Biochar as a protective adsorbent against cadmium and lead stress in parsley ( Petroselinum crispum ): a study on growth, nutrient uptake, and antioxidant enzyme activities

Background Heavy metal contamination, particularly from cadmium (Cd) and lead (Pb), poses serious threats to plant growth, nutrient uptake, antioxidant defense mechanisms, and food safety. Biochar (BC) is known to improve soil quality and reduce heavy metal toxicity in plants. This study aimed to assess the impacts of Cd and Pb stress on nutrient element concentrations and antioxidant enzyme activities in parsley ( Petroselinum crispum ), and to evaluate the extent to which BC mitigates these detrimental effects. Methods The study was conducted in 3 kg capacity pots arranged in a randomized factorial design with three replications. Parsley ( P. crispum ) was chosen as the test plant. Cd and Pb were applied separately as CdSO 4 and Pb(NO 3 ) 2 at concentrations of 0, 2.5, and 5 mg kg −1 . Biochar was incorporated into the soil at 0% and 1% (w/w) rates. Dry weight, heavy metal accumulation, nutrient element concentrations, and antioxidant enzyme activities—catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX)—were measured to determine the physiological and biochemical responses of plants to heavy metal stress and BC application. Results The results showed that the highest dry weight (11.05 g pot −1 ) was observed in the control group with 1% BC application, representing a 17.8% increase compared to the non-BC application. This increase in dry weight was found to be statistically significant. While the BC application reduced Cd concentration in the plant by 14.4%, it had no significant effect on Pb concentration. Under heavy metal stress, BC improved nutrient acquisition, elevating potassium (K), magnesium (Mg), iron (Fe), zinc (Zn), and manganese (Mn) concentrations by 15.9%, 5.9%, 69.3%, 5.5%, and 17.6%, respectively. Exposure to Cd and Pb inhibited CAT, POD, and APX activities; however, BC application alleviated this suppression, increasing enzyme activities by 20.4%, 66.7%, and 25.0%, respectively. Conclusions BC application increased dry weight, nutrient uptake, and antioxidant enzyme activities in parsley plants exposed to Cd and Pb stress. Overall, the findings indicate that BC enhances the growth performance and antioxidant defense capacity of parsley plants grown in Cd- and Pb-contaminated soils, highlighting its strong potential as a sustainable soil amendment for mitigating heavy metal toxicity and safeguarding crop productivity in contaminated agricultural systems.