Amelioration of lead (Pb) stress in Brassica campestris L. by synergistic application of biochar and compost under Pb-contaminated soil.

Lead (Pb) is one of the most toxic heavy metals adversely affecting agricultural productivity, soil health, and food safety. Organic amendments (OAs), particularly biochar (BC) and compost (C), can mitigate heavy metal toxicity through Pb (II) immobilization, improved soil physicochemical properties, enhanced nutrient availability, and activation of plant defense systems. Therefore, this study investigated the individual and combined effects of BC (1% w/w) and C (1% w/w), applied solely or in combination (0.5% w/w each), on Pb (II) tolerance in Brassica campestris grown under 500 and 1000 mg kg⁻¹ Pb (II) contamination. Lead stress markedly reduced plant growth, yield, photosynthetic pigments, gas exchange parameters, and overall physiological performance, with more severe effects observed at 1000 mg kg⁻¹ Pb (II). However, the combined application of BC and C significantly improved plant height, root length, seed production, grain yield, chlorophyll contents, transpiration, photosynthetic activity, and stomatal conductance compared with Pb-stressed treatments. The co-application of BC and C also enhanced antioxidant defense mechanisms by increasing catalase, superoxide dismutase, peroxidase, and glutathione S-transferase activities, thereby reducing Pb-induced oxidative stress. Moreover, significantly reduced AB-DTPA-extractable soil Pb and Pb accumulation in plant tissues, with the combined treatment showing the greatest reduction in Pb uptake. Overall, the findings demonstrate that synergistic application of biochar and compost alleviates Pb (II) toxicity through reduced Pb (II) availability and plant uptake, improved physiological performance, and strengthened antioxidant defense systems.