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

Tidal salt marshes produce and emit CH . Therefore, it is critical to understand the biogeochemical controls that regulate CH spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH production, and higher salinity concentrations inhibit CH production in salt marshes. Recent evidence shows that CH 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 CH and CO fluxes coupled with depth profiles of soil CH and CO pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH production and fate within a temperate tidal salt marsh. We found unexpectedly high CH concentrations up to 145,000 μmol mol positively correlated with S (salinity range: 6.6-14.5 ppt). Despite large CH production within the soil, soil-atmosphere CH fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m  s ). CH and CO within the soil pore water were produced from young carbon, with most Δ C-CH and Δ C-CO values at or above modern. We found evidence that CH within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH is produced, including diffusion into the atmosphere, CH oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH 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 CH production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH budgets and blue carbon in salt marshes.