Combined Effect of Biochar and Salicylic Acid in Alleviating Heavy Metal Stress, Antioxidant Enhancement, and Chinese Mustard Growth in a Contaminated Soil

Heavy metal (HM) pollution has become a serious issue globally, endangering both human health and agricultural production, particularly in acidic soils with high HM bioavailability. Although the exact mechanism of either rice husk biochar (RH) or salicylic acid (SA) in lowering HM levels in soil and plants has been discovered, their combined effect remains unknown. Therefore, this study mitigated HM stress and reduced their content in the Chinese mustard plants by adding different levels of RH alone or with foliar application of SA to produce safe foods. RH was added to the soil at 0%, 2.5%, and 5% either alone or with foliar application of SA, which was applied to the plant leaves at concentrations of 0, 0.5, and 1 mM in triplicate for each. The highest reductions of the DTPA-extractable metals were recorded in a high level of RH (5%), which were 32.16%, 43.22%, 49.72%, and 29.54% for Pb, Cd, Cu, and Zn, respectively, compared with the control. Additionally, the combined effect of RH and SA at a high rate demonstrated the highest increase in fresh (19-fold) and dry (threefold) shoot, and the greatest relative reductions in shoot Pb, Cu, Cd, and Zn concentrations were 74.72%, 69.19%, 54.16%, and 53.86%, respectively, compared with the control. Furthermore, the combination significantly improved all studied antioxidant enzymes and increased chlorophyll a and b and carotenoid contents by approximately 300%, 400%, and 250%, respectively, compared with the untreated soil. H 2 O 2 , malondialdehyde, and proline contents were significantly stimulated to the highest levels in the HM-contaminated soil, whereas that amended with RH alone or with sprayed SA significantly reduced their content. Adding RH with sprayed SA can remediate HM-contaminated soil and reduce the concentrations of potentially toxic metals in plants.