Quantifying soil biomechanics using X-Ray diffraction-imaging and physical modelling

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Desertification caused by climate change and soil compaction caused by land use intensification are exacerbating social issues ranging from food security to civil development and are expected to worsen dramatically in the near future. Soil compaction from intensified farming affects 25-45% of Europe's arable land area. This results inincreased density, which makes soil's more rigid and harder to break. Similarly, as temperatures rise near the equator and droughts become more common, previously arable land will become deserts. As land becomes drier due to desertification, soils again become more brittle and rigid. This will have severe impacts on agriculture, which restrict soil penetration by growing plant roots and burrowing earthworms. These organisms are vital for crop and ecosystem health. As drier climates may pose a mechanical threat to food security, a better understanding of the fundamental factors that facilitate root growth becomes crucial. Few studies have investigated key physical constraints that shape below ground biological activity, the strategies these biological organisms employ in order to modify their own habitats, or the resulting impact that their modifications have on soil structural suitability. Understanding this biophysical interplay may harness greater potential to provide more sustainable farming and civil design practices. I propose to develop tools to assess the mechanical potential for soil to support biological activity that promotes agriculture under changing climates and land use practices.In order to move through soil, plant roots and earthworms must exert pressures that exceed the elastic limitations of soil to achieve deformations large enough for penetration. Under wetter conditions, inelastic soil deformation is ductile, and soil's resistance to penetration is lower. This facilitates biological movement below ground. However, as soil dries, capillarity pulls soil aggregates densely together and finer clay particles begin to bind tightly, which creates a more brittle and resistant body that hinders earthworm activity. Field compaction experiments have demonstrated that increased soil mechanical impedance also reduces crop yields. Despite the reduced efficacy of root growth under mechanically limiting conditions, studies have demonstrated that some plant roots still manage to exert pressures great enough to grow. A hypothesis for how roots achieve this is via multi-scale processes where root turgor pressure allows axial extension while cells near the tip multiply and reorient themselves acting to reduce local frictional effects and assemble past the root cap. While this ensemble of processes is suggested to allow some roots to locally exert up to 1 MPa of pressure (capable of fracturing chalk), there is no clear understanding as to how this growth mechanism can enable this magnitude of pressure nor at what scales these loads are actually being applied. Knowledge of these processes may unlock plant traits which can be harnessed to reclaim desertified land and maintain healthy soils in semi-arid regions.Besides being able to see below ground, X-ray techniques can also be used to determine physical forces acting on objects. To this end, I intend to couple X-ray techniques in order to measure and monitor below ground activity with mathematical models to interpret and quantify the forces they apply below ground. The results from my work will ultimately outline mechanical constraints that hinder biophysical activity as well as unveil key biophysical processes that facilitate plant growth under harsh climatic conditions. These considerations could be used to help remediate damaged land caused by climate change or land use intensification and better inform future agricultural practices.