Effects of Rhizobioaugmentation with N-Fixing Actinobacteria Frankia on Metal Mobility in Casuarina glauca-Soil System Irrigated with Industrial Wastewater: High Level of Metal Exclusion of C. glauca

Agroforestry practices coupled with wastewater irrigation systems are sustainable strategies for water management. The performance of these practices could be improved by rhizobioaugmentation. This approach would be particularly useful in developing countries where it can be used as a low-cost tool to control widespread environmental contaminations. The main objectives of the present study are to (1) determine the effects of wastewater on metal/nutrient contents in soils, (2) assess the pattern of metals in Casuarina glauca, and (3) analyze the effects of rhizobioaugmentation of C. glauca growing in industrial wastewater–irrigated agricultural soil using N-fixing Frankia symbionts. Overall, the wastewater treatment significantly increases the levels of total Pb, B, Cr, Mn, Na, Sr, Zn, As, Co, Sb, Sn, and Fe. Only a small portion of total metals/nutrients were phytoavailable. The bioaccumulation in roots of all the metals/nutrients measured was high while the translocation from roots to aerial parts showed insignificant level of movement of the elements tested. Based on bioavailable metals/nutrients, the bioaccumulation factors were 34, 41, 94, 196, 584, 587, 1859, and 9917 for Mg, As, Ni, Mn, Cu, Co, Cr, and Pb, respectively. Hence, C. glauca is classified as a metal excluder. Rhizobioaugmentation with Frankia resulted in an increase or a decrease of metals/nutrients in soil depending on the bacterial strain used and the metal/nutrient element. It also increased significantly the bioaccumulation in roots of some metals and the uptake of key nutrients such as Ca, Na, and K by Casuarina plants. Overall, the results of the present study showed that C. glauca is suitable for phytoremediation of metal-contaminated soils. The use of Frankia represents a potential approach of managing Casuarina glauca wastewater–irrigated soil system.