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.