Surprising concentrations of hydrogen and non-geological methane and carbon dioxide in the soil

Due to its potential use as a carbon-free energy resource with minimal environmental and climate impacts, natural hydrogen (H₂) produced by subsurface geochemical processes is today the target of intensive research. In H₂ exploration practices, bacteria are thought to swiftly consume H₂ and, therefore, small near-surface concentrations of H₂, even orders of 10² ppmv in soils, are considered a signal of active migration of geological gas, potentially revealing underground resources. Here, we document an extraordinary case of a widespread occurrence of H₂ (up to 1 vol%), together with elevated concentrations of CH₄ and CO₂ (up to 51 and 27 vol%, respectively), in aerated meadow soils along Italian Alps valleys. Based on current literature, this finding would be classified as a discovery of pervasive and massive geological H₂ seepage. Nevertheless, an ensemble of gas geochemical and soil microbiological analyses, including bulk and clumped CH₄ isotopes, radiocarbon of CH₄ and CO₂, and DNA and mcrA gene quantitative polymerase chain reaction analyses, revealed that H₂ was only coupled to modern microbial gas. The H₂-CO₂-CH₄-H₂S association, wet soil proximity, and the absence of other geogenic gases in soils and springs suggest that H₂ derives from near-surface fermentation, rather than geological degassing. H₂ concentrations up to 1 vol% in soils are not conclusive evidence of deep gas seepage. This study provides a new reference for the potential of microbial H₂, CH₄ and CO₂ in soils, to be considered in H₂ exploration guidelines and soil carbon and greenhouse-gas cycle research.