Effects of resistant bacterial inoculant concentration on phytoremediation of cadmium–arsenic co-contaminated soil by Celosia argentea–Pteris vittata intercropping system and its microbial mechanisms

Cadmium–arsenic (Cd–As) co-contamination threatens soil ecology and food safety, and plant growth-promoting rhizobacteria (PGPR) offer a promising route to plant-based remediation, yet how PGPR inoculation dose governs remediation in intercropping systems remains unclear. Here, a Cd–As-tolerant Bacillus cereus strain was applied at four levels (CK; BS 4×10⁸, BM 8×10⁸, BL 1.6×10⁹ CFU mL⁻¹) to a Celosia argentea – Pteris vittata intercropping system, and plant physiology, Cd–As bioconcentration/translocation factors, rhizosphere physicochemistry, enzyme activities, metal fractions, and 16S rRNA gene amplicons were analyzed. The response was strongly dose-dependent. The medium-dose BM treatment performed best, with shoot BCF_Cd = 3.274 and BCF_As = 13.710: it enhanced root activity, antioxidant status, and plant biomass, reduced labile Cd fractions while mobilizing residual As, and enriched Proteobacteria and the indigenous functional genus Novosphingobium . Excessive inoculation (BL) was inhibitory, reducing root activity, phytoextraction performance, enzyme activities, and rhizosphere bacterial balance. The inoculated B. cereus did not dominate the rhizosphere; instead, it acted mainly by reshaping the rhizosphere microenvironment and selectively enriching indigenous functional microorganisms. These findings reveal the dose-dependent microecological mechanisms of PGPR-assisted phytoremediation and provide a practical dosage reference for remediating Cd–As co-contaminated soils.