Integrating fine root diameter and watershed mapping to characterize rhizosphere hydrology

Root morphology and soil hydraulic characteristics were integrated using watershed distance mapping to show water distribution and uptake across the plant-soil interface. Poplar (Populus deltoides, P. trichocarpa), maize (Zea mays), juniper (Juniperus virginiana), grape (Vitis rotundifolia) and maple (Acer saccharum) seedlings were grown in sand, after which root diameter and soil water dynamics were assessed via sequential neutron radiography. Three local soil regions (root-soil interface or edge, rhizosphere, bulk soil) were classified based on both radial distance from the root surface and diameter of the nearest root, from which changes in water content and distribution were characterized using digital image processing. Water content dynamics across the rhizosphere showed two different species-independent processes: a consistently elevated water content at the root-soil edge interface which increased with root diameter, and hysteresis as the rhizosphere transitioned to bulk soil (similar to 0.5 cm from the root), independent of root diameter. Water uptake per unit root surface area declined exponentially with root diameter, independent of species. Results highlight the species-independent hydrologic characteristics of the rhizosphere and the potential for evaluating them in a local spatially connected soil context. Avenues for improved integration of soil and root characteristics are discussed.

Automated summary

This study integrated root morphology and soil hydraulic characteristics to illustrate water distribution and uptake at the plant-soil interface. Different species of seedlings were grown in sand, and their root diameter and soil water dynamics were examined using neutron radiography. The rhizosphere showed species-independent processes, with consistently elevated water content at the root-soil edge interface and hysteresis as it transitioned to bulk soil. Water uptake per unit root surface area declined exponentially with root diameter, regardless of species. The findings emphasize the importance of considering species-independent hydrologic characteristics of the rhizosphere in a local spatial context and suggest avenues for improved integration of soil and root characteristics.