The rotation of white lupin (Lupinus albus L.) with metal-accumulating plant crops: A strategy to increase the benefits of soil phytoremediation

Most of the plants employed to remove metals from contaminated soils are annuals and have a seed-to- seed life cycle of a few months, usually over spring and summer. Consequently, for most of the year, fields are not actively cleaned but are completely bare and subject to erosion by water and wind. The objective of this study was to evaluate the benefits of using Lupinus albus as a winter crop in a rotation sequence with a summer crop ideally selected for phytoextraction, such as industrial hemp. Lupin plants were grown in two alkaline soil plots (heavy metal-contaminated and uncontaminated) of approximately 400 m 2 each after the cultivation and harvest of industrial hemp. A smaller-scale parallel pot experiment was also performed to better understand the lupin behavior in increasing concentrations of Cd, Cu, Ni and Zn. White lupin grew well in alkaline conditions, covering the soil during the winter season. In few months plants were approximately 40e50 cm high in both control and contaminated plots. In fields where the bioavailable fraction of metals was low (less than 12%), plants showed a high tolerance to these contaminants. However, their growth was affected in some pot treatments in which the concen- trations of assimilable Cu, Zn and Ni were higher, ranging from approximately 40e70% of the total concentrations. The lupin's ability to absorb heavy metals and translocate them to shoots was negligible with respect to the magnitude of contamination, suggesting that this plant is not suitable for extending the period of phytoextraction. However, it is entirely exploitable as green manure, avoiding the appli- cation of chemical amendments during phytoremediation. In addition, in polluted fields, white lupin cultivation increased the soil concentration of live bacteria and the bioavailable percentage of metals. On average live bacteria counts per gram of soil were 65 10 6 ± 18 10 6 and 99 10 6 ± 22*10 6 before and after cultivation, respectively. The percentages of bioavailable Cu, Pb, Ni, Zn and Cr, which were 5.7 ± 0.7, 5.3 ± 1.7, 1.2 ± 0.1, 12 ± 1.5 and 0.1 ± 0.02%, respectively, before lupin growth, increased to 9.6 ± 1.6, 7 ± 2, 2 ± 0.3, 14 ± 1.5 and 0.1 ± 0.02% after lupin harvest. On the whole, our results indicate that the winter cultivation of white lupin in sequence with a metal- accumulator summer crop can improve the recovery of soil quality during the phytoextraction period. It improves the safety of the area, limiting additional ecological and human health problems, and enhances soil health by avoiding the use of chemical amendments and by increasing the levels of viable microorganisms