Characterizing environmental drivers of the soil hydrogen sink through controlled laboratory experiments

Microbial uptake in soils is the dominant natural sink of atmospheric hydrogen (H2); however, the environmental controls governing this process remain poorly constrained across different soil types and land uses. This study investigates H2 fluxes with a diverse set of soils using controlled laboratory incubations designed to isolate the effects of soil moisture, soil physical properties, carbon pools, pH, and temperature. Topsoils from 11 sites were sieved, repacked, and subjected to a moisture gradient from saturation to near-dryness, with H2 fluxes regularly measured. Across all soils, moisture was the primary control of H2 uptake. Uptake initially increased as soils dried from saturation, peaked at intermediate moisture levels (10 to 40% water-filled pore space), and then declined again under both saturated and near-dry conditions. Organic content-rich systems behaved differently to mineral soils. Peatland soil showed exceptionally strong H2 uptake across a wider moisture range, driven by its porous structure and large dissolved organic carbon (DOC) pool. When included in regression models, DOC content emerged as a major predictor of flux, contributing significantly to an overall model explanatory power of R2 = 0.51 when paired with other variables. Investigations revealed that forest litter acted as a strong H2 sink, with uptake an order of magnitude higher than soils (by mass) and remained active even at sub-zero temperatures. These results demonstrate that H2 uptake is strongly regulated by soil physical structure and moisture in mineral soils, but also by labile carbon and organic-layer properties in high-carbon environments. The findings highlight the importance of explicitly representing peatlands, the availability of labile carbon pools, and surface organic layers in models of the global H2 budget and emphasise the need for more field measurements in carbon-rich and understudied ecosystems.