Synergistic application of canna-derived biochar and arbuscular mycorrhizal fungi enhances cadmium immobilization and soil ecological function in contaminated soils

Cadmium (Cd) is a highly toxic and persistent heavy metal pollutant that poses a serious threat to soil ecosystems and food safety. In this study, biochar derived from three wetland plants (reed, typha, and canna) at various pyrolysis temperatures were evaluated for their capacity to immobilize Cd in contaminated soil. Canna-derived biochar prepared at 500 °C (Ca500) exhibited the most favorable physicochemical properties, including a high surface area and a developed porous structure. Subsequently, ICP-MS analysis, antioxidant activity assays, and microbial community analysis were conducted to further investigate the effects of combined Ca500 and arbuscular mycorrhizal fungi (AMF) application on Cd immobilization in soil and on soil microbial communities. The results revealed that co-application of AMF and biochar (at 6% of soil dry weight) reduced the proportion of transportable Cd in soil from 62.9 to 35.42%, and significantly inhibited the translocation of Cd to plant stems and leaves by 31.6% and 22.6%, respectively, thereby effectively enhancing the edible safety of vegetables. Biochemical analysis showed that the combined application of AMF and biochar synergistically activated the antioxidant defense system, with the activities of SOD, POD, and CAT increasing by 8.9%, 79.1%, and 21.1%, respectively. Meanwhile, the malondialdehyde (MDA) level decreased by 47.2%, indicating alleviation of Cd-induced oxidative damage. Combined analysis of the microbial community using Kraken2 and Bracken revealed that the combined treatment enriched beneficial bacterial genera such as Streptomyces and Pseudomonas, and upregulated genes associated with stress response, sulfur metabolism, and ethanol metabolism, thereby improving soil fertility. In conclusion, the work highlights the ecological advantages of integrating AMF and biochar for sustainable remediation of Cd-contaminated soils, offering both mechanistic insights and practical implications for safe vegetable production.