The effectiveness of biochar in mitigating copper toxicity: the impacts of dose and particle size on soil properties and plant physiology

Heavy metal contamination, particularly copper (Cu) toxicity, poses a severe threat to global agricultural productivity and soil ecosystem stability. This study evaluated the effects of different doses (D0: control, D0.5: 0.5%, D1:1.0%, and D2: 2.0%) and particle sizes (PS1:<0.5, PS2:0.5-1.0, PS3:1.0–2.0, and PS4:>2.0 mm) of biochar on the chemical and biological properties of calcareous soil and the physiological responses of wheat under conditions of high Cu toxicity. Biochar applications increased the content of soil organic matter (SOM), with the highest value (3.48%) observed at the D2xPS1, corresponding to a 26.08% increase compared to the control. Similarly, cation exchange capacity (CEC) increased significantly, with the highest value (44.89 cmol kg− 1) obtained in the D2xPS2 combination, a 72.99% increase relative to the control. The highest values were observed at D0.5×PS2 for basal respiration (BR) (2.40 mg CO₂ g⁻¹ ODS 24 h⁻¹), at D1×PS2 for microbial biomass carbon (MBC) (112.64 mg MBC g⁻¹ ODS 24 h⁻¹), at D2×PS4 for dehydrogenase (DHA) (6.31 µg TPF g⁻¹ ODS 24 h⁻¹), and at D2×PS2 for catalase (CAT) (1.93 ml O₂ g⁻¹ ODS 3 min⁻¹). In terms of plant growth and yield, lower biochar doses combined with small or medium particle sizes produced more favorable outcomes. The highest grain yield (6.76 g pot− 1), representing a 13.23% increase over the control, was obtained in the D0.5xPS1 combination. Cu uptake decreased by 23% (7.83 mg kg− 1) at the lowest dose, whereas Mn and Zn uptake increased by 58% (12.58 mg kg− 1) and 50% (51.41 mg kg− 1), respectively, at the D1 dose. Malondialdehyde (MDA) content, an indicator of cellular damage, decreased by 29.5% at the D0.5 dose (0.86 µmol g− 1 FW), while higher doses strongly activated antioxidant defense systems, resulting in more than a 2-fold increase in ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities. Overall, the findings suggest that lower doses of biochar and smaller particle sizes, particularly combinations such as D0.5xPS1, can have a more balanced and beneficial effect on the chemical properties of calcareous soils and the physiological performance of wheat under high Cu toxicity. • Biochar application significantly improved soil chemical (OM and CEC) and biological (microbial biomass and activity) properties. • The effect of biochar dose and particle size on copper toxicity was found to be significant. • Lower application dose and relatively small particle size were most effective in improving wheat plant development under copper toxicity.