Illuminating root-soil mechanics

Soil compaction and increased global drought enhance soil mechanical strength and stiffness, which poses threat to soil health and ultimately adversely impacts society. Despite these restrictive conditions, some plant roots are able to grow under these harsh environments. This study was formed to develop a proof of concept measurement system that could be used to quaantify local plant biomechanical pressures during growth in in-situ assays. Using micro cone penetrometer root analogues, we tested the viability of combining X-ray computed tomography (XCT) with X-ray diffraction (XRD) measurements in order to understand whether we can characterize the mechanics during penetration associated with root growth. Cone penetrometers were used to penetrate into surrogate soil materials (e.g. gypsum), as these materials have porous and granular behaviour similar to natural soils but facilitate diffraction measurements due to their well defined atomic structures. Diffraction maps were generated across a sub region of the imaged space in order to quantify changes in the diffraction patterns. Diffraction maps are simplified representations by spectra peaks resulting from integrated Debeye-Scherrerr rings. Shifts in the spectra peaks are associated with deformations of the lattice crystals that comprise the surrogate soil material resulting from applied loads from the micro cone penetrometers. XCT images highlight compaction zones near the cone tip during penetration, with an increase in the compression zones radius as we push the cone deeper into the media. The XRD measurements peak shifts just outside of the local compacted zone of the cone. Qualitative finite element simulations illustrate that this is likely due to local inelastic deformations near the cone tip during penetration. However, the remote elastic deformations can be monitored as long as the inelastic zone remains local to the cone tip. While direct measurements near a root tip might be difficult due to the inelastic nature of the penetration process, the results show promise in terms of gaining insights into the locality of root growth biomechanical processes and the potential to elucidate root strategies under compacted soils in an in-situ test.