Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO2) and methane (CH4) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide (\( {\text{ER}}_{{{\text{CO}}_{2} }} \)) or methane (\( {\text{ER}}_{{{\text{CH}}_{4} }} \)), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m−2 year−1) and 210Pb radiodating (336 g C m−2 year−1), a minor sink for atmospheric CH4, and a GHG sink (~620 g CO2-eq m−2 year−1). The tidal freshwater marsh was a source of CH4 to the atmosphere (~22 g C–CH4 m−2 year−1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though 210Pb radiodating indicated modest C accretion (110 g C m−2 year−1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm−1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH4 at the oligohaline site (>80 g C–CH4 m−2 year−1), making this site a significant GHG source (>2,000 g CO2-eq m−2 year−1). The oligohaline site did not accumulate C over the 2 year study period, though 210Pb dating indicated long term C accumulation (250 g C m−2 year−1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.