Mitigating cadmium and salinity stress in lettuce through biochar application: impacts on growth and nutrient dynamics.

Heavy metal contamination and soil salinity pose significant threats to safe and sustainable vegetable production, particularly for sensitive crops such as lettuce. These combined stresses hinder nutrient uptake, accelerate toxicity, and underscore the need for effective soil amendments that can mitigate multi-stress conditions. However, the potential of low-cost, hazelnut husk-derived biochar to alleviate simultaneous cadmium (Cd) and salinity stress in lettuce remains poorly understood. This study evaluated the effects of biochar applications on lettuce (Lactuca sativa L.) grown under varying cadmium (Cd) concentrations and salt stress conditions. A greenhouse experiment was conducted using a factorial design with four levels of biochar (0, 0.5, 1, and 2%), four Cd treatments (0, 0.5, 1, and 2 mg kg⁻¹), and two salinity regimes (0 and 75 mM NaCl), with three replicates per treatment. Plant growth, yield, and the concentrations of macro-nutrients phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na), and micro-nutrients iron (Fe), copper (Cu), manganese (Mn), and boron (B), as well as Cd, were determined. Results showed that biochar significantly increased shoot biomass, improved nutrient concentrations, and reduced Cd accumulation in lettuce shoots. Under non-saline conditions, the highest biochar dose (2%) reduced shoot Cd concentrations by approximately 30-50% across Cd levels and increased shoot biomass by about 40-50% compared with the non-amended control. Under saline conditions, biochar still decreased shoot Cd concentrations by around 25-30% and increased shoot dry weight by up to nearly 60%. Across Cd and salinity levels, biochar markedly reduced shoot Na concentrations (by almost 90%) and enhanced K⁺, Mn, and B concentrations; under salt stress, Zn increased by approximately 40%. Principal Component Analysis (PCA) revealed that PC1 primarily reflected gradients associated with the effects of biochar and Cd on nutrient and Cd profiles, whereas PC2 captured variability driven by salinity. Overall, these findings demonstrate that hazelnut husk-derived biochar is an effective and low-cost soil amendment that can improve nutrient status, reduce Cd bioavailability, and mitigate salinity-induced ionic imbalances, offering a practical strategy for enhancing lettuce production in Cd-contaminated and saline soils.