Spartina alterniflora has the highest methane emissions in a St. Lawrence estuary salt marsh

Abstract

Salt marshes have the ability to store large amounts of ‘blue carbon’, potentially mitigating some of the effects of climate change. Salt marsh carbon storage may be partially offset by emissions of CH₄, a highly potent greenhouse gas. Sea level rise and invasive vegetation may cause shifts between different elevation and vegetation zones in salt marsh ecosystems. Elevation zones have distinct soil properties, plant traits and rhizosphere characteristics, which affect CH₄ fluxes. We investigated differences in CH₄ emissions between four elevation zones (mudflat, Spartina alterniflora, Spartina patens and invasive Phragmites australis) typical of salt marshes in the northern Northwest Atlantic. CH₄ emissions were significantly higher from the S. alterniflora zone (17.7 ± 9.7 mg C m⁻²h⁻¹) compared to the other three zones, where emissions were negligible (<0.3 mg C m⁻²h⁻¹). These emissions were high for salt marshes and were similar to those typically found in oligohaline marshes with lower salinities. CH₄ fluxes were significantly correlated with soil properties (salinity, water table depth, bulk density and temperature), plant traits (rhizome volume and biomass, root volume and dead biomass volume all at 0–15 cm) and CO₂ fluxes. The relationships between CH₄ emissions, and rhizome and root volume suggest that the aerenchyma tissues in these plants may be a major transport mechanism of CH₄ from anoxic soils to the atmosphere. This may have major implications for the mitigation potential carbon sink from salt marshes globally, especially as S. alterniflora is widespread. This study shows CH₄ fluxes can vary over orders of magnitude from different vegetation in the same system, therefore, specific emissions factors may need to be used in future climate models and for more accurate carbon budgeting depending on vegetation type.