Evidence that appropriately elevating Serratia nematodiphila ZP5 inoculation concentration alleviates heavy metal stress and enhances phytoremediation in Cajanus cajan L.

Lead (Pb)-cadmium (Cd) co-contamination in mining soils threatens soil ecological security and limits phytoremediation efficiency. Under severe contamination, conventionally dosed microbial inoculants may exhibit limited survival and activity, thereby reducing their contribution to plant-assisted remediation. This study examined how different inoculation concentrations of Serratia nematodiphila ZP5 affected phytoremediation by Cajanus cajan L. and the associated microbial mechanisms under light (LS), moderate (MS), and severe (HS) Pb-Cd co-contamination. ZP5 inoculation increased Pb and Cd accumulation in C. cajan L., with stronger responses as contamination intensified. Conventional inoculation (CM) was the most effective under LS and MS, whereas high-concentration inoculation (CH) produced the greatest remediation effect under HS. Under HS, CH increased total Pb and Cd accumulation in C. cajan L. by 60.52% and 61.95%, respectively. CH affected Cd and Pb translocation differently. It increased the Cd translocation factor from 0.516 to 0.564 but decreased the Pb translocation factor from 0.234 to 0.180. Co-occurrence network analysis indicated that CH reshaped endophytic bacterial networks and mitigated the loss of network complexity caused by heavy metal stress. Amplicon sequencing revealed that ZP5 inoculation increased the relative abundance of Pseudomonadota, including Ensifer and Hydrogenophaga, in roots and shoots. These taxa were associated with improved phytoextraction. These findings suggest that high-concentration ZP5 inoculation may alleviate severe Pb-Cd stress more effectively than conventional inoculation. Therefore, a concentration-adaptive microbial inoculation strategy may improve microbe-assisted phytoremediation in Pb-Cd co-contaminated mining soils.