Regulation of soil properties by amendments and their impact on Cd fractions and bacterial community structure: Exploring the mechanism of inhibition on Cd phytoavailability

The application of soil amendments is crucial for mitigating cadmium (Cd) phytoavailability in Cd-contaminated paddy fields, thereby promoting safer rice production. However, the mechanisms through which these amendments influence phytoavailable Cd by modifying soil properties have not yet been fully elucidated. A pot experiment was conducted to evaluate the effects of three soil amendments-sepiolite (SE), wollastonite (WO), and a composite (YY)-on the Cd concentrations in brown rice, soil Cd fractions, soil properties, and bacterial community structure. Additionally, the relationships among brown rice Cd concentration, soil properties, Cd speciation, and bacterial diversity were explored. The findings demonstrated that the YY, SE, and WO amendments significantly increased the soil pH, cation exchange capacity (CEC), and concentrations of exchangeable calcium (ExCa), magnesium (ExMg), and available silicon (ASi), facilitating the transformation of water-soluble and acid-extractable forms of Cd into reducible fractions and facilitating the formation of low-solubility Cd compounds, thereby significantly lowering the levels of CaCl₂-extractable Cd and DTPA-extractable Cd. The YY amendment also increased available potassium (AK) and available phosphorus (AP) while simplifying the bacterial community structure, notably increasing the abundance of Firmicutes and Bacteroidota. In contrast, SE amendment increased the abundance of Acidobacteriota. Both the YY and SE amendments reduced Cd phytoavailability by modifying Cd speciation and optimizing soil bacterial communities, whereas WO primarily lowered Cd phytoavailability by altering Cd speciation alone. These results underscore the regulatory role of soil amendments in modifying soil properties, influencing Cd speciation, and reshaping bacterial communities, ultimately reducing Cd accumulation in brown rice. This study enhances our understanding of the mechanisms by which amendments alter soil properties to reduce Cd phytoavailability, offering insights for developing in situ passivation technologies for Cd-contaminated soils.