メタンの謎(Methane mystery)

2024-02-202024-06-14

2024-02-19 デラウェア大学 (UD)

◆潮汐塩性湿地は、植物や鳥、魚の生息地を提供し、ハリケーンや海岸侵食から守る役割も果たす。これらの湿地は温室効果ガスである二酸化炭素を吸収し、土壌が炭素の貯蔵庫として機能している。しかし、一部の微生物がメタンを生産し、土壌から排出されることがあり、これが大気中の二酸化炭素よりも強力な温室効果ガスである。
◆最近の研究では、潮汐塩性湿地の土壌に高濃度のメタンが存在し、これが周囲の湾や河川に移動していることが示されている。この発見は、地球温暖化の観点から重要であり、温暖化対策の考え方を見直す必要があるかもしれない。

＜関連情報＞

潮汐のある塩性湿地の土壌に高濃度のメタン：メタンはどこへ行くのか？ High methane concentrations in tidal salt marsh soils: Where does the methane go?

Margaret Capooci, Angelia L. Seyfferth, Craig Tobias, Andrew S. Wozniak, Alexandra Hedgpeth, Malique Bowen, Jennifer F. Biddle, Karis J. McFarlane, Rodrigo Vargas
Global Change Biology Published: 30 November 2023
DOI:https://doi.org/10.1111/gcb.17050

Abstract

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 S²⁻ (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 Δ¹⁴C-CH₄ and Δ¹⁴C-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.