Evaluation of survival, growth, and phytoremediation potential of Populus and Salix seedlings grown in polluted soils
Authors: Salam Mir Md Abdus 
Published: February 2024

The aim of this thesis was to evaluate the survival, growth, and phytoremediation potential of Populus and Salix seedlings grown in polluted soils. More specifically, the following topics were studied: (1) the survival and growth of two European aspen clones and four hybrid aspen clones grown in control soil (pristine), old creosote soil polluted with hydrocarbons, and pristine soil spiked with fresh diesel oil at three different planting densities in a greenhouse over two growing seasons (Article I); (2) the survival, growth, and hydrocarbon removal of three European aspen clones and seven hybrid aspen clones grown in hydrocarbon-contaminated soil (including polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs)) under field conditions over 4 years (Article II); and (3) the growth and metal accumulation ability of Salix psammophila seedlings with bamboo biochar (BBC) amendment at ratios of 0–7% in soils heavily contaminated by Cd and Zn in a pot experiment over 180 days (Article III). In study I (Article I), the survival rates of European aspen and hybrid aspen clone seedlings were 70–100% in control soil, 99% in the old creosote-contaminated soil, and 22–59% in the diesel-contaminated soil across all planting densities. The heights of aspen seedlings were 5–44% and 9–38% lower and the stem dry biomass was 9–93% and 34–63% lower in diesel-contaminated and creosote-contaminated soils, respectively, compared to the control. Low plant density increased survival rates and growth compared to higher density treatments. Of all the clones, hybrid aspen clones 14 and 291 and European aspen clone R3 showed reasonable survival and growth across all treatments. Soil treatment, planting density, and clone type significantly affected survival rate, height, and stem dry biomass (p < 0.05). In study II (Article II), the highest survival rates in old creosote-contaminated soils were in clone 291 (72%) among hybrid aspen clones and clone R3 (70%) among European aspen clones. Hybrid aspen clones 14 and 34 had 16–211% greater heights than other hybrid aspen clones. The height of European aspen clone R3 was also 25‒35% greater than that of other European aspen clones. However, clone type did not significantly affect seedling survival or height (p > 0.05). Among hybrid aspen clones, clone 134 had the largest hydrocarbon removal at a depth of 5–10 cm and clone 191 at a depth of 10–50 cm. Clone 14 also showed potential for removing hydrocarbons at both soil depths. In European aspen clones, clone R2 had the highest hydrocarbon removal at both soil depths. However, all clones showed an ability to remove total PAHs and TPHs from the soil (but p < 0.05 only at a soil depth of 5–10 cm). The reduction in hydrocarbon levels in the soil was more prominent at a soil depth of 5–10 cm than at a depth of 10–50 cm. Based on studies I and II, European aspen and hybrid aspen clones can be considered candidates for the remediation of soils polluted with PAHs and TPHs. In study III (Article III), BBC ratios of 1% and 5% resulted in only slight decreases in characteristics, especially height (0.6‒1.3%) but also total dry biomass (2‒10%), of S. psammophila seedlings compared to the control, whereas BBC 3% increased these measurements slightly (2% increase). BBC 7% reduced the height (16%) and total dry biomass (26%) of seedlings compared to the control. BBC amendment increased the accumulation of Cu, Cd, and Zn in different plant tissues, especially Cd and Zn accumulation (23‒30% and 13‒24%, respectively), in the BBC 3% treatment compared to the control. Based on these findings, S. psammophila with BBC amendment can be considered a candidate for phytoremediation. However, metal accumulation in the roots, stems, and leaves was not significantly affected by the BBC 1‒7% treatments (p > 0.05), except for Pb accumulation in the roots and Cu accumulation in the stem (p < 0.05). Overall, hybrid aspen, European aspen, and S. psammophila seedlings showed reasonable survival and growth, photosynthetic activity, efficient hydrocarbon removal from soil and metal accumulation ability both under greenhouse conditions and in a field experiment. Therefore, these species could be used to depollute areas affected by a range of hydrocarbons or Cd and Zn. However, future research should be conducted in the field to verify the abilities of hybrid and European aspens and S. psammophila to remediate soil contaminated by hydrocarbons, Cd, or Zn, and such studies should also use different planting densities and soil amendments over longer periods.