Isotope effects associated with soil production and uptake of molecular hydrogen

Abstract

Molecular hydrogen (H2) is the second most abundant reduced gas in the atmosphere, but its biogeochemical cycle is not well understood. This thesis focuses on the soil production and uptake of H2. The biogenic soil sink of molecular H2 is the largest (~75% of total removal) and most uncertain term in the global atmospheric H2 budget. With the large uncertainty, it is difficult to predict how atmospheric H2 may respond to future changes in climate or anthropogenic emissions. The biological N2 fixation on land is a poorly understood source of H2, which contributes approximately 4% to the total source strength. Although it is globally a minor H2 source, it has a large local effect on the isotopic composition of H2, due to its very deuterium-depleted source signature. To constrain the biogenic source and sink, I collected air samples from a grass field at Cabauw and a forest site at Speuld in the Netherlands, and investigated the isotopic fractionation during H2 soil uptake and the isotopic signature of H2 emitted from the soil (δDsoil). The H2 emitted from the soil is expected to be from the N2 fixation within the soil. Our results show that the deposition velocity (vd) is higher on Speuld forest soil than on Cabauw grassland. The fractionation constant α is 0.943±0.013 for Speuld forest soil and 0.977±0.051 for the Cabauw grassland. A positive correlation between α and vd has been suggested in previous studies but is not found in our dataset. For selected experiments with strong emission and weak uptake of H2, the isotopic composition of soil emitted H2, δDsoil, obtained from a Keeling plot is -693±71‰. However, model calculations considering the influence of soil uptake on the Keeling plot suggests that this number may underestimate the actual δDsoil. An alternative mass balance approach suggests that the actual δDsoil emitted to the atmosphere varies over a range from -600‰ to +196‰. A possible reason could be strong fractionation during removal of H2 in the soil before it escapes to the atmosphere, which should be investigated in future studies.