A novel source of atmospheric H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: abiotic degradation of organic material

H Lee,T Rahn,H L Throop

doi:10.5194/bg-9-4411-2012

Abstract

Abstract. 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.

Highlights

Atmospheric H2 is one of the most abundant reduced gases in the atmosphere, with a seasonally varying dynamic equilibrium of approximately 530 ppb
The H2 production rates during thermal degradation and photodegradation of plant litter were normalized in two ways: per area and per mass: we suggest that H2 production rate by thermal degradation is most appropriate on a mass basis because temperature would affect litter biomass as a whole
Thermal degradation of plant litter typically led to a higher H2 production rate than did photodegradation alone (Fig. 2 and Supplement 1; range = 0.0036 to 1.01 nmol m−2 s−1)

Summary

Introduction

Atmospheric H2 is one of the most abundant reduced gases in the atmosphere, with a seasonally varying dynamic equilibrium of approximately 530 ppb. Once emitted to the atmosphere, H2 is either oxidized by OH (19 ± 5 Tg yr−1) or consumed through microbially-mediated soil uptake (56 ± 41 Tg yr−1), maintaining a seasonally dependant dynamic equilibrium in the troposphere (Novelli et al, 1999). Among the known source-sink dynamics at the soil-atmosphere interface, the dominant pathway for loss of H2 from the atmosphere is via microbially-mediated soil uptake (Novelli et al, 1999; Ehhalt and Rohrer, 2009) the magnitude of this loss is still regarded as highly uncertain (Constant et al, 2009; Ehhalt and Rohrer, 2009)

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Conclusion