Chemical staining of particulate organic matter for improved contrast in soil X-ray µCT images

Degradability of organic matter (OM) in soil depends on its spatial location in the soil matrix. A recent breakthrough in 3D-localization of OM combined dual-energy X-ray CT-scanning with OsO4 staining of OM. The necessity for synchrotron-based µCT and the use of highly toxic OsO4 severely limit applications in soil biological experiments. Here, we evaluated the potential of alternative staining agents (silver nitrate, phosphomolybdenic acid (PMA), lead nitrate, lead acetate) to selectively enhance X-ray attenuation and contrast of OM in CT volumes of soils containing specific mineral soil particle fractions, obtained via lab-based X-ray µCT. In comparison with OsO4, administration of Ag+ and Pb2+ resulted in insufficient contrast enhancement of OM versus fine silt (< 20 µm) or clay (< 2 µm) mineral particles. The perfusion procedure used in this work induced changes in soil structure. In contrast, PMA staining resulted in a selective increase of OM’s attenuation contrast, which was comparable to OsO4. However, OM discrimination from other soil phases remained a challenge. Further development of segmentation algorithms accounting for grey value patterns and shape of stained particulate OM may enable its automated identification. If successful in undisturbed soils, PMA staining may form an alternative to OsO4 in non-synchrotron based POM detection

Bodemverdichting in Vlaanderen : Gevolgen van bodemverdichting op het watertransport door een bodem

De gevolgen van bodemverdichting op het watertransport door een bodem zijn verkend. Een bodemfysische database is gecreëerd door naast historische metingen nieuwe metingen te verzamelen op verdichte percelen. Dat is gebeurd door op 26 percelen op 2 plekken op 3 diepten ongestoorde monsters te nemen en de bodemfysische eigenschappen te bepalen. Op 6 percelen zijn continue hydrologische metingen verricht van bodemvocht op 3 diepten en grondwaterstanden. De laatste metingen zijn gebruikt om te toetsen of het model SWAP met de gemeten bodemfysische eigenschappen in staat is het watertransport in het perceel te beschrijven. Met SWAP zijn voor 5 Vlaamse stroomgebiedjes de effecten van bodemverdichting verkend voor klimaatscenario’s door de verdichte en niet-verdichte toestand te vergelijken. De met de pedotransferfuncties en nieuwe data berekende effecten van verdere verdichting op de waterhuishouding blijken globaal gezien beperkt te zijn. Dit neemt niet weg dat de lokale effecten mogelijk aanzienlijk kunnen zijn, gelet op de grote variatie in bodemverdichting die binnen de percelen werd opgemeten. De verschillen tussen de verdichte en niet-verdichte situatie in stroomgebieden zijn beperkt. Klimaatscenario’s leiden op verdichte bodems tot meer oppervlakkige afstroming en meer droogtegevoelighei

DEM simulation of stress transmission under agricultural traffic part 1 : comparison with continuum model and parametric study

The discrete element method (DEM) is an appealing technique to study the effect of wheel loads and traction on the soil. Unlike in continuum-based soil mechanics, the soil is modelled as an assembly of particles, which move and interact with each other and transmit stress through force chains. In this study, the simulation of stress transmission under wheeling with the continuum method and with DEM was compared. Both methods showed a good agreement in the simulation of the vertical normal stress. The horizontal stresses were strongly different due to traction and draw-bar forces. Stresses in the DEM simulation were transmitted via a heterogeneous network of force chains: there was a coexistence of a distinct load-bearing and a dissipative force network in the shallow layers. Furthermore, the effect of the DEM material parameters (normal and shear stiffness and cohesion, and friction angle), wheel radius and traffic speed on the transmission of stress was evaluated. The normal and shear stiffness mainly influenced the depth of the stress transmission, and the friction angle influenced the horizontal stress transmission. The wheel radius influenced the stress distribution in the topsoil, but did not have a significant effect in deeper layers. Contrary to findings in literature on soil compaction, the increased traffic velocity caused higher stresses. DEM allows to simulate the geomechanical process of soil compaction and can give more insight in the role of heterogeneous stress transmission

