Zinc Alleviates Copper Toxicity to Lettuce and Oat in Copper-Contaminated Soils

Copper (Cu) mining operations have a global footprint and have led to contamination of soils and Cu toxicity to plants. Understanding the controls of plant Cu uptake, including competition with other metals, such as zinc (Zn), is essential for improving plant growth in Cu-contaminated soils. The objective of this study was to evaluate the capacity of Zn to alleviate the toxicity of Cu in crops grown in Cu-contaminated soils. Lettuce was grown in 27 soils with ranges of total Cu and Zn concentrations of 82–1295 mg Cu kg⁻¹ and 86–345 mg Zn kg⁻¹ for a period of 21 days. Oat was grown in 21 soils with the same total Cu and Zn concentration ranges for a period of 62 days. Regression analyses were used to evaluate the impact of total soil Zn on plant growth in the Cu-contaminated soils. We show for the first time that Zn alleviates Cu toxicity in lettuce and oat grown in soils. Specifically, we observed a negative (toxicity) effect of total soil Cu and a positive (protective) effect of total soil Zn on shoot growth response for lettuce and oat. The effective concentration 50% (EC₅₀) of Cu/Zn mass ratio was 7.0 ± 1.8 for lettuce shoot length and 5.9 ± 1.0 for oat shoot weight. These results indicate that the previously demonstrated efficacy of Zn in mitigating Cu phytotoxicity in hydroponic systems can extend to more complex soil systems. Further research should be done to evaluate specific Zn amendments for restoring vegetative growth in Cu-contaminated soils.