Abstract:[Background] Rhizosphere microorganisms are commonly employed in soil heavy metal remediation due to their ability to enhance soil fertility and boost crop growth. However, the efficacy of a solitary microbial agent in remediation is inconsistent and vulnerable to interference from indigenous microorganisms. Consequently, developing reliable composite microbial agents is crucial for facilitating plant remediation of heavy metals. [Objective] We conducted pot experiments to the study mechanism by which Burkholderia sp. DHC34 and/or Pseudomonas fluorescens fortified Ageratum conyzoides L. in the remediation of cadmium (Cd)-contaminated soil. [Methods] The single microbial agents BH (Burkholderia sp. DHC34) and PH (P. fluorescens), as well as the composite microbial agents BPH1-1, BPH1-2, and BPH2-1 (consisting of Burkholderia sp. DHC34 and P. fluorescens mixed at ratios of 1:1, 1:2, and 2:1, respectively), were used to treat A. conyzoides. The plants treated with sterile water were taken as the control group. The plant biomass and Cd accumulation were measured, and the micro-method was employed to determine the activities of antioxidant enzymes in plant leaves. Microbial diversity was assessed through Illumina PE250 high-throughput sequencing. [Results] In comparison to the control group, P. fluorescens alone significantly enhanced the activities of peroxidase, catalase, and superoxide dismutase, while increasing the plant height and fresh weight by 50.0% and 77.7%, respectively. Burkholderia sp. DHC34 resulted in weak acid extractable fraction of Cd comprising 60.0% of the total, while increasing the Cd accumulation per A. conyzoides plant by 188% relative to the control. The co-inoculation of the two strains in varying ratios significantly enhanced the Cd uptake by A. conyzoides plants. BPH1-2 exhibited the most significant performance, increasing the biomass and Cd accumulation of A. conyzoides by 71.2% and 166%, respectively. The bioenrichment coefficients of aboveground and underground parts reached 11.40 and 6.25, respectively. Furthermore, the co-inoculation of both strains induced substantial changes in the rhizosphere microbial community composition, markedly increasing the abundance of beneficial taxa such as Flavisolibacter, Adhaeribacter, and Streptomyces. By regulating the activity of the microbial groups involved in carbon and nitrogen cycling, the composite microbial agents fortified the Cd remediation efficiency and stablized the rhizosphere environment of A. conyzoides. [Conclusion] Burkholderia sp. DHC34 and P. fluorescens exhibit potential applications as growth-promoting agents for A. conyzoides. The co-inoculation of the two bacterial strains could create a functional pathway synergizing the plant growth-promoting effect of P. fluorescens and the Cd activing effect of Burkholderia sp. DHC34. The co-inoculation leads to the enrichment of beneficial rhizosphere microbial groups to mitigate Cd stress on A. conyzoides. Notably, the overall efficacy of BH surpassed that of PH, and BPH1-2 exhibited the best performance among the composite agents.
