Zinc and cadmium release from soil aggregate of different size fractions during repeated phytoextraction with Sedum plumbizincicola: Insight from stable isotope analysis

Abstract

Studies based on metal isotope changes at micro-scale interfaces within soils (i.e., between aggregates of different size fractions) can provide further insight into soil-plant interactions during long-term phytoextraction. Here, aggregate size separation was conducted on two contaminated soils that underwent phytoextraction using Zn/Cd hyperaccumulator Sedum plumbizincicola over six consecutive seasons. The dynamic changes in Zn and Cd concentrations, chemical fractionations and isotope compositions in soil aggregates were investigated. As phytoextraction proceeded from the first (C1) to the sixth (C6) season, shoot Zn displayed a marginally heavier isotope composition despite the essentially constant Zn isotope composition of bulk soils and the 50–250, 5–50, 1–5, and <1 ​μm aggregate size fractions. Most likely, this results from moderate depletion of Zn in the bulk soils (by ​≤ ​21%) and gradual release of heavy Zn isotopes into soil bioavailable pool due to soil acidification and Zn exchange during repeated phytoextraction. Light isotopes of Cd were significantly enriched in all aggregates of different size fractions (Δ^114/110Cd_C6–_C1 ​= ​−0.14 ​± ​0.04 to −0.02 ​± ​0.04‰) with similar decreases in Cd concentrations (50–64% and 87–92% in two soils) over the five consecutive seasons. Rayleigh modelling produced similar Cd isotope fractionation factors for aggregates of different size fractions, indicating that similar mechanisms controlled Cd release from soil aggregates. In contrast to other plants preferring light Cd isotopes, Zn/Cd hyperaccumulator continuously took up heavier Cd isotopes from soils. The results were due to the enhanced root exudation to mobilize more Cd from soil solids and organic ligands excreted from roots preferentially complexed heavy Cd isotopes based on density functional theory. The different isotopic behaviours of Zn and Cd suggest different processes controlling their migration in the soil-plant system.