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

气候变化造成的荒漠化和土地利用集约化造成的土壤压实正在加剧从粮食安全到民间发展等一系列社会问题，预计在不久的将来会急剧恶化。集约化耕作造成的土壤压实影响了欧洲25-45%的可耕地面积。这导致密度增加，使土壤更坚硬，更难破碎。同样，随着赤道附近气温上升，干旱变得更加普遍，以前的可耕地将变成沙漠。随着土地因沙漠化而变得更加干燥，土壤再次变得更加脆弱和坚硬。这将对农业产生严重影响，因为种植植物根系和穴居蚯蚓限制了土壤的渗透。这些生物对作物和生态系统的健康至关重要。由于干燥的气候可能对粮食安全构成机械威胁，因此更好地了解促进根系生长的基本因素变得至关重要。很少有研究调查影响地下生物活动的关键物理限制，这些生物有机体为了改变自己的栖息地而采用的策略，或它们的改变对土壤结构适宜性的最终影响。了解这种生物物理的相互作用可以利用更大的潜力来提供更可持续的农业和土木设计实践。我建议开发工具来评估土壤的机械潜力，以支持在不断变化的气候和土地利用方式下促进农业的生物活动。为了在土壤中移动，植物根系和蚯蚓必须施加超过土壤弹性限制的压力，以实现足够大的变形以穿透土壤。在较湿润的条件下，非弹性土的变形具有延性，土的抗侵彻性较低。这有利于地下生物的活动。然而，随着土壤变干，毛细作用将土壤聚集体紧密地聚集在一起，更细的粘土颗粒开始紧密结合，这就形成了一个更脆、更有抵抗力的土体，阻碍了蚯蚓的活动。田间压实试验表明，土壤机械阻抗的增加也会降低作物产量。尽管在机械限制条件下，根系的生长效率降低，但研究表明，一些植物的根系仍然能够施加足够大的压力来生长。关于根如何实现这一目标的假设是通过多尺度过程，其中根膨胀压力允许轴向延伸，而尖端附近的细胞繁殖和重新定向，以减少局部摩擦效应并在根帽外聚集。虽然建议这些过程的集合允许一些根在局部施加高达1mpa的压力（能够破裂白垩），对于这种生长机制如何能够产生如此大的压力，以及这些负载实际施加的规模，目前还没有明确的认识。了解这些过程可以揭示植物特性，利用这些特性可以在半干旱地区开垦荒漠化土地并保持健康的土壤。除了能够看到地下，x射线技术还可以用来确定作用在物体上的物理力。为此，我打算将x射线技术与数学模型结合起来，以测量和监测地下活动，以解释和量化它们在地下施加的力。我的工作结果将最终概述阻碍生物物理活动的机械限制，并揭示促进植物在恶劣气候条件下生长的关键生物物理过程。这些考虑可以用来帮助修复因气候变化或土地利用集约化造成的受损土地，并更好地为未来的农业实践提供信息。

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.