Hydrologic Context Alters Greenhouse Gas Feedbacks of Coastal Wetland Salinization

Changes in sea-level rise and precipitation are altering patterns of coastal wetland hydrology and salinization. We conducted paired laboratory (20 weeks) and field (15 weeks) marine salt addition experiments to disentangle the effects of hydrology (permanent versus intermittent flooding) and elevated marine salts (sulfate versus other salt ions) on greenhouse gas (GHG) emissions from freshwater forested wetland soils. Marine salt additions strongly affected GHG emissions in both experiments, but the magnitude, and even the direction, of GHG responses depended on the hydrologic context in which marine salt exposure occurred. Under permanent flooding, carbon dioxide (CO2) fluxes were unaffected by marine salts, whereas methane (CH4) fluxes were significantly suppressed by the addition of sulfate (as K2SO4) both with and without marine salts. In contrast, in intermittently flooded field and laboratory soils elevated salinity reduced carbon mineralization and CO2 fluxes, but enhanced CH4 fluxes relative to both controls and treatments with elevated sulfate. Thus, elevated salinity or alkalinity (and not sulfate) controlled both gaseous carbon fluxes under intermittent flooding. Nitrous oxide (N2O) fluxes had contrasting responses in the field and laboratory. In the laboratory, N2O fluxes were not significantly related to chemical treatment but increased with porewater ammonium concentrations, which increased in salinity treatments via cation exchange. In intermittently flooded field conditions, elevated salinity strongly suppressed N2O fluxes because ammonium did not accumulate in porewater; it was likely lost through advection, dispersion, or plant uptake. Understanding dynamic hydrologic and vegetation patterns across wetland landscapes will be critical for predicting both the magnitude and direction of wetland GHG responses to increasing marine salt across broad spatial scales.