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，预计在不久的将来还会急剧恶化。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，更难打破。同样，随着赤道附近气温上升，干旱变得更加普遍，以前可耕地将变成沙漠。随着土地因荒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，因为农业限制了植物根系和蚯蚓对土壤的渗透。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。很少有研究调查了形成地下生物活动的关键物理限制，这些生物有机体采用的策略，以改变自己的栖息地，或由此产生的影响，他们的修改对土壤结构的适合性。了解这种生物物理相互作用可能会利用更大的潜力，提供更可持续的农业和民用设计实践。我建议开发工具，以评估土壤的机械潜力，以支持生物活动，促进农业在不断变化的气候和土地利用practices. To移动通过土壤，植物根系和蚯蚓必须施加的压力，超过土壤的弹性极限，以实现变形足够大的渗透。在潮湿的条件下，非弹性土壤变形是延性的，土壤的抗渗透性较低。这有利于地下生物运动。然而，随着土壤干燥，毛细作用将土壤团聚体紧密地拉在一起，更细的粘土颗粒开始紧密结合，这产生了一个更脆和更有抵抗力的身体，阻碍了土壤的活动。田间压实实验表明，增加土壤机械阻抗也会降低作物产量。尽管在机械限制条件下根生长的效率降低，但研究表明，一些植物根仍然设法施加足够大的压力来生长。关于根如何实现这一点的假设是通过多尺度过程，其中根膨压允许轴向延伸，而尖端附近的细胞增殖并重新定向，以减少局部摩擦效应并组装通过根冠。虽然这一系列过程被认为允许某些根局部施加高达1 MPa的压力（能够压裂白垩），但对于这种生长机制如何能够实现这种压力大小以及这些负载实际上被施加的规模没有明确的理解。对这些过程的了解可能会揭示植物的特性，这些特性可以用来开垦荒漠化土地，保持半干旱地区土壤的健康。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，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约束，并揭示在恶劣气候条件下促进植物生长的关键生物物理过程。这些考虑因素可用于帮助补救因气候变化或土地利用集约化而受损的土地，并为未来的农业实践提供更好的信息。

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.