Redistribution of calcium and sodium in calcareous soil profile and their effects on copper and lead uptake: A poplar-based phytomanagement

Highlights: • Poplar afforestation led to the soil profile redistribution of calcium and sodium. • The cycling of calcium and sodium activated trace metals in the calcareous soil. • The content of trace metals in poplar leaves and litter increased with stand age. • Safe production of poplar wood can be achieved by strengthening leaf collection. Poplar serves as a phytostabilizator in phytomanagement of the trace metals (TMs) copper (Cu) and lead (Pb) contaminated land. In the process of long-term phytomanagement, it is not clear how the cycling of the mineral nutrients calcium (Ca) and sodium (Na) in calcareous soil will affect poplar remediation mechanisms. We selected a site contaminated by Cu and Pb and phytomanaged by Populus simonii Carr. stands of different ages (7, 14, and 28 years) to study the influencing mechanisms. The results showed that after afforestation, the Ca in the subsoil returned to the topsoil through fallen leaves, whereas the Na in the topsoil migrated downward to the subsoil by leaching, resulting in the redistribution of mineral nutrients in the soil profile. In addition, the Ca content in soil solution of the root-zone was significantly lower relative to that of the bulk soil, whereas the Na content in soil solution was significantly higher in all stands. As a result, because of the competitive adsorption of mineral nutrient and TM cations on the soil surface, the pool of bioavailable TM in root-zone soils did not significantly decrease with stand age. On the contrary, the TM content in poplar leaves (Cu: 31–37 mg kg⁻¹; Pb: 62–84 mg kg⁻¹) and litter (Cu: 230–790 mg kg⁻¹; Pb: 394–1366 mg kg⁻¹) increased significantly with stand age. Nevertheless, the TM content in poplar wood (Cu < 3 mg kg⁻¹; Pb < 12 mg kg⁻¹) remained at an extremely low level in all stands. Our results highlighted that strengthening leaf collection is necessary to eliminate ecological risks and ensure the safe production of poplar wood in the long-term phytomanagement of TM-contaminated land.