Enhanced Cd phytostabilization and rhizosphere bacterial diversity of Robinia pseudoacacia L. by endophyte Enterobacter sp. YG-14 combined with sludge biochar.

Robinia pseudoacacia L., a pioneer woody legume grown in mining areas, has been recognized as a remarkable accumulator of various heavy metals. Compared with other hazardous heavy metals (HMs), it is of low capacity in accumulating Cd, which, as a result, may hinder the phytoremediation efficiency. To enhance R. pseudoacacia's uptake efficiency of Cd, the individual effects of various rhizobia and arbuscular mycorrhizal fungi have been reported, however, the combined influence of endophytes and biochar receives little attention. In the current study, a Cd-adsorbing endophyte Enterobacter sp. YG-14 was inoculated to R. pseudoacacia, and its extraordinary effect on increasing R. pseudoacacia's Cd uptake was found, which was ascribed to the reinforced root Cd chelation by the strain through secreting siderophores/LMWOAs. Further, P-enriched sludge biochar was applied along with YG-14 to form a combined biochar-endophyte-accumulator system, in which biomineralization were reinforced (i.e. CdCO3 and Cd2P2O7 were generated), as the total and acid-soluble Cd in rhizosphere were reduced by 61.75% and 69.01% respectively, and soil's bacterial diversity was further improved with diversified N2-fixing microbial biomarkers. Multiple synergistic effects (E > 0) were also found, with the optimum performance on plant growth parameters (increased by 39.61%-561.91%) in comparison to the control group. Moreover, the system exhibited a preferable Cd phytostabilization capacity with the highest increase (81.42%) in Cd accumulation and a significant reduction (72.73%) in Cd root-to-shoot translocation.