Dissertation Research: Does Hydraulic Redistribution Increase Microbial Activity in the Rhizosphere?
Z.G. Cardon and P.M. Herron
The root systems of single plants can span a depth of soil exhibiting strong gradients of water availability. Often, in arid regions, dry soils are found at the surface and moister soils at depth. This gradient of water availability provides a driving force for the passive redistribution of water through the root systems from the deeper soils to the shallow, nutrient rich soils, usually at night. Such "hydraulic redistribution" is now recognized in almost 60 species worldwide, and has been shown to increase rates of transpiration, increase growth in neighboring plants, and alter the hydrology of entire regions. A number of hypotheses have emerged focusing on the potential importance of hydraulic redistribution of water into the shallow, nutrient-rich soil layers for maintenance of soil microbial activity, and thus potentially enhanced nutrient availability, around plant roots. This project explores whether hydraulic redistribution, and resulting patterns of water availability, affect rhizosphere microbial biomass, growth rates, and activity. We will test whether nutrient cycling around plant roots, and nutrient availability to plants, are enhanced by hydraulic redistribution in the greenhouse and in the field. The focal plant species is sagebrush (Artemisia tridentata), a deep-rooted shrub that is often a community dominant in semi-arid, western landscapes in the U.S. In field work, we will explore the relationship between the presence of hydraulic redistribution and the reproductive fitness of Artemisia; in the greenhouse, we will use soil bacteria that are genetically engineered to report soil water potentials to provide the first glimpse of soil water potentials at micro- and millimeter scales around roots. In both the greenhouse and the field, standard soil assays and soil psychrometers will be used to track microbial activity and bulk soil moisture. Results from this study will suggest mechanisms by which deep-rooted Artemisia tridentata can affect nutrient dynamics and water contents in shallow soils of semi-arid systems.
