The greenhouse gas flux and potential global warming feedbacks of a northern macrotidal and microtidal salt marsh

Conversion of wetlands by drainage for agriculture or other anthropogenic activities could have a negative or positive feedback to global warming (GWF). We suggest that a major predictor of the GWF is salinity of the wetland soil (a proxy for available sulfate), a factor often ignored in other studies. We assess the radiative balance of two northern salt marshes with average soil salinities > 20 ppt, but with high (macro-) and low (micro-) tidal amplitudes. The flux of greenhouse gases from soils at the end of the growing season averaged 485 ± 253 mg m^-2 h^-1, 13 ± 30 μg m^-2 h^-1, and 19 ± 58 μg m^-2 h^-1 in the microtidal marsh and 398 ± 201 mg m^-2 h^-1, 2 ± 26 μg m^-2 h^-1, and 35 ± 77 μg m^-2 h^-1 in the macrotidal marsh for CO₂, N₂O, and CH₄, respectively. High rates of C sequestration mean that loss of these marshes would have a radiative balance of - 981 CO₂_eq. m^-2 yr^-1 in the microtidal and - 567 CO₂_eq. m^-2 yr^-1 in the macrotidal marsh.