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

Journal Article · Thu Nov 30 00:00:00 EST 2023 · Global Change Biology

DOI:https://doi.org/10.1111/gcb.17050· OSTI ID:2228379

Capooci, Margaret  ^[1]; Seyfferth, Angelia L.  ^[1]; Tobias, Craig  ^[2]; Wozniak, Andrew S.  ^[3]; Hedgpeth, Alexandra ^[4]; Bowen, Malique ^[3]; Biddle, Jennifer F.  ^[3]; McFarlane, Karis J.  ^[5]; Vargas, Rodrigo  ^[1]

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1. Department of Plant and Soil Science University of Delaware Newark Delaware USA
2. Department of Marine Sciences University of Connecticut Groton Connecticut USA
3. School of Marine Science and Policy University of Delaware Lewes Delaware USA
4. Department of Geography University of California, Los Angeles Los Angeles California USA, Center for Accelerator Mass Spectrometry Lawrence Livermore National Laboratory Livermore California USA
5. Center for Accelerator Mass Spectrometry Lawrence Livermore National Laboratory Livermore California USA

Abstract Tidal salt marshes produce and emit CH ₄ . Therefore, it is critical to understand the biogeochemical controls that regulate CH ₄ spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH ₄ production, and higher salinity concentrations inhibit CH ₄ production in salt marshes. Recent evidence shows that CH ₄ 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 ₄ and CO ₂ fluxes coupled with depth profiles of soil CH ₄ and CO ₂ pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH ₄ production and fate within a temperate tidal salt marsh. We found unexpectedly high CH ₄ concentrations up to 145,000 μmol mol ⁻¹ positively correlated with S ²⁻ (salinity range: 6.6–14.5 ppt). Despite large CH ₄ production within the soil, soil–atmosphere CH ₄ fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m ⁻²  s ⁻¹ ). CH ₄ and CO ₂ within the soil pore water were produced from young carbon, with most Δ ¹⁴ C‐CH ₄ and Δ ¹⁴ C‐CO ₂ values at or above modern. We found evidence that CH ₄ within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH ₄ is produced, including diffusion into the atmosphere, CH ₄ oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH ₄ 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 ₄ production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH ₄ budgets and blue carbon in salt marshes.

• Sponsoring Organization: USDOE
• Grant/Contract Number: DE‐AC52‐07NA27344; DE‐SC0022185; DE‐SC0023099
• OSTI ID: 2228379
• Alternate ID(s): OSTI ID: 2228776
• Journal Information: Global Change Biology, Journal Name: Global Change Biology Vol. 30 Journal Issue: 1; ISSN 1354-1013
• Publisher: Wiley-Blackwell
• Country of Publication: United Kingdom
• Language: English