Vesicular Horizon Distribution, Properties, and Pedogenic Processes in Deserts of the Western United States
Vesicular horizons are common surface horizons in arid and semi-arid lands and are characterized by the prevalence of nearly spherical, non-interconnected vesicular pores. They regulate surface hydrology in water-limited ecosystems, but are easily disrupted by human land-use. In order to interpret potential changes to vesicular horizon distribution and properties in response to land-use and climatic change, we need to know more about the current distribution of vesicular horizons and how they are formed. The objectives of this dissertation are to: 1) evaluate the distribution and properties of vesicular horizons across the western U.S., 2) determine how disturbance and recovery of vesicular horizons impacts their pore morphology and hydraulic properties, and 3) determine the influence of microbial respiration and thermal expansion of gases on vesicular pore formation. The methods used in this research include analysis of soil databases, examination of vesicular horizon recovery from disturbance in the field, creation of vesicular pores in the lab, and X-ray computed tomography analysis of soil pores. Our results show that vesicular horizons cover 156,000 km2 of the western U.S. and are best expressed in the cold deserts (i.e., Central and Northern Basin and Range) relative to the warm deserts (i.e., the Sonoran and Mojave Deserts). We observed that vesicular horizons recover rapidly from disturbance, both in artificially disturbed soils and in tire tracks, and that the rate of recovery is dependent on the frequency of precipitation during the recovery period. Disturbance does not severely alter the hydraulic properties of the vesicular horizon, but leads to a slight reduction in Ksat and Gardner's α. In the lab, we found evidence that both biotic and abiotic processes are involved in vesicular pore formation; however, the role of thermal expansion of gases in growth of the vesicular pores was not supported. Through this work we have found that vesicular horizons occur extensively throughout arid and semi-arid regions of the western U.S. Vesicular horizon formation and recovery from disturbance is dependent on climatic conditions (e.g., precipitation frequency, soil temperature), biological factors (e.g., shrub cover, microbial respiration), and their interactions.