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 a fuel), 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 remain highly uncertain. Fluxes of H2 within soils are rarely measured, and data to inform global models are 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 uptake, in which exponential regression models could be fitted to concentration data over a 7 min 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 silt/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 silt/clay content of the grassland site (55 % silt, 20 % clay) decreased aeration when soils were wet, resulting in poor aeration and low H2 uptake. The well-drained forest site (60 % sand) 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.