Search for a command to run...
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.