Biochar amendment reduced the risk associated with metal uptake and improved metabolite content in medicinal herbs

Contaminations of heavy metals such as lead (Pb) and cadmium (Cd) in medicinal plants (MPs) not only restrict their safe consumption due to health hazards but also lower their productivity. Biochar amendments in the soil are supposed to immobilize the toxic metals, improve the soil quality, and agricultural productivity. However, the impact of biochar on growth attributes, metal accumulation, pharmacologically active compounds of MPs, and health risk is less explored. An experiment was performed on three medicinal plants [Bacopa monnieri (L.), Andrographis paniculata (Burmf.) Nees, and Withania somnifera (L.)] grown in a greenhouse in soil co-contaminated with Pb and Cd (at two concentrations) without and with biochar amendments (2% and 4% application rates). The fractionation of Pb and Cd, plant growth parameters, stress enzymes, photosynthetic capacity, pharmacologically active compounds, nutrient content, uptake and translocation of metals, antioxidant activities, and metabolite content were examined in the three MPs. The accumulation of Pb and Cd varied from 3.25–228 and 1.29–20.2 mg kg−1, respectively, in the three MPs, while it was reduced to 0.08–18 and 0.03–6.05 mg kg−1upon biochar treatments. Plants grown in Pb and Cd co-contaminated soil had reduced plant biomass (5%–50% depending on the species) compared to control and a deleterious effect on photosynthetic attributes and protein content. However, biochar amendments significantly improved plant biomass (21%–175%), as well as photosynthesis attributes, chlorophyll, and protein contents. Biochar amendments in Pb and Cd co-contaminated soil significantly reduced the health hazard quotient (HQ) estimated for the consumption of these medicinal herbs grown on metal-rich soil. An enhancement in secondary metabolite content and antioxidant properties was also observed upon biochar treatments. These multiple beneficial effects of biochar supplementation in Pb and Cd co-contaminated soil suggested that a biochar amendment is a sustainable approach for the safe cultivation of MPs.