Environmental controls of the soil sink of atmospheric hydrogen: a year-long study at a UK grassland using the aerodynamic gradient technique

Atmospheric hydrogen (H2) plays a significant role in tropospheric chemistry, yet substantial uncertainty remains surrounding the magnitude and controls of the global soil sink. We report the first year-long, continuous measurements of H2 exchange using the aerodynamic gradient (AG) method at a temperate grassland. Despite the challenges associated with resolving small vertical gradients and the slow response time of GC instrumentation, the AG system produced a coherent record of ecosystem-scale H2 uptake. Temporally aggregated data revealed that soil moisture was the dominant control of H2 uptake at our site, with stronger uptake during warm, dry periods and minimal uptake when water-filled pore space was high (>50%). Soil temperature showed little independent effect once covariance with moisture was accounted for. The annual flux of two adjacent grassland fields was estimated to be −3.00 and −7.67 kg H2 ha−1 over the measurement period of September 2024 to September 2025 (mean deposition velocity (Vds) of 0.20 and 0.51 mm s−1). This study provides further evidence that the H2 soil sink can be quantified accurately at the field scale using currently available micrometeorological methods but highlights the need for thorough temporal and spatial resolution measurements of soil parameters to better understand the drivers behind H2 flux in soils.