Nano-selenium modified green eggshell biochar reduces cadmium accumulation in shallots (Allium schoenoprasum L.)

Green eggshell biochar, a renewable biomass material, demonstrates promising potential for environmental remediation. This study systematically prepared biochar under varying pyrolysis conditions and identified nano-selenium-modified biochar (produced at 600 °C for 3 h, termed 6-3 S) as the optimal formulation for cadmium (Cd) immobilization. Compared to untreated soil, the 6-3 S biochar reduced bioavailable Cd content by 38.65 % in contaminated soil. Correspondingly, Cd accumulation in shallot tissues decreased by 56.64 % (white parts) and 82.69 % (green parts). Furthermore, the 6-3 S treatment enhanced plant selenium levels by 21.3–29.8 % and preserved leaf microstructure integrity, reducing stomatal deformation by 44.2 % compared to controls. Additionally, Nitro Blue Tetrazolium (NBT) staining area decreased from 39.03 % to 24.00 %, indicating reduced oxidative stress. These dual effects—Cd suppression and selenium enrichment—significantly improved shallot quality and safety. The findings establish a scientific foundation for deploying nano-selenium-modified biochar in heavy metal-contaminated agricultural systems.