Application of <i>Torreya grandis</i> peel biochar in cadmium contaminated soil remediation and pakchoi growth enhancement

This study aims to elucidate the effect of <i>Torreya grandis</i> peel biochar application on cadmium-contaminated soil and pakchoi (<i>Brassica chinensis L.</i>) growth. A pot experiment was designed involving four distinct biochar/soil ratio treatments: 0 (Control, CK), 1% (T1), 2.5% (T2), and 5% (T3). The results demonstrated that the incorporation of 5% effectively modulated the pH of acidic soil, substantially elevating soil organic matter, and available N, P, K content. Moreover, it augmented the activities of catalase, urease, and acid phosphatase in the soil, concurrently diminishing the Cd content. This treatment reduced the exchangeable and carbonate-bound Cd fractions by 45% while enhancing the iron-manganese oxide-bound, organic matter-bound, and residual Cd fractions by 26%, 29%, and 96%, respectively. Regarding the growth of pakchoi, the 5% biochar application significantly decreased the Cd content in the edible portion by 51%. It significantly enhanced the fresh weight per plant, soluble solids, soluble sugar, soluble protein, and vitamin C content of pakchoi. In conclusion, the application of <i>T. grandis</i> peel biochar is a viable approach for improving the properties of Cd-contaminated soil, passivating Cd fractions, and enhancing the yield and quality of pakchoi. A biochar pyrolysis temperature of 500 °C and a biochar/soil ratio of 5% is recommended.