Endophytic bacteria enhance cadmium remediation through siderophore production and soil microbial dynamics

Microbial remediation of heavy metals (HMs) is an environmentally friendly and cost-effective approach to soil restoration. This study aimed to identify the endophytic bacterial strain with the highest capacity to mobilize cadmium (Cd) among four isolates from Paulownia fortunei root nodules. We conducted soil incubation experiments under four Cd contamination levels (10, 40, 80 and 300 mg kg^-1) and three inoculation treatments, and measured soil extractable Cd, microbial community composition, and diversity. Results indicated that strain S7 exhibited the greatest Cd activation capability, attributed to its production of siderophores, organic acids and extracellular polymeric substances (EPS). Analysis of variance showed that inoculation treatment, Cd level and their interaction had significant effects on the relative abundances of the phyla Actinobacteria, Firmicutes, Gemmatimonadota and Bacteroidetes, as well as on bacterial diversity indices (Shannon, ACE, Chao). These dominant phyla were strongly correlated with soil physicochemical properties. Structural equation modelling (SEM) revealed that strain S7 directly enhanced soil Cd mobilization through siderophore production, and indirectly by altering soil pH, bacterial richness (ACE index), and the abundance of Chloroflexi. This study provides new insights into the mechanisms by which endophytic bacteria facilitate Cd remediation and enhances our understanding of microbe-assisted environmental restoration.