How Raw and Gypsum Modified Bauxite Residues Affect Seed Germination, Enzyme Activities, and Root Development of Sinapis alba

This study aimed to better understand the impact of different bauxite residues (BR) on the germination and the development of Sinapis alba (white mustard). Unamended BR from Provence, France (PRO), and Guinea (GUI) bauxite were selected, and modified bauxite residues from PRO and GUI (MBR-PRO and MBR-GUI) were obtained by gypsum application and repeated leaching, in order to reduce their pH, electrical conductivity (EC), and exchangeable sodium percentage (ESP). Germination rates were monitored in soil-BR mixtures with increasing concentrations of BR. A rhizotron experiment was done, where Sinapis alba was allowed to develop in a layer of soil on the top of a layer of bauxite residue. To assess the impact of the residue on root development, the WhinRhizo® software was used to measure the architectural traits of roots. Peroxidase and fluorescein hydrolase activities were also assessed on the roots grown either in the soil or in the residue layers. Results showed that (i) bauxite residue origin, (ii) bauxite residue modification by gypsum, and (iii) bauxite residue concentration had significant effects on the germination, the root development and architecture, and the enzymatic activities of the roots of Sinapis alba. The PRO residue had a far stronger phytotoxic effect compared to the GUI residue on every measured variable. The toxic effect was strongly modulated by the origin of bauxite residue. The gypsum application efficiently reduced the phytotoxic effect of the residues, but significant negative effects on the different variables were still recorded.