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Chemistry and Biochemistry


Soil underlying a natural fire develops a hydrophobic soil sub-layer. This hydrophobicity decreases with time although the mechanisms are unresolved, but are thought to be biotic and abiotic. Some of the compounds accounting for hydrophobicity are polycyclic aromatic hydrocarbons (PAHs) of which pyrene is an example.

Bacteria that grew on pyrene were isolated from burned soils at two sites in Utah in order to analyze the biotic microbial degradation of the hydrophobic soil sub-layer. The two sites were Wood Camp (Logan, UT) and Milford Flats (Central UT). Identifications of the genera of nine isolated bacteria were made through l6S rRNA sequencing and included: Pseudomonas, Microbacterium, Streptomyces, Stenotrophomonas, Achromobacter, and Variovorax. The presence of such microbes could be involved in modifying the PAH-component of the burned soils to decrease hydrophobicity.

Exposure to sunlight decreases the hydrophobicity of the burned soils and increases the solubility of the phenolics. Chemical differences between the control non-burned and burned soils were seen in FTIR spectra. Peak modifications were intensified by removing the contributions to the spectra from the soil mineral components. A modified bacterial population enriched in pseudomonads and streptomycetes survived the heat, drying and UV-irradiation caused by exposure to sunlight.

Dry heat at 121°C alone did not eliminate the hydrophobicity of the burned soils, but wetting to saturation removed most hydrophobicity. Hydrophobicity was not restored after drying.

These findings suggest that both biotic and abiotic mechanisms are involved in the reversal of hydrophobicity imposed by natural fires.


This work made publicly available electronically on January 3, 2011.

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Faculty Mentor

Anne Anderson

Departmental Honors Advisor

Astrid Jacobsen