Project

Predicting the effect of low molecular weight organic acids (LMWOAs)

Many plants exude low molecular weight organic acids (LMWOA) as a stress response under conditions of low (micro)nutrient availability. LMWOA are small molecules containing carboxylic acid functional groups and include compounds such as citric acid, oxalic acid, malic acid, etc. They can form soluble complexes with metals, thereby increasing their mobility and altering their direct bioavailability.

Problem

However, the effect of individual LMWOAs in root exudates on the dissolved metal concentrations in the rhizosphere, and how this depends on soil properties (e.g. pH, soil texture, organic matter and reactive metal content) remains poorly understood. Also, literature mostly reports studies in which LMWOA interact with soils under conditions remote from those occurring in the rhizosphere; typically, the amount of LMWOA per amount of soil is unrealistically large.

Mechanistic multi-surface soil models can accurately describe equilibrium metal speciation in soils. Thermodynamic constants for metal complexes of LMWOA have been reported. By including these constants into multi-surface models, the effect of individual LMWOAs on the mobility of metals in (rhizosphere) soil can be predicted. Such equilibrium models can provide a better understanding of which LMWOAs are capable of forming soluble complexes with individual metals under specified conditions. Still, the validity of these model needs to be experimentally verified.

Project goal

The aim of this MSc thesis project is to explore how accurately multi-surface models can predict metal speciation in soil solution when LMWOA are added to a soil. This will be examined for a set of soils that vary in soil properties (e.g. pH, organic matter and clay content), and for various LMWOAs including citrate, oxalate and malate. Also the effect of LMWOA concentration on metal speciation will be explored. Model predictions will be compared with experimental data from batch interaction experiments for selected soil-LMWOA combinations, to verify the validity of an equilibrium approach.

The outcomes of this study will improve our understanding of the role of LMWOA in nutrient acquisition and establish how well we can predict their effect on metal speciation in (rhizosphere) soil, depending on the soil conditions.

Used skills

  • Literature review;
  • Proposal writing;
  • Multi-surface modelling;
  • Laboratory work (soil batch interaction experiments);
  • Data processing;
  • Thesis writing.

Requirements

  • Some laboratory experience;
  • Required courses for the MSc thesis Soil Chemistry and Chemical Soil Quality (SOC-81336), preferably including:
    • Soil Quality (SOC21806);
    • The Applications (SBL-34806).

Supervisor