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.

The soil sink of molecular hydrogen is the largest and most uncertain term in the global atmospheric H₂ budget. Lack of information about the mechanisms regulating this sink limits our ability to predict how atmospheric H₂ may respond to future changes in climate or anthropogenic emissions. Here we present the results from a series of laboratory experiments designed to systematically evaluate and describe the temperature and soil moisture dependence of H₂ uptake by soils from boreal forest and desert ecosystems. We observed substantial H₂ uptake between −4°C and 0°C, a broad temperature optimum between 20°C and 30°C, a soil moisture optimum at approximately 20% saturation, and inhibition of uptake at both low and high soil moisture. A sigmoidal function described the temperature response of H₂ uptake by soils between −15°C and 40°C. Based on our results, we present a framework for a model of the soil H₂ sink.