Iron mobilization from mineral soil constituents driven by root exudates

Several mineral nutrients are present in unavailable forms in the soil and plants have to mobilize them, for instance by releasing organic and inorganic compounds (root exudates), which alter the chemical, physical and biological characteristics of the rhizosphere. Among these exudates organic acids (e.g. citric acid, malic acid, oxalic acid), phenolic compounds (e.g. flavonoids), amino acids and siderophores of microbial and/or plant origin can strongly influence and modify the biogeochemical cycles of sparingly soluble nutrients, thus causing changes in their availability for plant uptake. Chlorosis induced by Fe deficiency is a major nutritional disorder in crops and fruit trees, particularly when grown on calcareous soils. Plants react to iron deficiency stress adopting different kinds of adaptive responses, including the release of protons, organic acids and phenolic compounds or phytosiderophores, non-proteinogenic amino acids, highly efficient in binding Fe. The objective of this study was to assess the influence of several types of root exudates (citric acid, malic acid, oxalic acid, genistein, quercetin and phytosiderophores) on the mineralogy of two different soils (an agricultural calcareous soil and an acidic polluted soil) and to evaluate possible synergic or competitive behaviors. X-ray diffraction (XRD) coupled with Electron Probe Micro Analysis (EPMA) has been used to identify the crystalline and amorphous phases, which are subjected to mineral alteration when exposed to the action of root exudates. Solubilization of Fe and trace metals such as Cu, Zn, Ni, Cr, Pb, Cd as well as of major elements such as Si, Al, Fe and Mn have been assessed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). As expected soil microorganisms decrease the active fraction of root exudates due to degradation processes. In some cases exudates were completely degraded within 24 hours. First results seem to show that the exudate/soil interaction induced mainly a solubilization of amorphous mineral phases. Automated EPMA analyses on individual soil particles will give further details on the identification of the mineral phases involved in the weathering process. Results obtained contribute to a better understanding of the biogeochemical cycles of Fe within the rhizosphere which play an important role in its availability for plant uptake.