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Content 2

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Micro-scale spatial distribution of xenobiotics and trace elements in soils

Diversity in soil characteristics exists from landscape to nano-scale volumes of soil. At scales relevant to microorganisms, the highly heterogeneous physical structure of soil is compounded by dynamic fluxes in solute, substrate and energy flows that can change the micro-environments in fractions of a second or remain spatially isolated for centuries. As a result micro-volumes of soil may contain many different types of habitats or reaction environments (Mummey et al., 2006, Microbial Ecol. 51:404-411).

In the last 20 years, advances in analytical techniques has resulted in a number of studies that show the microscale spatial heterogeneity of metals in soil aggregates and soil thin sections. For example, Jacobson et al. (ES&T, 2007, 41(18):6343-6349) used microfocused synchrotron-based x-ray fluorescence spectroscopy (μ-SXRF) and electron microprobe spectroscopy to map the distribution of copper (Cu) in thin sections of intact Cu-contaminated soils and found that the metal was heterogeneously distributed at both milli- and micro-meter scales. Cu “hotspots”, which were found to be associated with particulate organic matter, were identified within micrometers of areas with little to no Cu. In the figure below 1000-fold differences in relative Cu intensity can be observed among neighboring pixels ( 20 μm or 0.3 mm diameter) mapping the distribution of Cu in a soil thin section.

Microorganisms that are sensitive to Cu could avoid metal hotspots entirely. Other micgroorganisms could preferentially populate the areas with high Cu concentrations.

Although the spatial distributions of microbes and heavy metals at the micro- to centimeter scale can and have been demonstrated,we are now interested in linking the spatial distribution of a contaminant such as Cu to the spatial distribution of a microbial population in an intact soil sample or aggregate to unequivocally demonstrate that the two distributions are related.With a seed grant from the Office of the Vice-President of Research we are developing a method that allows for the analysis of both microbial and elemental spatial distributions at the microscale.

Cofactor effects on contaminant fate and transport through soils

Abiotic influences on the reversal of soil hydrophobicity in fire affected soils

Every year thousands of hectares of the western US are impacted by wildfires. Along with the wildlife and vegetation, soils are affected when the incompletely combusted organic matter is deposited on the soil surface or volatilized organics condense on soil particles. The process causes the soils to become water-repellent or hydrophobic. The fire induced water repellency of soils, which lasts anywhere from several months to several years, is thought to play a key role in slope failure and erosion resulting in significant economic and environmental losses every year. Our lab is part of an interdisciplinary group of scientists at USU, including Anne Anderson and Helga Van Miegroet, who were awarded a research catalyst grant from the office of the Vice President of Research to study different environmental mechanisms which, over time reduce fire-induced soil hydrophobicity. Our group is focused on abiotic factors and characterizing the chemical changes that occur on particle surfaces as the water-repellency is reversed.

The effect of surfactants on Cryptosporidium parvum transport through soils

The microscopic protozoan Cryptosporidium parvum is a priority pathogen that may be transmitted between animals and humans. It is responsible for many waterborne disease outbreaks within the United States and probably the most important water-borne pathogen in developed countries. Oocysts typically enter water systems from the fecal waste of infected hosts either directly or after transport through soil. Major sources include municipal wastewater treatment facilities and infected agricultural livestock and wildlife populations. On the left is an immunofluorescence image of C. parvum oocysts isolated from murine (rodent) fecal matter. The US EPA microbiology website is the source of the image (Photo Credit: H.D.A. Lindquist, US EPA).

The management of C. parvum transport through soil to avoid ground and/or surface water contamination is a challenge and many studies are being conducted to better understand the mechanisms involved. Laboratory experiments have shown that the amount of water in a soil affects the retention of the oocysts. As the soil dries, the oocysts tend to accumulate increasingly at air-water interfaces (Darnault et al., 2001, Water Resources Research 37(7):1859-1868). It follows then, that a substance that would alter the properties of the gas-liquid interface in soils could affect the fate and transport of C. parvum oocytes sorbed to the interfacial surface. One such class of chemicals is surfactants. Because of their ampiphilic nature, and because they decrease water tension, surfactants have the potential to influence water flow and the retention properties of soils. Furthermore, surfactants occur widely in soils due to waste-water irrigation, land application of sewage-sludge, and agricultural practices such as the application of pesticides in surfactant solution sprays or the application of surfactants as soil wetting agents. Even naturally occurring soil humic acids have surfactant properties. In order to better understand the mechanisms that control the fate and transport of C. parvum in the vadose zone, it is paramount that the effect of surfactants on the air-water interface at which C. parvum sorb be investigated. With a grant from the USDA, Dr. Christophe Darnault's laboratory at the University of Illinois, Chicago, and my laboratory are investigating the effect of natural and industrial surfactants on C. parvum transport in soil environments representative of the agricultural watersheds of the Midwestern and Western United States.

Effect of elevated CO2 on the fate and transport of trace elements through soils

Soils contain large stores of global carbon (C) in the form of soil organic matter, dissolved organic compounds and gases like CO2 and methane that result from plant and microbe respiration. Many studies are being conducted to determine whether soils may ultimately be an additional source of atmospheric CO2 or a sink for atmospheric carbon.

In soils, CO2 is a source of acidity that results from its dissolution in the soil solution to form carbonic acid. Continuous additions of even low level acidity will affect soil chemistry. For example the buffering capacity of the soil will decrease, base or nutrient cations will be released from exchange sites and may be transported below the rooting zone, and soil mineral weathering will increase. Although the calcium carbonate in Utah's calcareous soils will mitigate the increased acidity, many of the trace elements associated with the calcite could be released as the carbonate weathers. With funding from the USU Agricultural Experiment Station, my lab is investigating the effects of elevated atmospheric CO2 on the fate and transport of plant micronutrients in soils. Some of the research involves greenhouse microcosms in which plants are grown in soil columns carved out of the ground, as seen in the image on the right, and set in lysimeters. In this way the complex interactions between plants, microbes and their soil environment is preserved.

Chinampas VFT

For educational copyright reasons access to the Chinampas virtual field trip is restricted to instructors at educational institutions as an example of an instuctional module used in Soils and Civilizations courses. Instructors who wish to view the site for a limited time should contact Astrid Jacobson at USU for a temporary password.

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Carbon and Nitrogen Cycling

Dr. Stark studies microbial controls on carbon & nitrogen cycling in forest and rangeland ecosystems. For more details on the type of research conducted in his lab, visit his lab poster.

Biogeochemistry of wildland soils

Dr. Van Miegroet research focuses on mineral cycling, nutrient transport mechanisms, and carbon sequestration in wildland soils. Among her current projects is a pilot study to investigate the mechanisms leading to the reversal of hydrophobicity in soils that have been affected by wildfires.