Effect of Biochar on the Growth, Photosynthesis, Antioxidant System and Cadmium Content of <i>Mentha piperita</i> ‘Chocolate’ and <i>Mentha spicata</i> in Cadmium-Contaminated Soil

Cadmium (Cd) is a common heavy metal contaminant which seriously affects plant growth and environmental safety. Biochar, as an organic soil amendment, has been shown to effectively mitigate Cd damage to plants. To study the effectiveness of biochar on mitigating Cd stress, <i>Mentha piperita</i> ‘chocolate’ and <i>Mentha spicata</i> were used in a pot experiment of Cd stress with a CdCl₂ solution (10 mg Kg⁻¹), while a biochar suspension (0, 40, 80, and 160 g Kg⁻¹) was applied to the soil. The effects of Cd on the growth, physiological and biochemical properties, and Cd content in plant tissues of both mint species were found to be significant. The application of 40 g Kg⁻¹, 80 g Kg⁻¹, 160 g Kg⁻¹ biochar significantly alleviated Cd damage to both mint species, increased plant height, leaf length, leaf width, biomass, photosynthetic rate, transpiration rate, stomatal conductance, and chlorophyll content, and decreased antioxidant enzyme activities (including superoxide dismutase, catalase, peroxidase, and polyphenol oxidase) and non-enzymatic antioxidant content (including flavonoids and total phenols). Biochar effectively reduced the Cd uptake by plants and decreased the migration and transformation capacity of Cd in the soil–plant system. In addition, the available nitrogen (available N), available phosphorus (available P), available potassium (available K), and pH in the soil increased after biochar application compared to non-biochar amended soil. The addition of 160 g Kg⁻¹ biochar was shown to have the best performance of the application rates in this experiment and may be considered as an effective way to reduce the damage caused by Cd contamination to <i>M. piperita</i> ‘chocolate’ and <i>M. spicata</i>.