The synergistic utilization of Chinese milk vetch and immobilized materials can reduce Cd bioavailability in paddy soil through strengthening materials adsorption, soil properties and microbial changes

Immobilized materials (IMs) and Chinese milk vetch ( Astragalus sinicus L., MV) have been widely employed to remediate cadmium (Cd) pollution in paddy fields. However, the combined immobilization effect and mechanism of MV with IMs (limestone, biochar, and sepiolite) on soil Cd contamination are unclear. To investigate the combination impact, an adsorption and a pot experiment were conducted. The results revealed that during the decomposition of MV, protein-like substances were more abundant in the early stages and gradually transformed into humic substances in the later stages. MV enhanced the Cd adsorption capacity of IMs by increasing the abundance of functional groups on their surfaces. Consequently, soil available Cd was significantly reduced by 36.86% to 68.17% compared to the control. In the combined treatment, Cd concentrations in brown rice were significantly decreased by 88.60% to 94.31%. Compared to individual IMs, the combined application exhibited better performance in Cd reduction. Moreover, the co-utilization significantly improved soil fertility (soil organic matter, total nitrogen, etc). These changes contributed to increased soil microbial diversity and soil microbiota stability, particularly in treatments containing MV. Notably, MV significantly improved the abundance of Desulfobacterota which helped to Cd immobilization and it was significantly negatively correlated with Cd availability . Partial least squares path model indicated that soil dissolved organic matter helped to reduce soil Cd availability. The co-utilization could not only strengthen the improvement of soil fertility, but also reduced Cd bioavailability. This study will provide a low-cost and high-efficiency method for the remediation of Cd contamination in paddy soil. • More humus was produced in the later stage of MV decomposition • MV DOM increased the adsorption capacity of the material to Cd • The co-utilization of MV and IMs improve soil fertility and microbial diversity • The changes of sulfate reducing bacteria contribute to Cd fixation