Technical Note: Disturbance of soil structure can lead to release of entrapped methane in glacier forefield soils

摘要

Investigations of sources and sinks of atmospheric CH₄ are needed tounderstand the global CH₄ cycle and climate-change mitigation options.Glaciated environments might play a critical role due to potential feedbackswith global glacial meltdown. In an emerging glacier forefield, anecological shift occurs from an anoxic, potentially methanogenic subglacialsediment to an oxic proglacial soil, in which soil-microbial consumption ofatmospheric CH₄ is initiated. The development of this change inCH₄ turnover can be quantified by soil-gas profile analysis.We found evidence for CH₄ entrapped in glacier forefield soils whencomparing two methods for the collection of soil-gas samples: a modifiedsteel rod (SR) designed for one-time sampling and rapid screening (samplescollected ∼1 min after hammering the SR into the soil), and anovel multilevel sampler (MLS) for repetitive sampling through a previouslyinstalled access tube (samples collected weeks after access-tubeinstallation). In glacier forefields on siliceous bedrock, sub-atmosphericCH₄ concentrations were observed with both methods. Conversely,elevated soil-CH₄ concentrations were observed in calcareous glacierforefields, but only in samples collected with the SR, while MLS samples allshowed sub-atmospheric CH₄ concentrations. Time-series of SR soil-gassampling (additional samples collected 2, 3, 5, and 7 min after hammering)confirmed the transient nature of the elevated soil-CH₄ concentrations,which were decreasing from ∼100 μL L?1 towardsbackground levels within minutes. This hints towards the existence ofentrapped CH₄ in calcareous glacier forefield soil that can be releasedwhen sampling soil-gas with the SR.Laboratory experiments with miniature soil cores collected from two glacierforefields confirmed CH₄ entrapment in these soils. Treatment bysonication and acidification resulted in a massive release of CH₄ fromcalcareous cores (on average 0.3–1.8 μg CH₄ (g d.w.)−1) (d.w. – dry weight);release from siliceous cores was 1–2 orders of magnitude lower (0.02–0.03 μg CH₄(g d.w.)−1). Clearly, some form of CH₄ entrapmentexists in calcareous glacier forefield soils, and to a much lesser extent insiliceous glacier forefield soils. Its nature and origin remain unclear andwill be subject of future investigations.