Biochar amendment affects phenolic composition and antioxidant capacity restoring the nutraceutical value of lettuce grown in a copper-contaminated soil

To evaluate the effects of biochar amendment on the nutritional value of lettuce (Lactuca sativa L.) grown in a Cu-contaminated soil, both antioxidant activity and phenolic composition of leaves were investigated. The results showed that Cu decreased phenolic acid and flavonoid concentrations as well as total antioxidant activity, whereas anthocyanins were not affected. The addition of biochar to the contaminated soil either restored (antioxidant activity and flavonoids) or exceeded (total phenols, phenolic acids and anthocyanins) the control values. The amendment did not influence the composition of the different phenolic classes. Chlorogenic acid and quercetin derivatives were the main free phenolic acids and flavonoids, respectively. When added to the uncontaminated soil, biochar did not exert any negative effect on phenolics and antioxidative activity of leaves. The results indicate that lettuce grown in a moderately metal-contaminated soil and suitable for human consumption maintains its beneficial health properties following biochar addition.