Dissertation Research: Rhizodeposition Control on Spatial Compartmentalization of Active Microorganisms and Nitrogen Mineralization/Immobilization

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The movement of plant carbon from roots into the soil and the effects of this carbon on nitrogen mineralization and immobilization processes in the soil will be studied. Experiments will be performed using radioactive and stable isotope tracers to follow the movement and transformations of 14CO2 and 5NH4 in root chambers (rhizotrons) planted with Pinus ponderosa and Bromus mollis. The mass and activity of microorganisms will be a central focus of the research. The spatial distribution of microbial biomass will be determined in relation to plant roots. The metabolic state of these microorganisms will be assessed using vital stains and microautoradiography. Nitrogen mineralization and immobilization will be determined using short-term 15N pool dilution experiments. Results from these studies will be used to compare the metabolic status of rhizosphere and bulk soil microorganisms and to determine the importance of microbial competition for plant nutrients at the soil-root interface for these model systems. The significance of this project is that it should yield a valuable data base for future work on the relationship between plant carbon from roots and the amount of N available to plants; that is, it will lead to insight as to what controls N availability in soil systems. This is an important issue since N is often the nutrient limiting plant growth in ecosystems world- wide. The facilities at the University of California, Berkeley, are excellent for this type of research. Dr. Firestone is a first-rate scientist and teacher and this area of research is one of her specialties. Ms. Norton shows all evidence of being a promising student with three publications to her credit even before beginning her doctoral work.