High methane concentrations in tidal salt marsh soils: Where does the methane go?

Tidal salt marshes produce and emit CH₄ . Therefore, it is critical to understand the biogeochemical controls that regulate CH₄ spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH₄ production, and higher salinity concentrations inhibit CH₄ production in salt marshes. Recent evidence shows that CH₄ is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH₄ and CO₂ fluxes coupled with depth profiles of soil CH₄ and CO₂ pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH₄ production and fate within a temperate tidal salt marsh. We found unexpectedly high CH₄ concentrations up to 145,000 μmol mol^-1 positively correlated with S^2- (salinity range: 6.6-14.5 ppt). Despite large CH₄ production within the soil, soil-atmosphere CH₄ fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m^-2  s^-1 ). CH₄ and CO₂ within the soil pore water were produced from young carbon, with most Δ¹⁴ C-CH₄ and Δ¹⁴ C-CO₂ values at or above modern. We found evidence that CH₄ within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH₄ is produced, including diffusion into the atmosphere, CH₄ oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH₄ production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH₄ production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH₄ budgets and blue carbon in salt marshes.