Impacts of CaAl- and MgAl-Layered Double Hydroxides and Conventional Amendments on Cd Immobilization, Bacterial Communities, and Rice Cd Accumulation in Tropical Paddy Soils

Cadmium (Cd) pollution of paddy soils poses a serious threat to food safety and human health. Calcium- and magnesium-aluminum layered double hydroxides (CaAl-/MgAl-LDHs) have recently been deployed to remediate Cd-contaminated soils; nevertheless, the responses of soil properties and microbial communities to CaAl-/MgAl-LDHs are not well elucidated. Herein, a field trial was implemented to evaluate the impacts of CaAl-/MgAl-LDHs and conventional amendments (lime (LM) and biochar (BC)) on soil Cd availability and bacterial community structure, together with the consequent impacts on rice grain Cd enrichment. Compared with CK without amendments, CaAl-LDHs, MgAl-LDHs, LM, and BC amendments significantly raised rice yield-related agronomic traits, with the greatest yield increment observed in the CaAl-LDHs treatment. The four amendments enhanced soil pH, cation exchange capacity (CEC) and available phosphorus (AP), while reducing DTPA-extractable Cd concentration. Notably, CaAl-LDHs and MgAl-LDHs treatments outperformed LM and BC in suppressing DTPA-extractable Cd at rice-grain filling and maturity stages. The four amendments substantially decreased rice Cd concentration, with the lowest values observed in CaAl-LDHs and MgAl-LDHs treatments. Furthermore, the four amendments boosted bacterial α-diversity and reconstructed bacterial community structure by elevating the phyla Thermodesulfobacteriota, Bacteroidota, and Firmicutes. Based on redundancy and Pearson correlation analyses, soil pH, CEC, available Cd, ExCa, and AP were identified as critical factors governing bacterial community structure. Partial least squares path model suggested that soil amendments modified pH and CEC, which influenced bacterial communities and available soil Cd, thereby suppressing rice Cd accumulation. These findings suggest that bacterial communities and soil chemical properties play critical roles in Cd immobilization upon amendment addition, and support the application potential of CaAl-/MgAl-LDHs to achieve safe rice production in Cd-polluted soils.