Study on the Interaction Effect of Heavy Metal Cadmium in Soil–Plant System Controlled by Biochar and Nano-Zero-Valent Iron

The accumulation of heavy metal cadmium (Cd) in farmland soil in edible parts of crops seriously threatens plant growth, human health, and even the global ecological environment. Finding stabilization remediation technology is an important means to treat Cd-contaminated soil. This study comprehensively evaluated the synergistic effects of independent or combined application of biochar (BC) (10, 30 g kg^-1) and nano zero-valent iron (nZVI) (0.1% <i>w</i>/<i>w</i>) on soil properties and morphological and physiological traits of pakchoi (<i>Brassica rapa</i> L. subsp. <i>chinensis</i>) under Cd (1, 3 mg kg^-1) stress by pot experiments. It was shown that Cd toxicity negatively affected soil properties, reduced pakchoi biomass and total chlorophyll content, and increased oxidative stress levels. On the contrary, the combined application of BC (30 g kg^-1) and nZVI (0.1%, <i>w</i>/<i>w</i>) reduced the Cd accumulation in the shoot parts of pakchoi from 0.78 mg·kg^-1 to 0.11 mg·kg^-1, which was lower than the Cd limit standard of leafy vegetables (0.20 mg kg^-1) in GB 2762-2017 "National Food Safety Standard". Compared with the control, the treatment group achieved a 61.66% increase in biomass and a 105.56% increase in total chlorophyll content. At the same time, the activities of catalase (CAT) and superoxide dismutase (SOD) increased by 34.86% and 44.57%, respectively, and the content of malondialdehyde (MDA) decreased by 71.27%. In addition, the application of BC alone (30 g·kg^-1) increased the soil pH value by 0.43 units and the organic carbon (SOC) content by 37.82%. Overall, the synergistic effect of BC (30 g kg^-1) and nZVI (0.1% <i>w</i>/<i>w</i>) helped to restore soil homeostasis and inhibit the biotoxicity of Cd, which provided a new option for soil heavy metal remediation and crop toxicity mitigation.