Root-Induced Compaction and its Effects on Soil Microstructure and Soil Hydraulic Properties Using X-ray Micro-Tomography and Numerical Simulations
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Soil is a challenging environment where resources, such as water, nutrients and air, are scarce and patchy. Plants and soil microorganisms have limited ability to move toward nutrient-enriched zones, compared with animals. Plants and roots have evolved to adapt and influence their physical environment as a way to create favorable conditions for its development. On the other hand, the structure of the soil controls its ability to receive, store and transmit water, to cycle carbon and nutrients, and to disperse anthropogenic and natural contaminants. As roots grow, soil structure changes; the pressure exerted produces a decrease in soil porosity and an increase in soil density, creating complex soil-root interactions.The scales of interest in the study of soil structure range from angstroms to kilometers. To study processes such as plant growth, root penetration, storage of water, and movement of nutrients, the most relevant scales range from microns to centimeters.Recent advances in X-ray micro-tomography (XMT) imaging have allowed the study of rhizosphere on spatial scale hitherto unreachable. This dissertation investigates utilization of XMT to study root-induced compaction on soil micro-structure and its effect on soil hydraulic properties. The current state of the art in soil-plant-root interactions using XMT is presented. Techniques to improve sensitivity of XMT images when roots are being scanned, are discussed. Feasibility of various segmentation techniques and examples of their use to quantify soil properties are addressed. XMT data quantification is then used in the study to feed numerical flow models.At bulk-scale, soil compaction has been associated with decrease in porosity and hydraulic conductivity and thus a reduction in soil productivity and fertility. However, at the aggregate scale, this study shows that natural root-induced compaction increases contact areas between aggregates, which may lead to an increase in unsaturated hydraulic conductivity of the soil adjacent to the roots. An analytical model to estimate the effect aggregate compaction on its effective hydraulic conductivity was developed. It was found that the effective hydraulic conductivity of a pair of aggregates, undergoing uniaxial stress, increased following a non-linear relationship as inter-aggregate contact area increased.Additionally, this study presents numerical modeling using actual XMT images of aggregated soil around a root surrogate to demonstrate how root-induced deformation increases unsaturated water flow towards the root, providing insight into the growth and water uptake patterns of roots in natural soils.Finally, this study presents a novel procedure that allowed coupling mechanical induced compaction with fluid flow simulation to model root-induced compaction in the rhizosphere. XMT images were used to describe in detail the rhizosphere microstructure. Then, finite element simulations were used to study the effect of an expanding root on root water uptake. The effect of increase aggregate connectivity, increase in effective hydraulic conductivity, root-soil contact area, and increase in local and global hydraulic gradient were evaluated. These results contribute to a better understanding of soil-water-root interactions.