Performance of poplars and willow grown on soil under wastewater irrigation during the first 2 years of establishment.

Recycling wastewaters as irrigation and fertilization of tree species is a market-driven action for purpose-grown timber plantations that promotes the circular economy. A greenhouse experiment was carried out to investigate the performance of poplar clones (Populus alba L. "20/45", P. nigra L. "62/154", and P. euramericana (Dode) Guinier "92/40") and willow (Salix excelsa S.G. Gmel) grown on soils under tap water irrigation (TWI) or wastewater irrigation (WWI) during the first 2 years of establishment. Results showed that at 2 years after planting, poplars and willow exhibited significantly greater performance when grown in WWI versus TWI (P < 0.05). Likewise, phytoremediation of nutrients along with some heavy metals increased in plant organs of WWI-grown plants relative to their TWI counterparts (P < 0.05). After 2 years of growth and under both conditions (TWI and WWI), P. nigra plants showed the greatest growth and biomass; however, leaf area was the largest for P. alba and P. euramericana plants. Also, P. nigra plants had the highest total contents of copper (Cu) and manganese (Mn), P. alba had the most zinc (Zn), and differences for iron (Fe) were negligible across species. In contrast, total contents of nickel (Ni) and chromium (Cr) were higher for S. excelsa, P. nigra, and P. alba plants, while the lowest values were measured in P. euramericana plants. Across all species and soil treatments, the translocation factor (TF) was greater than 1 for Mn, Cu, and Zn, indicating phytoextraction and phytoaccumulation of these elements into leaves and stems, while TF was less than 1 for Fe, lead (Pb), Ni, and Cr, resulting in phytoremediation of these elements as phytostabilization. Differences in tolerance index (TI) values were negligible across species, and TI was greater than 100% after 2 years of growth, indicating that all four species are suitable for phytoremediation applications with urban wastewaters having similar heavy metal concentrations as in the present study. Nevertheless, given the combination of enhanced growth, biomass, and heavy metal accumulation (Mn, Cu, Cr, and Ni), P. nigra plants exhibited the greatest phytoremediation potential of four tested species.