Spatial variation of methane emissions and methane-cycling communities in a coastal salt marsh.

Coastal salt marshes are highly valuable ecosystems that function as significant methane (CH₄) sources. CH₄ emissions in salt marsh ecosystems result from a range of production and consumption pathways affected by biotic factors, including plant types and microbial communities. Although salt marsh ecosystems are known to be important, there is still a notable lack of understanding of the specific microbial processes and mechanisms that result in spatial variability of CH₄ production and emission rates. Thus, this study was to identify the spatial variability of CH₄ fluxes across different vegetation zones, determine potential CH₄ production rates across varying soil depths, and characterize microbial communities associated with CH₄ cycling in a salt marsh ecosystem on Goodwin Island, York River, VA. We found that the edge of the salt marsh, dominated by Sporobolus alterniflorus, exhibited higher CH₄ fluxes, reaching up to 59.19 μmol m^-2 h^-1, compared to marsh interiors dominated by Spartina patens and Distichilis spicata. The top 5 cm of soils at the marsh edge showed the highest CH₄ production rate, up to 6.0 μmol g^-1 d^-1 based on laboratory incubation experiments. The microbial community analysis further suggested the coexistence of methanogenic archaea and sulfate-reducing bacteria in low-sulfate environments, which might be influenced by groundwater discharge. Accordingly, the integrated findings demonstrated salt marsh ecosystems, especially at the marsh edge, as CH₄ emission hotspots by specific microbial communities and dominant vegetation types. This also highlights the importance of incorporating these spatially explicit mechanisms into global CH₄ emission assessments.