Root Induced Changes of Soil Physical Properties Using Synchrotron X-Ray Microtomography (CMT) and Micromechanical Simulations

NSF Ecosystem Sciences and Hydrology Research Grant Awarded to Drs. Berli, Menon, Ghezzehei, Nico, Young and Tyler: July 2008

The rhizosphere, i.e. the zone of soil immediately surrounding plant roots, plays a prominent role in supplying plants with water and nutrients. However, surprisingly little is known about rhizosphere physical properties and how they affect root growth, water and nutrient uptake. The lack of non-invasive and non-destructive imaging techniques necessary to observe living roots growing in undisturbed soil have been a main reason for this shortcoming. Recent advances in synchrotron X-ray microtomography (CMT) provide the potential to directly observe soil physical properties around living roots in-situ. Goal of the proposed study is to quantify rhizosphere physical properties by (1) employing CMT to visualize physical root-soil structure interactions, (2) simulating root-induced structural alterations using various micro-mechanical approaches (analytical, finite element, discrete element modeling), and (3) estimating changes in rhizosphere hydraulic properties (water retention and hydraulic conductivity functions) based on CMT imaging and inverse modeling. The proposed study seeks to provide transformative insights into the role of rhizosphere physical properties for water and nutrient uptake by living plants. It serves as a stepping stone for better understanding the role of plants in the critical zone at the soil-atmosphere interface. The project cuts across disciplinary boundaries of biology, soil physics, and soil mechanics to offer new insights on surface runoff, soil compaction and erosion, losses to agricultural productivity, land reclamation, and first-principles of soil-plant interactions.

Research Areas

• Hydrology - research related to scaling of transport behavior of chemicals and microorganisms; remediation of contaminants; and scaling phenomenon in heterogeneous porous material.
• Biogeosciences - research related to biotic and abiotic controls on mineral formation; dissolution and transport in Mojave Desert ecosystems; influence of atmospheric CO2, nitrogen deposition, and soil moisture on root activity; stomatal conductance and C sequestration; and investigations of biocomplexity.
• Mathematics - computational aspects of kinetic effects on very long desorption time frames, and the changes to forms of the transport equations (e.g. fractional derivationes).

Research Hypotheses and Questions

• The nature of sandbox preconditioning that may be required to yield flow and transport results comparable to field conditions.
• Biotic and abiotic controls on CaCO3 formation on desert ecosystems.
• The extent of light, non-aqueous phase smearing with water table fluctuations.
• Implications of the existence of an enormous nitrate reservoir in the arid West (and other deserts).
• Energy and mass partitioning in arid soil ecosystems and the role of biogeochemical cycling.
• How new computational and theoretical methods, tested with data collected at this site can be used to understand energy and mass transport phenomenon.
• Creation of facility to test and benchmark sensors for subsurface monitoring. What are the environmental limitations and constraints to new classes of sensors, developed as part of the SEPHAS program.
• What is the role of soil structure in water movement and plant uptake in arid settings? Can we use that knowledge to predict future pedological development?
• Wetting front instability: no understanding of behavior of wetting front instability beyond ~10 cm; this has delayed progress in our understanding of gravitational fingering as far-reaching fluid flow and transport process in the unsaturated zone.
• Capillary barriers: theory and bench-scale experiments have shown this to occur, but performance in systems larger than several tens of centimeters is unknown.