Greenhouse Gas Fluxes along a Tidal Gradient of Low Salinity Freshwater Forested Wetlands

A dynamic closed chamber system was used to measure in situ efflux rates of CO2, CH4, and N2O to the atmosphere from wetland soils along the Mattaponi and Pamunkey rivers.  These greenhouse gas (GHG) flux measurements were taken quarterly from December 2015 to September 2017, at replicated hummock and hollow microtopography locations (where present) within each site. CO2 flux was measured directly from permanent soil collars by a LICOR-8100 system, and gas samples were collected from the LICOR-8100 for lab analysis of CH4 and N20 by gas chromatograph to be used in calculations of flux. Water level, temperature, and conductivity (converted to salinity) were measured every 20 minutes by a sonde deployed in a surface-groundwater well at each site. Sites along each river included a nontidal floodplain forested wetland, tidal freshwater forested wetland near the head-of-tide, tidal freshwater forested wetland along the lower tidal freshwater river, tidal freshwater forested wetlands under stress and converting to tidal marsh, and oligohaline marsh. By measuring GHG fluxes and identifying their underlying drivers in these wetland ecosystems along a tidal gradient of low salinity conditions, we may refine the salinity-GHG flux response of coastal wetlands. This work will also help inform about potential changes in GHG soil efflux rates associated with sea-level rise, salinization, and tidal extension.