The Rhizosphere Microbial Mechanism of Endophyte SaMR12 in Regulating the Intercropping Effect Between Sedum alfredii Hance and Pakchoi in Cadmium-Contaminated Soil

Endophyte SaMR12 from Sedum alfredii benefits vegetable–Sedum intercropping, but whether this effect varies with vegetable cultivars remains unclear. This rhizo-box study thus investigated how pakchoi cultivars and endophyte inoculation affect intercropping performance and rhizosphere bacterial communities. Under monoculture, shoot cadmium (Cd) concentrations differed twofold between the two pakchoi cultivars. Relative to monoculture, Sedum intercropping elevated the shoot Cd concentration in the low-Cd-accumulating pakchoi by 20%, whereas intercropping plus SaMR12 application significantly decreased levels by 18%. In contrast, the high-Cd-accumulating pakchoi showed no significant changes across treatments, confirming cultivar-dependent intercropping effects. After intercropping with pakchoi, the shoot Cd concentration in Sedum decreased significantly, while SaMR12 inoculation restored concentrations to monoculture levels. 16S rDNA sequencing revealed that the dominant bacterial phyla in the rhizosphere soil of the intercropping systems were Proteobacteria, Actinobacteria, and Bacteroidetes. Plant species were the decisive factor influencing the diversity of rhizosphere soil microbial communities. Intercropping and the endophyte had consistent effects on the microbial community diversity of the two pakchoi cultivars but differed in their impact on microbial community composition. The SaMR12 inoculation enriched plant-growth-promoting bacterial genera, such as Arthrobacter, Lysobacter, and Rhodanobacter, in the rhizosphere soil of Sedum, which may be the reason why the endophyte effectively alleviated the negative impacts of intercropping and improved its phytoextraction efficiency. In summary, the low-Cd-accumulating pakchoi–Sedum–endophyte SaMR12 intercropping system can establish a “Phytoextraction Coupled with Agro-safe production” (PCA) model for Cd-contaminated soils.