Spent mushroom substrate and citric acid promote the remediation of zinc and cadmium polluted soil by in situ Ricinus communis L . phytoremediation

Heavy metal contamination of agricultural soils, particularly by zinc (Zn) and cadmium (Cd), threatens food security and ecosystem health. This study evaluated the in situ phytoremediation potential of Ricinus communis L. in Zn- and Cd-contaminated field soils amended with citric acid (CA), spent mushroom substrate (SMS), and their combination (CA+SMS). Across contamination levels, SMS and CA+SMS significantly increased total biomass to 164.70 ± 5.61 and 162.80 ± 4.11 g per plant, respectively, compared with 77.38 ± 3.40 g in the unamended control (one-way ANOVA with Tukey's HSD, p < 0.05). Accordingly, total Zn extraction increased by 114.86% (SMS) and 104.89% (CA+SMS), while total Cd extraction increased by 112.80% and 99.22%, respectively (p < 0.05). Cd bioconcentration factors (BCF) remained > 1 across all treatments, whereas Zn BCF remained < 0.33. At high contamination, CA+SMS enhanced soil enzyme activities (urease and catalase (CAT)), with CAT reaching 1.98 ± 0.01 mL 0.1 mol L⁻¹ KMnO₄ g⁻¹ h⁻¹). SMS maintained seed oil content (∼ 55.71 ± 1.78%). Overall, R. communis is a high-biomass, metal-tolerant candidate for field phytoremediation, and CA+SMS is a practical, low-cost strategy that enhances plant uptake while promoting metal sequestration into less labile reducible/oxidizable/residual fractions relative to exchangeable/carbonate-bound pools.