Is the harvest of Salix and Populus clones in the growing season truly advantageous for the phytoextraction of metals from a long-term perspective?

The phytoextraction potential of short-rotation field coppice plantations was investigated on soil historically contaminated with risk elements (REs), especially Cd, Pb and Zn. The main objective of the study was to assess the effect of biomass harvest time (summer harvest versus traditional winter one) on biomass yield, REs accumulation and removal in the long-term study. The precise field experiment with two Salix clones and two Populus clones was established in April 2009. Shoots of all clones were harvested in February 2012 for the first time, and then identical branches and leaves were harvested every two years in September (summer harvest = SH) and the branches every two years in February (winter harvest = WH). The first summer harvest seemed to be more promising compared to the winter one, but the yields in the second and third harvests were inconsistent. The total phytoextraction efficiency of the SH declined in second and third harvests due to a decrease of leaves/aboveground biomass ratio, and the RE concentrations in leaves. Clonal Salix smithiana was the most productive one in the SH, with a dry matter yield of 15.1 t ha-1 year-1 and showed promising extraction ability for Cd (11.65 %) and Zn (4.59 %) over a 6-year field experiment. A lower portion of Cd (6.97 %) and Zn (2.38 %) was removed by this clone in the WH (calculated from the total soil content of REs). SH was more reasonable for phytoextraction compared to WH. Higher RE concentrations were found in leaves of Salix compared to Populus. Populus accumulated the highest Pb content in the branches; unfortunately, the Pb extraction was low, due to extremely high soil Pb contamination. Locally bred willows and poplars performed substantially better than internationally recognised ones, indicating the importance of phytoremediation optimisation, including clone selection, for suitable climatic conditions.

Effects of summer and winter harvesting on element phytoextraction efficiency of Salix and Populus clones planted on contaminated soil.

The clones of fast-growing trees (FGTs) were investigated for phytoextraction of soil contaminated with risk elements (REs), especially Cd, Pb, and Zn. As a main experimental factor, the potential effect of biomass harvesting time was assessed. The field experiment with two Salix clones (S1 - (Salix schwerinii × Salix viminalis) × S. viminalis, S2 - S. × smithiana) and two Populus clones (P1 - Populus maximowiczii × Populus nigra, P2 - P. nigra) was established in April 2009. Shoots of all clones were first harvested in February 2012. After two further growing seasons, the first half of the trees was harvested in September 2013 before leaf fall (summer harvest) and the second half in February 2014 (winter harvest). Remediation factors (RFs) for all clones and all REs (except Pb for clone S1) were higher in the summer harvest. The highest annual RFs for Cd and for Zn (1.34 and 0.67%, respectively) were found for clone S2 and were significantly higher than other clones. Although no increased mortality of trees harvested in the summer was detected in the following season, the effect of summer harvesting on the phytoextraction potential of FGTs clones should be investigated in long-term studies.