Siderophore-producing bacteria reduce soil cadmium bioavailability and alleviate cadmium stress in alfalfa

Reducing cadmium (Cd) bioavailability is a sustainable approach to mitigating Cd exposure risks in contaminated agricultural soils. This study evaluated whether siderophore-producing bacteria (SPB) alleviate Cd stress in alfalfa (Medicago sativa L.) by regulating plant performance, soil properties, and rhizosphere microbial communities. A pot experiment was conducted under different soil Cd levels with or without SPB inoculation. Cadmium stress markedly inhibited alfalfa biomass production, iron uptake, and nitrogenase activity, while increasing soil ammonium accumulation and soil available Cd concentrations. In contrast, SPB inoculation significantly enhanced plant biomass and Fe accumulation, increased soil available phosphorus, and reduced ammonium accumulation as well as soil Cd bioavailability. Although SPB slightly decreased nodule number, it substantially increased nitrogenase activity and the predicted abundance of nitrogen fixation-related functional genes, suggesting that SPB may improve N-fixation-related activity without increasing nodule number under Cd stress. Microbial community analyses showed that SPB partially restored bacterial diversity and community structure disrupted by Cd contamination. Structural equation modeling further revealed that coordinated plant, soil, and microbial pathways jointly contributed to reductions in soil Cd bioavailability. Overall, these results demonstrate that SPB inoculation reduces Cd exposure risks and promotes Cd stabilization processes, representing a promising microbial strategy for improving ecological safety in Cd-contaminated agroecosystems.