Biomass yield and metal phytoextraction efficiency of Salix and Populus clones harvested at different rotation lengths in the field experiment

Abstract

Background

Phytoextraction belongs to environmentally well-accepted remediation technologies to remove metals from contaminated soils. Due to long-time requirement, sufficient data for proper phytoextraction evaluation are missing. Four clones of fast-growing trees: two willow species (S1), Salix viminalis L. (Salix schwerinii E.L.Wolf × S. viminalis) × S. viminalis) and (S2)—Salix smithiana (Salix × smithiana Willd.), and two poplar clones (P1), Populus Max-4 (Populus nigra L. × Populus maximowiczii A. Henry) and (P2) Wolterson (P. nigra L.) were cultivated under field conditions at medium-to-high Cd and Pb, and low Zn soil contamination to assess trees’ long-term ability of biomass production and removal of potentially toxic elements (PTEs). The biomass yield and PTE uptake were measured during 8 years of regular growth under three rotation lengths: four harvests following 2-year periods (4 × 2y), two harvests in 4-year periods (2 × 4y), and one harvest representing 8 years of growth (1 × 8y).

Results

In most cases, the highest annual dry biomass yield was achieved with a 2 × 4y rotation (P1 = 20.9 t ha⁻¹ y⁻¹, S2 = 18.4 t ha⁻¹y⁻¹), and the yield decreased in order 2 × 4y > 1 × 8y > 4 × 2y of harvesting periods. Only clone S1 showed a different pattern. The differences in biomass yield substantially affected the PTE phytoextraction. The greatest amount of Cd and Zn was removed by willow S2, with the highest biomass yield, and the strongest ability to accumulate PTEs. With 2 × 4y rotation, S2 removed a substantial amount of Cd (9.07%) and Zn (3.43%) from the topsoil horizon (0–20 cm) and 5.62% Cd and 2.04% Zn from horizon 20–40 cm; phytoextraction rate was slightly lower for 1 × 8y rotation. The poplar P1 removed the most Pb in the 1 × 8y rotation, but the overall Pb phytoextraction was negligible. The results indicated that lignin and cellulose contents increased, and hemicellulose content decreased with increased concentrations of Cd, Pb and Zn in poplars wood.

Conclusions

The data confirmed that phytoextraction over longer harvest periods offered promising results for removing Cd from medium- to high-level contaminated soils; however, the ability of Pb removal was extremely low. The longer harvest period should be more economically feasible.