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

Heavy metal contamination of agricultural soils, particularly by zinc (Zn) and cadmium (Cd), threatens food security and ecosystem health. This study evaluated the <i>in situ</i> phytoremediation potential of <i>Ricinus communis</i> 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, <i>p</i> < 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 (<i>p</i> < 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, <i>R. communis</i> 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.