Molecular hydrogen uptake by soils in forest, desert, and marsh ecosystems in California

The mechanism and environmental controls on soil hydrogen (H 2 ) uptake are not well understood but are essential for understanding the atmospheric H 2 budget. Field observations of soil H 2 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 H 2 fluxes using flux chambers from September 2004 to July 2005. Mean H 2 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 H 2 mixing ratios were between 3% and 51% of atmospheric levels at 10 cm and that the penetration of H 2 into deeper soil layers increased with soil drying. Soil removal experiments in the forest demonstrated that the litter layer did not actively consume H 2 , the removal of this layer increased uptake by deeper soil layers, and the exposure of subsurface soil layers to ambient atmospheric H 2 levels substantially increased their rate of uptake. Similar soil removal experiments at the desert site showed that extremely dry surface soils did not consume H 2 and that fluxes at the surface increased when these inactive layers were removed. We present a model of soil H 2 fluxes and show that the diffusivity of soils, along with the vertical distribution of layers that actively consume H 2 regulate surface fluxes. We found that soil organic matter, CO 2 fluxes, and ecosystem type were not strong controllers of H 2 uptake. Our experiments highlight H 2 diffusion into soils as an important limit on fluxes and that minimum moisture level is needed to initiate microbial uptake.