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

Tidal salt marshes are known to generate and release methane (CH₄), a potent greenhouse gas. While acetoclastic methanogenesis was long considered the main production pathway, recent findings suggest alternative mechanisms may be at play. It was already known that high salinity can suppress methane production in these ecosystems. However, a gap remained in understanding how methane is produced and transported in salt marsh soils under varying salinity conditions. Prior research has shown that sulfate reduction typically limits methanogenesis, but new evidence indicates that methylotrophic pathways might bypass this limitation. This uncertainty drove the need to investigate methane dynamics more comprehensively. The role of microbial communities and carbon sources in methane formation was also unclear. Additionally, the fate of methane within the soil matrix and its eventual release to the atmosphere or water bodies was not fully resolved. These unresolved questions motivated the current investigation into methane production and transport in salt marsh soils.

This study aimed to clarify the mechanisms behind methane production and transport in tidal salt marsh soils. The specific problem addressed was the apparent contradiction between high methane concentrations observed in soils and low emissions to the atmosphere. The motivation stemmed from the need to understand how methane is generated and transported in salt marshes, especially under conditions where acetoclastic methanogenesis should be limited. The research sought to determine whether alternative pathways like methylotrophic methanogenesis contribute significantly to methane production. Additionally, the study aimed to identify the primary routes through which methane leaves the soil, such as diffusion, oxidation, or lateral transport. The investigation also aimed to assess the role of microbial communities and carbon sources in methane formation. Understanding these processes is essential for improving methane budget estimates in salt marshes. The findings could help refine models of greenhouse gas emissions from coastal wetlands.

The study combined multiple analytical approaches to investigate methane dynamics in a temperate tidal salt marsh. Soil-atmosphere fluxes of methane and carbon dioxide were measured using chamber-based techniques. Depth profiles of methane and carbon dioxide concentrations in soil pore water were obtained through sampling at various depths. Stable and radioisotopic analyses were conducted to determine the age and sources of carbon in methane and carbon dioxide. Pore water chemistry was analyzed to assess salinity, sulfide concentrations, and other relevant parameters. Microbial community composition was examined using molecular techniques to identify methanogenic pathways. The researchers also tracked seasonal variations in methane production and transport. Data collection was conducted across different times of the year to capture temporal changes. These methods allowed the team to evaluate the biogeochemical controls on methane production and fate in salt marsh soils.

The study revealed unexpectedly high methane concentrations in salt marsh soils, reaching up to 145,000 μmol mol⁻¹. These concentrations were positively correlated with sulfide levels despite the presence of high salinity (6.6–14.5 ppt). Methane production was found to be linked to methylotrophic and hydrogenotrophic methanogenesis rather than acetoclastic pathways. The age of methane and carbon dioxide in pore water was determined using radiocarbon analysis, with most samples showing Δ¹⁴C values at or above modern. This indicated that methane was produced from recently fixed carbon sources. Soil-atmosphere methane fluxes were generally low but showed extreme variability, particularly during plant senescence, with emissions reaching 84.3 ± 684.4 nmol m⁻² s⁻¹. The study also identified lateral transport to tidal creeks as a likely dominant methane flux pathway. Methane oxidation within the soil was observed but was not the primary fate of produced methane.

The findings demonstrate that methane production in salt marsh soils is biogeochemically heterogeneous and influenced by multiple processes. Methylotrophic and hydrogenotrophic methanogenesis were identified as key pathways contributing to methane formation, even in the presence of high salinity. The study highlights the importance of considering alternative methanogenesis pathways when assessing methane dynamics in salt marshes. Methane fate is governed by several processes, including diffusion, oxidation, and lateral export to tidal creeks. The latter appears to be the dominant pathway for methane removal from the soil. The study underscores the challenges in evaluating methane budgets and blue carbon in salt marshes due to the complex interplay of biogeochemical factors. The observed variability in methane fluxes emphasizes the need for long-term monitoring to capture seasonal and environmental influences. These results provide new insights into methane cycling in coastal wetlands and suggest that current models may need to be revised to account for the full range of methane production and transport mechanisms.