Moisture Controls on Hydrogen Oxidizing Bacteria: Implications for the Global Soil Hydrogen Sink

Assessing the impact of increasing anthropogenic H2 emissions on Earths radiative balance depends on understanding the soil microbial H2 sink--the largest and the most uncertain term in the global H2 budget. A primary control regulating the soil sink is soil moisture, with a relationship that remains poorly constrained. Here, we assess the sensitivity of microbial H2 oxidation to soil moisture in laboratory experiments with three temperate soils--silty loam, sandy loam, and loamy sand. Using genome-resolved metagenomics, we link H2 oxidation dynamics in these soils to specific microbial taxa adapted to withstand desiccation that have differential contributions to H2 uptake along the moisture gradient. The experiments reveal a notably low moisture threshold for H2 oxidizer activity, at water potentials between -70 and -100 MPa across all soil types, including in an arid sandy soil. These measurements, which represent some of the lowest water potentials reported for soil microbial activity, point to atmospheric H2 as a vital resource for microbial survival under stressful conditions. Through global simulations, we further show that the low moisture threshold for microbial activation increases the contribution of arid and semi-arid regions for soil H2 uptake by 4-7pp, while decreasing the contribution of temperate and continental regions (-7pp), even when assuming a linear scaling between uptake potential and soil organic carbon, as suggested by our experiments. Our results highlight the importance of H2 uptake under extreme hydrological conditions, particularly the roles of desertification, dryland expansion, and H2-oxidizer ecophysiology in modulating long-term changes in H2 uptake.