Effect of cadmium, zinc and substrate heterogeneity on yield, shoot metal concentration and metal uptake by Brassica juncea: implications for human health risk assessment and phytoremediation.

•  Heavy metal contaminants are usually heterogeneously distributed in soils. However, their effects on plants are usually studied under homogeneous conditions. Here we examined the effects of Cd, Zn, and their spatial distribution on shoot yield, shoot metal concentrations, and total metal uptake by Brassica juncea. •  One Cd concentration and three Zn concentrations were used. Metals were applied to the substrate either singly or in combination. •  Heterogeneous metal distribution enabled growth reduction to be avoided, even at concentrations that were highly phytotoxic when distribution was homogeneous. Moderate Zn contamination reduced Cd uptake by 40%. With high Zn contamination, metal concentrations were two to four times lower when metals were heterogeneously, rather than homogeneously, distributed; shoot yields were up to 24-times greater and total shoot Cd and Zn uptakes were on average six-times higher. •  It is suggested that human health risk from consuming plant parts grown on Cd-contaminated substrates is lower when Zn is also present and metal distribution is heterogeneous, and that phytoremediation potential is greater when contaminant distribution is heterogeneous.