Salinity Influence on Methane Emissions from Tidal Marshes

The relationship between methane emissions and salinity is not well understood in tidal marshes, leading to uncertainty about the net effect of marsh conservation and restoration on greenhouse gas balance. We used published and unpublished field data to investigate the relationships between tidal marsh methane emissions, salinity, and porewater concentrations of methane and sulfate, then used these relationships to consider the balance between methane emissions and soil carbon sequestration. Polyhaline tidal marshes (salinity > 18) had significantly lower methane emissions (mean +/- sd = 1 +/- 2 gm(-2) yr(-1)) than other marshes, and can be expected to decrease radiative forcing when created or restored. There was no significant difference in methane emissions from fresh (salinity=0-0.5) and mesohaline (5-18) marshes (42 +/- 76 and 16 +/- 11 gm(-2) yr(-1), respectively), while oligohaline (0.5-5) marshes had the highest and most variable methane emissions (150 +/- 221 gm(-2) yr(-1)). Annual methane emissions were modeled using a linear fit of salinity against log-transformed methane flux (log(CH4) - -0.056 x salinity + 1: 38; r(2) = 0.52; p < 0.0001). Managers interested in using marshes as greenhouse gas sinks can assume negligible methane emissions in polyhaline systems, but need to estimate or monitor methane emissions in lower-salinity marshes.