Quantifying the soil sink of atmospheric Hydrogen: a full year of field measurements from grassland and forest soils in the UK

Abstract. Emissions of hydrogen (H₂) gas from human activities are associated with indirect climate warming effects. As the hydrogen economy expands globally (e.g. the use of H₂ gas as an energy source), the anthropogenic release of H₂ into the atmosphere is expected to rise rapidly as a result of increased leakage. The dominant H₂ removal process is uptake into soils; however, removal mechanisms are poorly understood and the fate and impact of increased H₂ emissions remains highly uncertain. Fluxes of H₂ with soils are rarely measured, and data to inform global models is based on few studies. This study presents soil H₂ fluxes from two field sites in central Scotland, a managed grassland and a planted deciduous woodland, with flux measurements of H₂ covering full seasonal cycles. A bespoke flux chamber measurement protocol was developed to deal with the fast decline in headspace concentrations associated with rapid H₂ fluxes, in which non-linear regression models could be fitted to concentration data over a 7-minute enclosure time. We estimate annual H₂ uptake of -3.1 ± 0.1 and -12.0 ± 0.4 kg H₂ ha^-1 yr^-1 and mean deposition velocities of 0.012 ± 0.002 and 0.088 ± 0.005 cm s^-1 for the grassland and woodland sites, respectively. Soil moisture was found to be the primary driver of H₂ uptake at the grassland site, where the high clay content of the soil resulted in anaerobic conditions (near zero H₂ flux) during wet periods of the year. Uptake of H₂ at the forest site was highly variable and did not correlate well with any localised soil properties (soil moisture, temperature, total carbon and nitrogen content). It is likely that the high clay content of the grassland site (55 % clay) decreased aeration when soils were wet, resulting in poor aeration and low H₂ uptake. The well-drained forest site (25 % clay) was not as restricted by exchange of H₂ between the atmosphere and the soil, showing instead a large variability in H₂ flux that is more likely to be related to heterogeneous factors in the soil that control microbial activity (e.g. labile carbon and microbial densities). The results of this study highlight that there is still much that we do not understand regarding the drivers of H₂ uptake in soils and that further field measurements are required to improve global models.