Cadmium Isotopic Fractionation in the Soil-Plant System during Repeated Phytoextraction with a Cadmium Hyperaccumulating Plant Species

Analysis of stable metal isotopes can provide important information on biogeochemical processes in the soil-plant system. Here, we conducted a repeated phytoextraction experiment using the cadmium (Cd) hyperaccumulator Sedum plumbizincicola X. H. Guo et S. B. Zhou ex L. H. Wu (Crassulaceae) in four different Cd-contaminated agricultural soils over five consecutive crops. Isotope composition of Cd was determined in the four soils before and after the fifth crop, in the plant shoots harvested in all soils in the first crop, and in the NH4OAc extracts of two contrasting soils with large differences in soil pH (5.73 and 7.32) and clay content (20.4 and 31.3%) before and after repeated phytoextraction. Before phytoextraction NH4OAc-extractable Cd showed a slight but significant negative isotope fractionation or no fractionation compared with total Cd (Delta Cd-114/110(extract-soil) = -0.15 +/- 0.05 (mean +/- standard error) and 0.01 +/- 0.01 parts per thousand), and the extent of fractionation varied with soil pH and clay content. S. plumbizincicola preferentially took up heavy Cd from soils (Delta Cd-114/110(shoot-soil) = 0.02-0.14 parts per thousand), and heavy isotopes were significantly depleted in two soils after repeated phytoextraction (Delta Cd-114/110(soil: P5-soil: P0) = - 0.15 +/- 0.02 and -0.12 +/- 0.01 parts per thousand). This provides evidence for the existence of specific Cd transporters in S. plumbizincicola, leading to positive isotope fractionation during uptake. After phytoextraction by five sequential crops, the NH4OAc-extractable Cd pool was significantly enriched in heavy isotopes (Delta Cd-114/110(extract: P5-extract: P0) = 0.07 +/- 0.02 and 0.18 +/- 0.05 parts per thousand) despite the preferential uptake of heavy isotopes, indicating the occurrence of root-induced Cd mobilization in soils, which is supposed to favor heavy Cd in the organo-complexes with root exudates. Our results demonstrate that Cd is taken up by S. plumbizincicola via specific transporters, partly after active mobilization from the more strongly bound soil pool such as iron/manganese (hydr)oxide-bound Cd during repeated phytoextraction. This renders S. plumbizincicola a suitable plant for large-scale field phytoremediation.