Microbial driving mechanism of soil conditioner on reducing cadmium uptake by rice and improving soil environment

Soil conditioners can reduce the transfer of heavy metal ions from the soil to crops and play a crucial role in safe food production. However, it remains unclear how soil conditioners reduce cadmium (Cd) accumulation in rice (Oryza sativa L.). In this study, five soil conditioners (U-, N-, T-, L-, and C-type) commonly used in China were selected, and the effects of soil conditioners on bacterial and fungal community diversity, soil enzyme activity, soil pH, and organic matter content were investigated through field experiments in two sites. The Cd content of rice was significantly reduced by 32.4–55.1 % in site I and 16.0–33.3 % in site II under the five soil conditioner treatments. The decrease in Cd content was related to the increase in soil pH, organic matter content, and Cd fixation-related flora caused by soil conditioner application. U-, N- and C-type conditioners increased soil pH, inducing an increase in the relative abundances of microbial populations related to Cd sorption and sequestration (Firmicutes, Nitrospirota, Actinobacteriota, and Methylomirabilota). U-, T-, and L-type conditioners increased soil organic matter, causing an increase in the relative abundance of microbial flora associated with macromolecule degradation, including bacteria (Acidobacteriota and Chloroflexi) and fungi (Ascomycota and Basidiomycota). Conditioner application reduced the availability of soil Cd; indirectly promoted the increased abundance of beneficial soil bacteria; and increased the activities of invertase, urease, dehydrogenase, catalase, and acid protease. In conclusion, soil conditioner reduces the effective soil Cd by adjusting soil pH and organic matter content, increasing Cd-passivating flora and Cd-tolerant flora. Soil conditioner also increases the abundance of other beneficial flora, enhances soil enzyme activity, promotes the microecological balance of soil under Cd stress, and finally reduces Cd accumulation in rice.