DEM simulation of stress transmission under agricultural traffic part 2 : shear stress at the tyre-soil interface

Although it has been proven to be of considerable importance, it remains unclear how to model the effect of traction on soil compaction. This study used multiple approaches to simulate the stress under an active wheel with the pseudo-continuous Sohne model, and compared it to a discrete element method (DEM) model of the same scenario. It was found that the differences in normal horizontal stress (sigma(y)) due to these different boundary conditions in the Sohne model were limited. According to these simulations, the effect on normal vertical stress (sigma(z)) was negligible. This was in contrast with the DEM simulations, where the impact of traction on sigma(y) was also stronger and less shallow. Advanced numerical models can be used to gain insight in the importance of traction, and could be used to improve the boundary condition of the Sohne model. However, further experimental validation is needed

Accounting for surface refraction in velocity semblance analysis with air-coupled GPR

The aim of this study is to evaluate the velocity semblance analysis technique for air-coupled common midpoint (CMP) measurements with a small antenna offset. The technique was originally developed for seismic surveys, assuming the small spread approximation. Owing to the strong refraction at the surface and shallow investigation depth, this assumption is not valid in the case of air-coupled ground-penetrating radar (GPR). To overcome this assumption, a modification to the method is proposed accounting for the refraction at the surface. Synthetic experiments were executed to demonstrate that the traditional method resulted in a persistent overestimation, whereas the modified method improved the results significantly. Two field experiments have been conducted to test the method under different field conditions. In a first experiment on a road test site, the modified method improved the estimation of depth and propagation velocity significantly. However, the technique failed to estimate the propagation velocity and depth or objects in a second field test, due to rough terrain conditions and noise in the data. Therefore, an additional modi- fication was proposed, by incorporating in-line data as well. This improved the depth and velocity estimations significantly. Overall, this study demonstrates that the traditional velocity semblance analysis (TRAD) is not valid for air-coupled GPR. By accounting for the refraction at the surface and incorporating inline data, it is possible to successfully estimate depth and propagation velocity with small offset air-coupled GPR configurations

DEM simulation of stress transmission under agricultural traffic part 3 : evaluation with field experiment

In a concurrent paper, we compared a continuum model and a discrete element method (DEM) model in simulating stress transmission in soil under a wheel. Here, those models are evaluated with measurements of vertical normal stress (sigma(z)) under the wheels of a tractor-slurry spreader setup. It was found that the variation in the measured sigma(z) could be explained by the heterogeneous stress distribution in our structured soil, similar to what is observed in the DEM simulation. Furthermore, comparison of the continuum and DEM model showed that the lack of horizontal forces and dynamic load transfer at the boundary condition in the continuum model lead to a systematic underestimation of the measured sigma(z). Traction and drawbar forces have a significant impact on the stress state under a wheel. The continuum model and its boundary conditions should be modified to include these forces accurately

The relevance of measuring saturated hydraulic conductivity : sensitivity analysis and functional evaluation

The saturated hydraulic conductivity (K-s) is considered to be one of the main soil hydraulic properties to simulate water transport in the vadose zone, yet it is notoriously difficult to assess due to its large field-scale variability and sensitivity to preferential flow paths. With the increasing availability of convenient methods to measure the unsaturated hydraulic conductivity (K-u), the additional value of measuring K-s could be questioned. This study investigates the relevance of measuring K-s in two ways. Firstly, a sensitivity analysis was performed to evaluate which part of the hydraulic conductivity curve impacts the simulation of unsaturated water transport most. It was found that hydraulic conductivity curve between -10 cm and -1000 cm matric head was affecting the simulation more than at saturation. Secondly, a functional evaluation was performed by simulating water flow in six test fields according to three scenarios, in which the hydraulic properties were determined with the measured K-s and/or K-u. The simulation of saturation degree at three depths was the least accurate in the scenario where only K-s was measured. Remarkably, the most accurate simulation was found in the scenario with only the measured K-u. According to these results, measuring K-u would be of greater importance than measuring K-s for the simulation of water flow in the vadose zone