Microbial masking of deep hydrogen signals in soil-gas measurements

Natural hydrogen (H₂) produced by deep mantle and/or crustal processes has emerged as a promising source of carbon-free energy. Most current exploration methods rely on soil-gas sampling at one meter depth, where soil biological activity largely interact with H₂ through both biological production and, predominantly, biological consumption. Assessing the magnitude of microbial consumption and its drivers is therefore crucial in the context of natural hydrogen exploration.  In this study we developed a novel microcosm assay to discriminate the potential kinetics of aerobic H₂ consumption, anaerobic H₂ consumption and anaerobic H₂ production in soils. In parallel, we characterized soil physical, chemical, and biological properties (granulometry, pH, redox state, soil respiration, and enzymatic activities) to identify the factors controlling biological H₂ consumption and production. Experiments were conducted on soil sampled at 1 m depth in three sites exhibiting high soil H₂ concentrations near the North Pyrenean Fault Thrust in the southwest of France. We found that net H₂ production was consistently negligible confirming that biological H₂ and accumulation in soil is unlikely under most conditions. However, we found that H₂ consumption can reached up to 0.3 mmol g^-1 d^-1, indicating that microbial activity has the potential to deplete accumulated H₂ soil within seconds under optimal lab conditions. Three main metabolic pathways were identified for H₂ consumption: aerobic H₂ oxidation, anaerobic acetogenesis and denitrification. H₂ consumption rates were correlated with soil H₂ concentration within a site, suggesting that H₂ consumption directly influences our soil gas measurements. Our results demonstrate that biological consumption may be one of the major drivers of near-surface H₂ concentration and call for additional data to constrain true consumption kinetics across depths and sites.