Modeling of rhizosphere processes

The rhizosphere, i.e. the soil in the vicinity of a plant root, is a highly dynamic system, where many different processes contribute to the uptake of certain elements. Plant roots exploiting their surrounding medium can be of benefit when used to extract metals/metalloids from contaminated soil. Mathematical modeling is a powerful tool to investigate the underlying processes in the rhizosphere contributing to metal/metalloid bioavailability and uptake. Scope of this project is to improve the understanding of rhizosphere processes contributing to metal/metalloid bioavailability to selected hyperaccumulator and phytoremediation crops. An appropriate tool to address this are mechanistic rhizosphere models that simulate concentration gradients in the rhizosphere and uptake. Objectives of this project to set up mathematical formulations of single root models for metal/metalloid uptake, to find numerical algorithms, to create a computer simulation model, to validate this model using independent experimental data and to carry out sensitivity analyses. Innovations expected are the use of a pde-solver, a software package that solves partial differential equations (pde`s) automatically, which makes the cycle of model building-simulation-validation more efficient ("rapid prototyping"). With this tool, the most significant processes related to metal/metalloid bioavailability will be evaluated. Based on these results, a computer simulation program will be created. The mathematical formulations used will combine existing modeling approaches and include processes that have not been mathematically described in rhizosphere models before. Results of this project will help to gain an understanding of processes occurring in the rhizosphere of plant species taking up inorganic contaminants, in particular, phytoextraction. Understanding and prediction of processes in the rhizosphere and uptake strategies are considered key features in developing phytoremediation technologies.

The rhizosphere, i.e. the soil in the vicinity of the root, is a highly dynamic system where many different processes contribute to the uptake of nutrients and also contaminants from soil. The overall goal of this project was to use mathematical modeling in order to investigate the underlying processes in the rhizosphere contributing to contaminant bioavailability and uptake. Rationale is the potential use of plants for the clean-up of contaminated soils. Close collaboration between modelers and experimentalists enabled us to achieve this. We mainly studied nickel (Ni) bioavailability to the Ni hyperaccumulating plant Gösing Penny-cress (scientific name: Thlaspi goesingense), as well as heavy metal and water uptake by willows. Experimental data for model validation and parameterization were measured or taken from literature. Due to its small biomass, the Gösing Penny-cress is not likely to be applicable for practical purposes, but rather serves as a model plant to investigate the important rhizosphere processes on a small scale (single root scale). The plants were grown in Ni contaminated soil and it was found that under these conditions the uptake rate was most sensitive to the plant parameters. Furthermore, results suggest that root exudates my trigger replenishment of plant available Ni due to dissolution of Ni-bearing minerals that are available in the soil. Willows have been found to accumulate high amounts of heavy metals in their leaves. Their high biomass makes them attractive for practical purposes and we studied them on a larger (root system) scale. In a pot experiment with willows, we measured the water content at different depths. A qualitative agreement was found with the results of a mathematical model predicting a drying zone in the middle of the root zone. This may have an important impact on the selection of genotypes so as to make phytoextraction more efficient, since water is a prerequisite for solute and hence heavy metal uptake. Rhizosphere models and input parameter values used were compiled in a database which is available at http://rhizo.boku.ac.at/start. One of the main goals of this database is to make rhizosphere models accessible to the research community and to offer useful information to scientists wishing to use rhizosphere models. Accurate parameterization is a prerequisite for successful modeling. Methodological difficulties include the use of appropriate sampling schemes and technologies. In our project, this concerned for example the root analysis and determination of desorption isotherms and strategies to deal with this were worked out. Root morphological parameter are of fundamental importance for root uptake models. Their measurement can be time consuming and costly. Analysis of such data revealed a log-linear relationship between root radius and root length. This would provide modelers with an enormous advantage since parameters could be inferred from an established relationship rather than be by extensive measurements. Work to confirm this is in progress. An important benefit of modeling is to bring different disciplines such as soil and plant sciences, engineering or applied mathematics closely together. As collaborators in the modeling working group of COST Action 631 we participated in establishing one accepted simulation tool for all rhizosphere research groups. Several specialized tools, such as ORCHESTRA or HP1, exist. On the other hand, numerical solvers such as COMSOL (FEMLAB) are available that provide accurate and user friendly solutions to models based on partial differential equations. Results of this project support an understanding and prediction of processes in the rhizosphere and uptake strategies that are key features in developing phytoremediation strategies.