The role of root exudates in changing soil mineralogy and mobilising trace elements

Root exudates such as low molecular weight organic acids (e.g. citric acid, malic acid, oxalic acid), phenolic compounds (e.g., flavonoids), and siderophores of microbial or plant origin can alter soil mineralogy in the rhizosphere thus releasing trace elements in solution, both important micronutrients and dangerous pollutants. In this research, the effect of various root exudates (e.g. citric acid, malic acid, oxalic acid, genistein, quercetin and microbial siderophores) on the mineralogy of two different soils (an agricultural calcareous soil and an acidic polluted soil) has been assessed, also evaluating possible synergic or competitive behaviours. X-ray diffraction (XRD) coupled with Electron Probe Micro Analysis (EPMA) was used to identify the crystalline and amorphous phases which were subjected to mineral alteration when exposed to the action of root exudates. Solubilisation of trace metals such as Cu, Zn, Ni, Cr, Pb, Cd as well as of major elements such as Si, Al, Fe and Mn was assessed by means of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). It was also found that soil microorganisms attenuated mineral weathering by reducing the concentration of active root exudates in solution. These data are an important cornerstone to better understand the biogeochemical processes acting in the rhizosphere and the mechanisms involved in plant mineral nutrition and in the mobilization of toxic elements of environmental concern.