Stable isotope fractionation of cadmium in the soil-rice-human continuum

Consumption of rice (Oryza sativa) grain is a major pathway by which humans are exposed to Cd, especially in non-smoking Asian populations. Although the stable isotope signatures of Cd offer a potential tool for tracing its sources, little is known about the isotopic fractionation of Cd across the entire soil-rice-human continuum. Cadmium isotope ratios were determined in field soils, rice grain, and human urine collected from two Cd-contaminated regions in southern China. Additionally, Cd isotopic fractionation in rice plants was investigated using two transgenic plants differing in Cd uptake and accumulation. Analysis of isotope ratios revealed a preferential enrichment of the heavy Cd isotopes from soil to rice grain (delta Cd-114/110(grain-soil)=+0.40 parts per thousand) and from grain to urine (delta Cd-114/110(urine-grain)=+0.40 parts per thousand) in both regions. The first increase was mainly caused by partitioning between the soil solid phase and the soil solution, with heavier Cd preferentially enriching in the soil solution. Within the rice plant, we identified multiple processes that alter the isotope ratio, but the net effect throughout the plant was comparatively small. Cd fractionation in humans is presumably due to the preferential enrichment of heavier Cd isotopes by metal transporters DMT1 and ZIP8 (responsible for the absorption of Cd into body from the foods). These findings provide important insights into the Cd isotopic fractionation through the soil-rice-human continuum and are helpful for tracing the sources of Cd. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The analysis of Cd isotope ratios in soil, rice grain, and human urine samples from two Cd-contaminated regions in southern China revealed a preferential enrichment of heavy Cd isotopes from soil to rice grain and from grain to urine. The fractionation of Cd isotopes in the soil-rice-human continuum is likely due to processes such as partitioning between the soil solid phase and soil solution, as well as preferential enrichment by metal transporters in humans.