Biochar Derived from Cow Bones and Corn Stalks Reduced the Release of Cd and Pb and the Human Health Risk Index of Quinoa Grown in Contaminated Soils

Pollution of agricultural soil, which is a non-renewable natural resource, represents a major problem that must be worked on to develop practical solutions to prevent pollutants from reaching the food chain. A pot experiment was conducted to investigate the effect of biochar derived from cow bones (BON) or corn stalks (BIO) on reducing the release of Cd and Pb and the human health risk index (HRI) of quinoa grown in contaminated soils. Biochar derived from cow bones or corn stalks was added to the contaminated soil at the doses of 0, 2, and 4% (w/w). The pot experiment consisted of the following treatments: control, BIO2, BIO4, BON2, and BON4. BIO and BON significantly improved the soil cation exchange capacity (CEC), soil organic carbon (SOC), and soil microbial biomass carbon (SMBC). The highest rate of the two biochar types (BON and BIO) significantly enhanced quinoa growth and seeds quality. The addition of BIO4 and BON4 increased the oil, protein, and saponin in quinoa seeds by 33 and 37%, 28 and 31%, and 66 and 70%, respectively, compared to the control soil. BON and BIO treatments significantly reduced HRI. Quinoa seeds, based on the results of HRI, were safer when the contaminated soil was amended with cow bones biochar than the soil treated with corn stalks biochar. Quinoa plants grown in the polluted soils without the addition of biochar contained high concentrations of malondialdehyde (MDA) and low concentrations of chlorophyll, peroxidase (POD), and superoxide dismutase (SOD). Corn stalks and cow bones biochar increased the synthesis of antioxidant enzymes and decreased the synthesis of malondialdehyde (MDA). The results of sequential extraction revealed that corn stalks and cow bones biochar induced the transformation of Cd and Pb from the soluble fractions to less toxic stable forms. Corn stalks and cow bones biochar significantly reduced the daily release rate of Cd and Pb in the contaminated soil, with a clear superiority in the ability of cow bones biochar in this regard. Corn stalks and cow bones biochar increased the quality characteristics of the polluted soils and improved the ability of quinoa plants to resist the negative effects of toxic metals by improving the antioxidant system defense. The addition of high levels (4% w/w) of the two biochar types resulted in the best quinoa growth as well as seed quality. Cow bones biochar is more capable in reducing the health risks compared to corn stalks biochar, as it reduces the rate of release of Cd and Pb in the soil solution and reduces their transfer to seeds, it also improves the quality characteristics of quinoa seeds in terms of its protein and oil content.