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 CH_4 are needed to understand the global CH_4 cycle and climate-change mitigation options. Glaciated environments might play a critical role due to potential feedbacks with global glacial meltdown. In an emerging glacier forefield, an ecological shift occurs from an anoxic, potentially methanogenic subglacial sediment to an oxic proglacial soil, in which soil-microbial consumption of atmospheric CH_4 is initiated. The development of this change in CH_4 turnover can be quantified by soil-gas profile analysis. We found evidence for CH_4 entrapped in glacier forefield soils when comparing two methods for the collection of soilgas samples: a modified steel rod (SR) designed for one-time sampling and rapid screening (samples collected ~ 1 min after hammering the SR into the soil), and a novel multilevel sampler (MLS) for repetitive sampling through a previously installed access tube (samples collected weeks after accesstube installation). In glacier forefields on siliceous bedrock, sub-atmospheric CH_4 concentrations were observed with both methods. Conversely, elevated soil-CH_4 concentrations were observed in calcareous glacier forefields, but only in samples collected with the SR, while MLS samples all showed sub-atmospheric CH_4 concentrations. Time-series of SR soil-gas sampling (additional samples collected 2, 3, 5, and 7 min after hammering) confirmed the transient nature of the elevated soil-CH_4 concentrations, which were decreasing from ~ 100 μL L~(?1) towards background levels within minutes. This hints towards the existence of entrapped CH_4 in calcareous glacier forefield soil that can be released when sampling soil-gas with the SR. Laboratory experiments with miniature soil cores collected from two glacier forefields confirmed CH_4 entrapment in these soils. Treatment by sonication and acidification resulted in a massive release of CH_4 from calcareous cores (on average 0.3-1.8 μg CH_4 (g d.w.)~(?1)) (d.w. - dry weight); release from siliceous cores was 1-2 orders of magnitude lower (0.02-0.03 μg CH_4 (g d.w.)~(?1)). Clearly, some form of CH_4 entrapment exists in calcareous glacier forefield soils, and to a much lesser extent in siliceous glacier forefield soils. Its nature and origin remain unclear and will be subject of future investigations.