Field Measurement of the Fate of Atmospheric H2 in a Forest Environment: from Canopy to Soil

Meredith, L.K.
CGCS Report Series, Sc.D. Thesis, Department of Earth, Atmospheric and Planetary Sciences, MIT, 254 pages, Report Nr. 86

Atmospheric hydrogen (H2), an indirect greenhouse gas, plays a notable role in the chemistry of the atmosphere and ozone layer. Current anthropogenic emissions of H2 are substantial and may increase with its widespread use as a fuel. The H2 budget is dominated by the microbe-mediated soil sink, and although its significance has long been recognized, our understanding is limited by the low temporal and spatial resolution of traditional field measurements. This thesis was designed to improve the process-based understanding of the H2 soil sink with targeted field and lab measurements.

In the field, ecosystem-scale flux measurements of atmospheric H2 were made both above and below the forest canopy for over a year using a custom, automated instrument at the Harvard Forest. H2 fluxes were derived using a flux-gradient technique from the H2 concentration gradient and the turbulent eddy coefficient. A ten-fold improvement in precision was attained over traditional systems, which was critical for quantifying the H2 concentration gradients above the turbulent forest canopy. Soil uptake of atmospheric H2 was the dominant process in this forest ecosystem. Rates peaked in the summer and persisted at reduced levels in the winter season, even across a 70 cm snowpack. We present correlations of the H2 flux with environmental variables (e.g., soil temperature and moisture). This work is the most comprehensive attempt to elucidate the processes controlling biosphere-atmosphere exchange of H2. Our results will help reduce uncertainty in the present-day H2 budget and improve projections of the response of the H2 soil sink to global change.

In the lab, we isolated microbial strains of the genus Streptomyces from Harvard Forest and found that the genetic potential for atmospheric H2 uptake predicted H2 consumption activity. Furthermore, two soil Actinobacteria were found to utilize H2 only during specific lifecycle stages. The lifecycle of soil microorganisms can be quite complex as an adaptation to variable environmental conditions. Our results indicate that H2 may be an important energetic supplement to soil microorganisms under stress. These results add to the understanding of the connections between the environment, organismal life cycle, and soil H2 uptake.

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