BIOGEOCHEMICAL TRANSFORMATIONS OF NUTRIENTS AND METALS IN SOIL SYSTEMS

Two critical challenges for agricultural and environmental stewardship are to reduce pollution by nutrients and metals and to restore productivity of degraded soils. This project, which is a collaboration between a microbiologist (Bruns), soil chemist (Martinez), and biogeochemist (Kaye), aims to elucidate fundamental biogeochemical interactions between soil nutrients, microorganisms and metals to improve soil nutrient retention, plant health, and environmental quality. Soil nitrogen (N) transformations are a major focus of this project, because N is the most limiting nutrient for plant growth and is subject to losses by leaching and denitrification. An important goal for U.S. agricultural research is to increase nitrogen use efficiency by crops, which currently take up only 40-50% of fertilizer N (National Research Council, 1993). One expected outcome of this work is enhanced fundamental knowledge of how agricultural management and soil microbial community composition/activity affect the oxidation state(s) and chemical form(s) of nitrogen. Better understanding of the biogeochemical mechanisms associated with fertilizer type and method of application will increase soil N retention and help reduce N fertilizer losses to surface and ground waters. Improved fertilizer efficiency will help mitigate environmental impacts of agricultural N transport, such as the formation of dead zones in the Chesapeake Bay and Gulf of Mexico. A second major focus of this project is metal interactions with soil nutrients. Cadmium (Cd) is an example of a heavy metal that occurs in soils as a result of both natural and anthropogenic activities. Cadmium sources to soils include bedrock weathering, industrial emissions, fossil fuel burning, and application of metal contaminated sludges and phosphate-fertilizers. Cadmium soil concentrations have increased in the last 60 years due to industrial activities, fertilizer and pesticide production and application, and electroplating. Overall, Cd toxicity in plants results from reduced uptake, transport, and use of nutrients such as Ca, Mg, P, and K and water. The main route of entry for Cd into humans is ingestion, and plant foods are the predominant sources of Cd in human diets. Cadmium uptake by food crops is of specific concern because of its toxicity to humans if enough is consumed. Understanding the mechanisms controlling Cd transport and uptake in soils is an important step for environmental remediation and protection of human and ecosystem health. The expected outcome of this part of the project is a better fundamental understanding of the interactions between heavy metals, microbes, and organic matter in soils. Because microbial activity can mediate changes in oxidation states and mineral phases containing metals, microorganisms play critical roles in metal availability for plant uptake and human exposure. Research results will provide fundamental knowledge that can be used to develop and evaluate soil management approaches that reduce biological availability of harmful metals.