Impacts of Hydraulic Redistribution on Root Exudation and Rhizosphere Biogeochemistry

Authors

Richard Marinos¹* (rmarinos@buffalo.edu), Scott Mackay¹, Angela Possinger², Fiona Ellsworth¹, Mitchell Hitchcock¹

Institutions

¹The State University of New York–Buffalo, NY; ²Virginia Polytechnic Institute and State University, Blacksburg, VA

Abstract

Hydraulic redistribution (HR) is the passive movement of water from wet soil regions to dry soil regions (for example from deep soils at the water table to dry surface soils) using plant roots as conduits. It is well-appreciated that HR plays an important role in plant growth and survival during periods of drought but less is known about how HR sustains microbial life in the soil. HR provides water to microbes in the rhizosphere, and it also may flush root exudates into the rhizosphere. The central hypotheses behind this research are: (1) plants produce higher quantities of root exudates when undergoing HR; (2) the flow of water during HR carries these exudates farther than they would normally diffuse into the soil, expanding the rhizosphere, and; (3) HR-delivered exudates and water sustain microbial activity during drought with important implications for carbon cycling in soils. In this project, researchers are using greenhouse experiments to quantify how HR impacts the quantity, timing and location of root exudation in two temperate hardwood tree species. The results from these experiments are used to parameterize a root-scale biogeochemical model and a plant-scale ecohydrological model that will permit researchers to examine the effects of HR-driven exudation on rhizosphere microbial activity.

Initial experimental work on this project has focused on developing and refining analytical methods for the analysis of root exudates under both HR and non-HR conditions. Researchers have developed a workflow that allows imaging of root exudation using anodic aluminum oxide as a blotting medium to transfer exudates from the root-soil system in a spatially preserved manner. Analysis by FT-ICR-MSI confirmed that a broad suite of exudates can be analyzed in this manner. In upcoming work, researchers will apply this method to roots that are experiencing both HR and non-HR conditions to identify how HR changes the flux of exudates at the root scale.

Initial modeling work on this project has focused on expanding the TREES ecohydrological model to include subroutines for root exudation, including variable exudate fluxes based on user-defined parameters and variable C:N stoichiometry of exudate fluxes. Future work will focus on incorporating experimental data into this model, using measured root architectures to scale from individual root observations to whole-plant exudate fluxes.