P23 - Physics & Mechanics
Localization and quantification of physical and mechanical rhizosphere properties using X-ray microtomography and micro-sensing techniques
The aim of this project is to non-invasively image and study the evolution of structural patterns in the rhizosphere using soil column experiments (SCE) via X-ray computed tomography and quantitative image analysis. Two textures and genotypes from the central experimental platform will be used to investigate hydraulic and mechanical processes during root growth in a series of experiments. Aim is to use two different moisture contents: an “ideal” and a water-stressed treatment. The water content of this treatment will be determined with a pre-test and should allow sufficient, yet distinctly modified root growth. Our hypothesis is, that the resulting root structure under drought conditions, will be specific to soil texture and maize genotype. Tool-IP will be used for image analysis of pore and root system. Furthermore, deformation patterns will be identified using DaVis, a specialized software based on digital image correlation.
The distribution and transport of oxygen in the rhizosphere has a great impact on microbial activity and bio-chemical reactions and will be analysed by using micro-sensors after the harvest of the plants of the SCE. In collaboration with the working group at the University of Bayreuth the effect of mucilage and soil moisture on micro-mechanical processes during root penetration will be studied. For this workpackage a series of mechanical tests is planned, including penetration resistance, compression and shear vane tests. Another cooperation is planned with the Technical University of Munich regarding rhizodeposition and its effect on structure formation in the rhizosphere by applying novel staining techniques to visualize micro-scale soil organic matter distribution.
Phase 2
In the second phase we study the evolution of mechanical soil parameters in a combination of field- and lab-based experiments with a special focus on stability and penetration resistances (PR), including in-situ measurements with a customized micro-penetrometer and relating PR values to spatial root data. Our soil column experiment comprises different moisture contents and bulk densities in accordance with the approaches of other PP members and aims to quantify the effect of PR on root growth, as well as the impact of physical boundary conditions on root geometries.
Deformation patterns around growing roots and a quantification of the mechanical extent of the rhizosphere in the contrasting PP substrates are studied by employing image analysis of root networks and digital volume correlation with state-of-the-art software (ImageJ and LaVision-DaVis).
A comparison of stability of aggregates according to their origin (rhizosphere or bulk soil) is performed via dry crushing approach using aggregates taken at the complex sampling of the field experiment. This cooperation with the Technical University of Munich and Thünen Institute Braunschweig also includes the evaluation of spatial information of single macro-aggregates in their function as microbial habitats.
Outcome
We evaluated the spatiotemporal development of several soil mechanical parameters over both project phases starting from a homogeneous condition in the field. The pre-consolidation stress as an indicator for soil stability increased considerably in loam during the first few years whereas in sand no clear temporal development could be observed (
Rosskopf et al., 2022a
). Higher pre-consolidation stresses and lower compressibility were found in loam compared to sand. Measurements of penetration resistance (PR) in an artificial root growth experiment showed high local variations within the sample, whereas neither depth nor genotype had a significant effect on PR development. Together with P21 we investigated PR in relation to local soil density derived from Xray-CT scans (
Phalempin et al., 2023
). A field micro-penetrometer which will allow us to bridge the gap between field and lab scale is under construction.
The effect of mucilage mixed with substrates at different concentrations and water contents on mechanical soil properties showed that the interaction between these two factors was crucial for mechanical behaviour (
Rosskopf et al., 2022b
). In a dry loam the addition of 2 g mucilage per kg substrate led to a significant decrease in energy needed for root penetration whereas the contrary was observed at higher water contents. The overall stability of both substrates increased with mucilage concentration and decreased with higher water contents.
Deformation patterns along tap and seminal roots of both genotypes were investigated by Xray-CT in a real root growth experiment. Using digital volume correlation analysis (Davis), differences in the extent of the “mechanical” rhizosphere between sand and loam were observed (Rosskopf et al., in preparation).
The mechanical stability of aggregates derived from the rhizosphere and bulk soil was studied in collaboration with Technical University Munich (P19) and Thünen Institute Braunschweig (P17). Based on crushing tests we found that higher mechanical energies were needed to disrupt aggregates from the bulk soil compared to the rhizosphere aggregates. This could be due to the different structural organisation of the aggregates and the effect of root exudation on aggregate stability. Data on aggregate structure is available in form of Xray-CT scans but remains to be analysed.
Link to English scientific abstract
Link to German scientific abstract