Objectives To explore the effects of indole-3-acetic acid (IAA)-producing bacteria on promoting the growth of soybeans and reducing cadmium (Cd) in Cd-contaminated farmland soil, as well as the correlations between Cd and biomass and macronutrients, in order to provide a new effective bioremediation technology for alleviating Cd pollution in farmland soil.

Methods Six strains capable of producing indole-3-acetic acid (IAA) and exhibiting Cd resistance were isolated and screened from Cd-contaminated soil: Serratia sp. NQ, Citrobacter sp. SG, Bacillus sp. J2, J3, and Priestia sp. NQ2-7, NQ2-11 capable of IAA and exhibiting Cd resistance. Cultivation assays revealed that strains NQ, SG, J2, and J3 produced IAA at concentrations of 115.3, 134.4, 152.8, and 142.5 µg/mL respectively, demonstrating strong IAA-producing capacity. The minimum lethal concentrations (LC₅₀) for Cd were 150 mg/L for J2 and J3, and 100 mg/L for NQ and SG. In the potting soil for the soybean pot experiment, CdCl₂ was added and aged to prepare simulated Cd-stressed soils with Cd²⁺ concentrations of 2 mg/kg and 4 mg/kg respectively. After soybean seed germination, strains NQ, SG, J2, and J3 bacterial suspensions (bacterial concentration approximately 10⁷ CFU/mL) into the soil surrounding the seedlings. An equal volume of culture medium served as the control. Soybeans were harvested after 40 days of growth, and plant growth indicators, nutrient content, and Cd concentration were measured.

Results Soil Cd contamination inhibited the growth and nutrient uptake of soybean plants. Four strains of IAA-producing bacteria exhibited varying degrees of cadmium-reducing efficacy. Under Cd²⁺ stress at 2 mg/kg, soybean plants inoculated with four IAA-producing bacterial strains exhibited increased plant height by 24.96%−33.30%, the aboveground and underground biomass increased by 100.0%−111.5% and 47.71%−84.64%, respectively, compared to the uninoculated control. The contents of N, P and K in the aboveground and underground parts increased by 23.73%−39.61%, 37.05%−55.3%, 12.99%−25.16% and 17.48%−25.92%, 9.62%−30.77%, 28.06%−41.84%, respectively, while the Cd contents in the above-ground and underground parts decreased by 8.30%−23.50% and 5.54%−29.93%, respectively. Under Cd²⁺ stress at 4 mg/kg, the four IAA-producing bacterial strains increased soybean plant height by 29.76% to 37.15%, the aboveground and underground biomass increased by 111.86% to 144.63% and 94.44% to 113.89%, respectively, the contents of N, P, K in the aboveground and underground parts increased by 49.53%−59.81%, 31.32%−63.02%, 19.20%−31.73% and 32.32%−59.11%, 7.61%−36.96%, 43.99%−59.13%, respectively. The aboveground and underground Cd content decreased by 3.18%−18.39% and 9.44%−29.05% respectively. Under Cd²⁺ stress conditions of 2 mg/kg and 4 mg/kg, no significant correlation was observed between Cd content in soybean plants and biomass, N, P, or K content. However, a highly significant (P<0.001) positive correlation existed between N, P, and K content and biomass in soybean plants.

Conclusions IAA-producing bacteria alleviated the toxic effects of Cd on soybean plants through promoting growth, reducing cadmium and enhancing the absorption of nutrients, providing a theoretical basis and strain resources for the safe production of soybeans in cadmium-contaminated soil by using IAA-producing bacteria.