Soil water uptake from plants is a poorly understood question in the plant and soil sciences. A joint venture involving both plant and soil structure, this process is often rate-limited by the thin layer of soil surrounding the root, labeled as the rhizosphere, which has been shown to have physical, chemical, and biological properties that are significantly different from that of the bulk soil. Despite its importance disproportionate to its size and extent, little remains known about its characteristics, and furthermore under what particular biophysical conditions that the rhizosphere is influential, and to what extent. Our study analyzed the role of belowground plant morphology on soil structural and water dynamics of 4 different plant species (juniper, grape, maize, poplar), grown in sandy soils. The poplar system was extended to capture a full drying profile. Neutron radiography was used to characterize in-situ dynamics of the soil-water-plant system. A joint map of root morphology and soil moisture was created for the plant systems using digital image processing, where soil pixels were connected to associated root structures via minimum distance transforms.
Results show interspecies emergent behavior - a sigmoidal relationship was observed between root diameter and bulk/rhizosphere soil water content difference. Evaluating over the full range of drying, the extent of water retained near the thicker roots (i.e. the "slope" of the sigmoid) showed a rapid increase after a minimum bulk water content threshold was reached, suggesting the ability of the rhizosphere to maintain an optimal soil moisture regime for the roots, and its presence only in the thicker and more mature roots. Evaluating root diameter and water content difference as a proxy for the respective extent of root and rhizosphere development, we observed a logistic growth pattern for the rhizosphere: minimal development in the early stages is superceded by rapid onset of rhizosphere formation, which then stabilizes/decays with the likely root suberization. Our results provide an interspecies characterization of plant rhizosphere and soil water dynamics based on generalized plant structure and morphology, which provide an ideal avenue for providing a physical framework of species-independent belowground plant water uptake characterization.