Co-application of cadmium-immobilizing bacteria and organic fertilizers alter the wheat root soil chemistry and microbial communities

Cadmium contamination poses a significant risk to soil ecosystems in certain parts of the world. Using eco-friendly fertilizers alongside beneficial microorganisms offers a viable solution to mitigate Cd pollution in agricultural soil. This study used an outdoor experiment to evaluate the impact of administering a Cd-immobilizing bacterial (Bacillus) inoculant with two biologically-enriched organic fertilizers (either fermentative edible fungi residue or fermented cow dung) on wheat plants and associated microbial populations in a field contaminated with Cd. The mixed application of fermentative cow dung with the Cd-immobilizing bacterium reduced the effective Cd content of wheat root-soil by 13,0 %. Application of Cd-immobilizing Bacillus inoculant reduced the Cd enrichment of wheat roots by 0.07 mg/kg. Co-application of fermentative cow dung with the bacterial inoculant reduced the Cd enrichment of wheat seeds by 20,0 %. Co-application of the two organic fertilizers could improve some of the nutrients related to wheat and soil fertility; however, the diversity of the soil microbial community changed less and its species richness decreased. Applying the Bacillus inoculant inhibited the growth of native pathogenic bacteria, such as Proteobacteria. Whether administering it with either fermented cow manure or fermented edible fungus residue, the relative abundance of nitrate-reducing bacteria such as Rhodobacter increased, which should promote the soil nitrogen cycle. The main factors influencing soil microbial community structure of wheat plants were pH, available potassium, and available Cd content. Symbiotic network analysis revealed bacterial inoculant and organic fertilizer inoculum further altering the ecological relationships of microbial communities. According to the FAPROTAX functional prediction, Rhodanobacter may play a key role in nitrate respiration in the soil nitrogen cycle. In conclusion, this study provides a comprehensive, timely reference for understanding microbial changes caused by the combined application of this type of bacterial inoculant and organic soil amendments in Cd-contaminated fields.