Soils found in dry parts of the Earth, including large areas of the western United States, that have a layer of distinctive bubble like pores at or near the soil surface, are called vesicular soils. These layers are relatively thin, usually less than 10 cm, but can act as a water repellent skin that determines whether water infiltrates, runs off, or evaporates from the soil surface. Plans for solar and wind power facilities in the deserts of the southwest have raised concern about increased soil disturbance in previously undeveloped lands. The formation of these soils is still a mystery. This project will evaluate the formation of vesicular layers in deserts, including their distribution, formation and relation to soil water movement. An index will be used to quantify the relative development of the layers. The study also considers new ways to explain the formation of vesicular pores, such as modeling their expansion using basic chemical principals relating the temperature and volume of gases. The study will include analyses of soil data bases, laboratory experiments and field studies. One new approach will be to use high resolution CAT scans to look inside soil samples and examine the size and shape of the pores without disrupting soil structure.
Results will be presented at professional meetings and in high school classes. Results will also be used in a proposal for the formal recognition of vesicular soils. Management of arid lands is at a critical moment right now, with plans for extensive energy facilities under development and review. Since the vesicular layer is a fragile surface that is highly susceptible to erosion by wind and water when it is disturbed, adoption of this terminology will increase awareness and highlight management options for the arid west.
In this study, we examine a feature seen in soils of desert regions around the world: a thin surface layer filled with bubble-like pores, like a foam at the surface of the soil. These pores are referred to as vesicular pores. The presence of vesicular pores is not only a distinctive and interesting feature of the desert landscape, but it also tells us a great deal about the way that a soil behaves. Soils with vesicular pores are crusting soils that slow the entry of water, causing water to run-off instead of soaking into the soil during infrequent desert rainstorms. This is a critical concern in the desert environment, where water is the most limiting resource for plant growth. Furthermore, most vesicular horizons are formed in a layer of dust that has been deposited at the soil surface. This dust layer is stabilized by the crusting surface of the vesicular horizon; but when disturbed, it is easily returned to the desert winds. Dust released from desert soils is both a hazard to human health and to the health of ecosystems in mountain ranges that surround the deserts. Disturbance of desert soils is a growing concern, as the population of desert cities grow and solar and wind power facilities are developed on remote desert lands that previously saw very little human activity. We used three different approaches to study the vesicular horizon: analysis of soil databases, field studies, and laboratory experiments. By using these different approaches we were able to integrate information across different scales of study, from the distribution of vesicular horizons across the western United States to the micro-scale processes that promote entrapment of gases that form the vesicular pores. In order to collect data on the size, shape, and distribution of vesicular pores in soil samples, we used a high resolution X-ray computed tomography (CT) scanner. This instrument collects three-dimensional data on the internal pore structure of samples, without breaking them apart to look at the pores inside, much like a medical CT scanner. We found that vesicular horizons cover 156,000 km2 of the western United States, according to the USDA soil databases. When we examine the broad-scale patterns in vesicular horizons, we see that vesicular porosity varies between different ecological zones of the desert. The vesicular pores are better expressed, having larger pores that occupy more of the soil volume, in the Great Basin Desert, which is colder and receives more precipitation than the lower latitude Mojave and Sonoran Deserts. We observed that the vesicular pores reform following soil disturbance, but the rate of water entry into the soils occurs at an even slower rate when vesicular soils are disturbed. Lastly, we found that the formation of vesicular pores in disturbed vesicular horizons is related to precipitation frequency and that soil respiration (CO2 released as soil microorganism "breath") increases the rate of vesicular pore growth. Data collected in this study has been used to support a proposal for the introduction of official USDA terminology for the recognition of vesicular horizons in soils. The use of this terminology in USDA soil survey reports would greatly increase the recognition of the vesicular horizon among those people and organizations that are directly responsible for the management of arid lands. By understanding more about the current distribution of the vesicular horizon, and the processes of vesicular horizon formation that lead to this distribution, we can better predict how the fragile skin of desert soils will respond the increasing human activity in desert lands.
