Contamination Level Determines Whether Soil-Applied Zinc Oxide Nanoparticles Limit Cadmium Uptake in Maize

Cadmium (Cd) persists in agricultural soils and maize, a staple cereal, takes it up readily. Zinc oxide nanoparticles (ZnO NPs) are widely proposed as a remedy, yet most supporting evidence comes from foliar sprays or from soils with little Cd. We grew maize (Zea mays L.) in a strongly acidic loamy sand amended with Cd at 150, 250 and 350 mg kg−1 and treated it with soil-applied ZnO NPs at 100, 150 and 200 mg kg−1, in a screen-house factorial experiment with an untreated control. Cd reduced growth in proportion to dose; plants at the highest rate were 42% shorter than those at the lowest. No ZnO NP rate restored growth to control levels, and 100 mg kg−1 gave taller plants and longer roots than 150 or 200 mg kg−1. The effect of ZnO NPs on tissue Cd depended on contamination. At 150 mg kg−1 Cd, raising ZnO NPs from 100 to 200 mg kg−1 lowered shoot Cd by 51%. At 250 and 350 mg kg−1 Cd, higher ZnO NP rates were instead associated with higher root and shoot Cd. The translocation factor stayed below 1 in every treatment and was lowest at 200 mg kg−1 ZnO NPs. Soil-applied ZnO NPs cannot be relied on as a general control on Cd uptake by maize. Where they lowered shoot Cd, at the lowest contamination level tested, they did so at a cost to growth.