Soil Uptake of Molecular Hydrogen and Remote Sensing of Soil Freeze and Thaw

Citation

Smith Downey, Nicole V. (2007) Soil Uptake of Molecular Hydrogen and Remote Sensing of Soil Freeze and Thaw. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BXV8-HH61. https://resolver.caltech.edu/CaltechETD:etd-08182006-105638

Abstract

Soils play a large role in the cycling of atmospheric trace gases and are an important component of the climate system. The bulk of my thesis was directed at the role of soils in the global molecular hydrogen (H₂) cycle. I conducted field measurements of H₂ uptake in three Southern California ecosystems, and found that both the diffusion of H₂ into soils and the distribution of biological activity with depth controlled uptake rates at the surface. I then moved into the laboratory, where I mapped out the temperature and moisture controls on the biological uptake of H₂ in both desert and boreal forest soils. These experiments yielded simple relationships between moisture, temperature, and uptake rate, which I then used to constrain H₂ uptake by soils in a mechanistic model. The model is based on the 1D diffusion equation with a sink term, and is driven by a combination of remote sensing products and land surface modeling output. I calculated a mean annual soil H₂ sink of 67.3 ± 5.5 Tg. The model was able to reproduce the seasonal cycle at high northern latitudes, and implies that seasonal variability in snow cover is a key process controlling H₂ uptake. I found that snow cover and soil moisture control the uptake of H₂ globally, which may have important implications for the hydrogen budget in future climate change scenarios.

My second thesis topic involved the development of a remote sensing technique using passive microwave brightness temperatures to identify the freeze-thaw status of soils, which I applied to areas north of 45°N. I found a significant increase in the growing season length in North America by 3.8 days/decade, driven by both an earlier spring thaw and later fall freeze. The lengthening of the growing season may affect the carbon and hydrogen cycles at high northern latitudes, and is a new metric of global change.

Thesis Files