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 (H) produced by subsurface geochemical processes is today the target of intensive research. In H exploration practices, bacteria are thought to swiftly consume H and, therefore, small near-surface concentrations of H, even orders of 10 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 H (up to 1 vol%), together with elevated concentrations of CH and CO (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 H seepage. Nevertheless, an ensemble of gas geochemical and soil microbiological analyses, including bulk and clumped CH isotopes, radiocarbon of CH and CO, and DNA and mcrA gene quantitative polymerase chain reaction analyses, revealed that H was only coupled to modern microbial gas. The H-CO-CH-HS association, wet soil proximity, and the absence of other geogenic gases in soils and springs suggest that H derives from near-surface fermentation, rather than geological degassing. H 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 H, CH and CO in soils, to be considered in H exploration guidelines and soil carbon and greenhouse-gas cycle research.