soils from a coastal freshwater forested wetland

Abstract. A major concern for coastal freshwater wetland function and health is saltwater intrusion and the potential impacts on greenhouse gas production. Coastal freshwater wetlands are likely to experience increased hydroperiod with rising sea level, as well as saltwater intrusion. These potential changes to wetland hydrology may also alter forest structure and lead to a transition from forest to shrub/marsh wetland ecosystems. Loss of forested wetlands is already evident by dying trees and dead standing trees ("ghost" forests) along the Atlantic Coast of the US, which will result in significant alterations to plant carbon (C) inputs, particularly that of coarse woody debris, to soils. We investigated the effects of salinity and wood C inputs on soils collected from a coastal freshwater forested wetland in North Carolina, USA, and incubated in the laboratory with either freshwater or saltwater (2.5 or 5.0 ppt) and with or without the additions of wood. Saltwater additions at 2.5 ppt and 5.0  ppt reduced CO2 production by 41 and 37 %, respectively, compared to freshwater. Methane production was reduced by 98 % (wood-free incubations) and by 75–87 % (wood-amended incubations) in saltwater treatments compared to the freshwater treatment. Additions of wood resulted in lower CH4 production from the freshwater treatment and higher CH4 production from saltwater treatments compared to wood-free incubations. The δ13CH4-C isotopic signature indicated that in wood-free incubations, CH4 produced from the freshwater treatment was from the acetoclastic pathway, while CH4 produced from the saltwater treatments was more likely from the hydrogenotrophic pathway. These results suggest that saltwater intrusion into subtropical coastal freshwater forested wetlands will reduce CH4 fluxes, but long-term changes in C dynamics will likely depend on how changes in wetland vegetation and microbial function influences C inputs to the soil.