Mineral weathering and mobilization of trace metals in the rhizosphere: the role of root exudates

The mobilization of trace elements in the soil-plant system takes place essentially in the rhizosphere, where roots are intimately in contact with soil. In the rhizosphere, conditions such as pH modification, changes in red-ox potential, high concentrations of organic ligands (either of vegetal or microbial origin), and the modulation of soil-enzyme and microorganism activities, can strongly influence and modify the biogeochemical cycles of several elements, thus causing changes in their availability for plant nutrition. One class of these elements is composed by the trace metals; some of them are essential for plants even if in small concentrations and are amongst the micronutrients, such as Fe, Zn, Mn. Trace-metal solubility and availability can be influenced, among other factors, by the presence in soil of organic complexing agents that can induce metal mobilization from stable mineral phases. Among these compounds, root exudates such as low molecular weight organic acids (e.g. citric acid, malic acid, oxalic acid), phenolic compounds (e.g. flavonoids), siderophores, can be cited. These molecules can strongly contribute to mineral weathering in rhizosphere and, therefore, to the mobilization of trace metals. The mobilized trace metals, in function of the element and of its concentration, can be either essential nutrients or toxic elements for plants. Within this context, the objective of this research is to study the mineral weathering processes occurring in the rhizosphere, with particular attention addressed toward the processes involved in trace-metal mobilization and the role of natural organic chelating agents, such as low molecular weight organic acids, phenolic compounds and siderophores. Possible synergic or competitive effects, using pure chemicals as well as exudates collected from plants, on trace-metal solubilization and on mineral weathering occurring among these compounds are being evaluated.