Variable element transfers from an illite-rich substrate to growing plants during a three-month experiment

Abstract The element uptake by spinach was quantified during a three-month experiment in growing the species in a substrate consisting mostly of illite clay, with some quartz and hematite under strictly controlled laboratory conditions. The purpose of this experiment was a precise evaluation of the element transfers in avoiding all side effects related to the specific features of a natural soil, the atmospheric pollution and the climate variations. Three general uptake patterns were found for most elements. The most occurring consists of a decrease in Al, P and most analyzed trace elements (Rb, Zr, Ti, Pb, Ni, Co, Zn, Cu, Cr, U, Th and REEs) during early growth and of an increase afterwards. The second pattern reproduces a curvilinear increase in Mg, Ca, Sr and Ba in the plants during the three-month growth. The third pattern is characterized by a starting increase in K and Na followed by a decrease during the late growth, also obtained for Rb and Mn. The taken up of the various elements by the plants occurs by way of ion exchange and particle-surface modifications during the initial growth stage. The major change in the uptake is coincident, after one-month experiment, with the start of ion complexation by interaction with organics released by the plant roots. Only detected after one month of growth, the latter process appears to have been limited to the particles directly in contact with root exudates, as no detectable changes were observed in the crystallographic and chemical composition of the bulk illite component. Changes in the Ca/Sr and K/Rb ratios of the plants also record an enzyme contribution in the element transfers from substrate to the plants. The REE concentrations and distributions in the plants remained unchanged relative to their contents and distribution in the substrate, except for Eu that was progressively enriched in the successive uptake steps. Enzyme activity causing a selective uptake in anoxic conditions near the plant roots probably promoted the conversion of Eu 3 + to Eu 2 + and its preferential uptake relative to the other REEs that were transferred without fractionation.