Isotopic signatures of production and uptake of H.sub.2 by soil

Molecular hydrogen (H.sub.2) is the second most abundant reduced trace gas (after methane) in the atmosphere, but its biogeochemical cycle is not well understood. Our study focuses on the soil production and uptake of H.sub.2 and the associated isotope effects. Air samples from a grass field and a forest site in the Netherlands were collected using soil chambers. The results show that uptake and emission of H.sub.2 occurred simultaneously at all sampling sites, with strongest emission at the grassland sites where clover (N.sub.2 fixing legume) was present. The H.sub.2 mole fraction and deuterium content were measured in the laboratory to determine the isotopic fractionation factor during H.sub.2 soil uptake (α.sub.soil) and the isotopic signature of H.sub.2 that is simultaneously emitted from the soil ([delta]D.sub.soil). By considering all net-uptake experiments, an overall fractionation factor for deposition of α.sub.soil = k.sub.HD / k.sub.HH = 0.945 ± 0.004 (95 % CI) was obtained. The difference in mean α.sub.soil between the forest soil 0.937 ± 0.008 and the grassland 0.951 ± 0.026 is not statistically significant. For two experiments, the removal of soil cover increased the deposition velocity (v.sub.d) and α.sub.soil simultaneously, but a general positive correlation between v.sub.d and α.sub.soil was not found in this study. When the data are evaluated with a model of simultaneous production and uptake, the isotopic composition of H.sub.2 that is emitted at the grassland site is calculated as [delta]D.sub.soil = (-530 ± 40) ‰. This is less deuterium depleted than what is expected from isotope equilibrium between H.sub.2 O and H.sub.2.