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.