Tidal marsh methane dynamics: Difference in seasonal lags in emissions driven by storage in vegetated versus unvegetated sediments

Estuarine and coastal wetlands exhibit high rates of carbon burial and storage in anaerobic sediments, but the extent to which carbon sequestration is offset by methane (CH₄) emissions from these ecosystems remains unclear. In this study we combine measurements of sediment-air CH₄ fluxes with monitoring of belowground CH₄ pools in a New Jersey tidal marsh in order to clarify mechanistic links between environmental drivers, subsurface dynamics, and atmospheric emissions. Measurements were conducted in an unvegetated mud flat and adjacent low marsh vegetated with Spartina alterniflora and Phragmites australis. Pore water measurements throughout the year revealed long-term CH₄ storage in mud flat sediments, leading to a seasonal lag in emissions that extended into winter months. CH₄ reservoirs and fluxes in vegetated sediments were well described by an empirical temperature-response model, while poor model agreement in unvegetated sediments was attributed to decouplings between production and flux due to storage processes. This study highlights the need to incorporate sediment gas exchange rates and pathways into biogeochemical process models. Key Points The subsurface methane pool explains the seasonality of methane emissions Plant venting stops bubble saturation and long-term subsurface methane storage Temperature models may fail due to decoupled production and emissions of methane