Effects of kolanut pod-derived biochar on the growth and nutritional quality of Solanum macrocarpon L. grown on cadmium-polluted soil

Abstract

Cadmium (Cd) contamination of agricultural soils severely limits crop productivity and poses health risks through the food chain. This study evaluated the effect of kolanut pod-derived biochar on the growth and mineral nutrient content of Solanum macrocarpon L. grown in Cd-polluted soil. A screen house experiment was conducted using a Completely Randomized Design with four treatments: Control (no Cd, no biochar), Cd only (27 mg kg⁻¹), Cd + 17 g biochar, and Cd + 50 g biochar, each replicated thrice. Growth parameters (plant height, number of leaves) were recorded weekly for four weeks, and mineral concentrations (Fe, Mg, Zn, Mn, Ca) in leaves and roots were quantified via Atomic Absorption Spectrophotometry after harvest. Results indicated that Cd stress reduced plant height (Control: 8.50 ± 2.18 cm; Cd: 9.63 ± 2.47 cm at week 4 compared to Cadmium + 50 g Biochar) and number of leaves (Control: 8.00 ± 2.00; Cd: 5.33 ± 1.53) relative to control, though differences were not statistically significant (p > 0.05). Biochar addition, particularly at 50 g, enhanced growth, with week 4 mean heights of 10.33 ± 5.27 cm and 8.07 ± 0.61 cm for 50 g and 17 g treatments, respectively. Biochar also influenced nutrient uptake: in roots, Fe content increased significantly with 17 g biochar (160.67 ± 12.85 mg kg⁻¹) compared to Cd only (104.67 ± 0.29 mg kg⁻¹, p ≤ 0.05), while Mg was highest in Cd-only roots (167.00 ± 19.94 mg kg⁻¹). In leaves, Mg peaked with 17 g biochar (165.33 ± 2.68 mg kg⁻¹), and Fe remained highest in the control (129.83 ± 24.56 mg kg⁻¹). Biochar application increased soil pH (7.35), cation exchange capacity (21.42 mEq 100g⁻¹), and organic matter (8.2%), reducing Cd bioavailability. The results showed that kolanut pod biochar at higher rates alleviated Cd toxicity, improve nutrient composition, and enhance physiological performance of S. macrocarpon. This offers a sustainable, low-cost amendment for Cd remediation and food safety improvement.