Department of Earth System Science

The mechanism and environmental controls on soil hydrogen (H₂) uptake are not well understood but are essential for understanding the atmospheric H₂ budget. Field observations of soil H₂ uptake are limited, and here we present the results from a series of measurements in forest, desert, and marsh ecosystems in southern California. We measured soil H₂ fluxes using flux chambers from September 2004 to July 2005. Mean H₂ flux rates and standard deviations were −7.9 + −4.2, −7.6 + −5.3 and −7.5 + −3.4 nmol m⁻² s⁻¹ for the forest, desert, and marsh, respectively (corresponding to deposition velocities of 0.063 + −0.029, 0.051 + −0.036, 0.035 + −0.013 cm s⁻¹). Soil profile measurements showed that H₂ mixing ratios were between 3% and 51% of atmospheric levels at 10 cm and that the penetration of H₂ into deeper soil layers increased with soil drying. Soil removal experiments in the forest demonstrated that the litter layer did not actively consume H₂, the removal of this layer increased uptake by deeper soil layers, and the exposure of subsurface soil layers to ambient atmospheric H₂ levels substantially increased their rate of uptake. Similar soil removal experiments at the desert site showed that extremely dry surface soils did not consume H₂ and that fluxes at the surface increased when these inactive layers were removed. We present a model of soil H₂ fluxes and show that the diffusivity of soils, along with the vertical distribution of layers that actively consume H₂ regulate surface fluxes. We found that soil organic matter, CO₂ fluxes, and ecosystem type were not strong controllers of H₂ uptake. Our experiments highlight H₂ diffusion into soils as an important limit on fluxes and that minimum moisture level is needed to initiate microbial uptake.