A novel source of atmospheric H₂: Abiotic degradation of organic material

Molecular hydrogen (H₂) plays an important role in atmospheric chemistry by competing for reactions with the hydroxyl radical (OH·) and contributing to the production of H₂O in the stratosphere, indirectly influencing stratospheric ozone concentrations. The dominant pathway for loss of H2 from the atmosphere is via microbially-mediated soil uptake, although the magnitude of this loss is still regarded as highly uncertain. Recent studies have shown that abiotic processes such as photochemically mediated degradation (photodegradation) of organic material result in direct emissions of carbon (C) and nitrogen (N)-based trace gases as well as H₂. This H₂ production has important implications on source-sink dynamics of H₂ at the soil-atmosphere interface and thus it is important to quantify its variability over a range of plant types and materials. Here, we show laboratory observations of H₂ production and its temperature dependence during abiotic degradation of four plant litter types as well as pure cellulose and high lignin content woody material. A greater amount of H₂ was produced in the absence of solar radiation than from photodegradation alone, verifying that low temperature thermal degradation of plant litter is a source of H₂. In addition, we measured a significant release of H₂ both in the presence and absence of O₂. Our results suggest that abiotic release of H₂ during organic matter degradation is ubiquitous in arid ecosystems and may also occur in other terrestrial ecosystems. We propose that because these processes occur at the soil-atmosphere interface, they provide a previously unrecognized proximal source of H₂ for microbial uptake and confound interpretation of direct measurements of atmospheric uptake that are important for constraining the global H₂ budget.