Humates and Chlorides Synergistically Increase Cd Phytoaccumulation in Strawberry Fruits, Heightening Health Risk from Cd in Human Diet

Salt-affected and sandy pedospheres low in complex organic and mineral matrices critical for metal sorption (e.g. humics, aluminosilicates) could exacerbate metal transfer into the food chain. To test this hypothesis, a 3-factor study with salinity (0-50 mM NaCl), humates (HA; 0-150 mg/kg) and Cd contamination (0-9 mg/kg) was conducted in sandy substrate with strawberry. Cadmium phytoaccumulation decreased in the order roots > crowns > leaves > fruits. In comparison to the control, tissue Cd concentration was influenced by the NaCl x HA x Cd interaction, increasing Cd in leaves (up to 241-fold) and fruits (up to 135-fold) and exceeding the European maximum limit of 0.05 mg Cd/kg w wt. Surface analyses (XRD, SEM-EDX, FTIR, SIMS) revealed that the growth substrate rich in SiO2 (> 87% w/w) had uniform, nonporous and chemically unreactive surface structure. In contrast, the more complex HA matrix featuring abundant and heterogeneous micro-porosity and a large content of reactive radicals. Chemical speciation modelling of the rhizosphere solutions showed that almost all Cd was dissolved and distributed among the bioavailable Cd2+, Cl-complexed and HA-complexed pools, with small amounts of Cd adsorbed to K/Na-aluminosilicates. Slightly acidic pH (5.4-6.2) and complexation with Cl and HA in the rhizosphere favoured Cd solubility and its transfer to plants. The assessment of health risk of strawberry fruit consumption indicated a relatively higher the Estimated Daily Intake (EDI) in children (5% of provisional tolerable daily Cd intake) vs adults (< 1%), with the Dietary Risk Coefficient (DRC) < 0.1 in both populations, suggesting a low risk. However, given Cd intake from other sources and its cumulative effects, precautions are needed when consuming strawberries grown in salt-affected sandy soils.

Automated summary

The study found that salt-affected and sandy soils can exacerbate the transfer of the metal cadmium into the food chain. Salinity, humates, and cadmium contamination significantly influenced the accumulation of cadmium in strawberry plants, with elevated concentrations exceeding the European maximum limit in leaves and fruits. Surface analyses revealed the different properties of the substrates, with the SiO2-rich substrate having a nonporous and chemically unreactive surface, and the humates matrix characterized by abundant micro-porosity and reactive radicals. Chemical speciation modeling showed that most of the cadmium in the rhizosphere solution was dissolved and distributed among various pools, including bioavailable Cd2+, Cl-complexed, and HA-complexed forms.