N fertilizers promote abscisic acid-catabolizing bacteria to enhance heavy metal phytoremediation from metalliferous soils

Bacterial-assisted phytoextraction is an attractive strategy to enhance the phytoremediation efficiency of heavy metal (HM)-contaminated soils. In the present study, we investigated the synergistic effects of N fertilizers and abscisic acid (ABA)-catabolizing bacteria on the HM (Cd, Zn, and Pb) phytoremediation efficiency of Brassica juncea L. (B. juncea). Compared with Rhodococcus qingshengii (R. qingshengii) alone, urea, ammonium‑nitrogen (NH4+-N), and nitrate‑nitrogen (NO3--N) fertilizers combined with R. qingshengii increased HM concentrations in B. juncea by 13.8 %-48.2 %, 44.2 %-54.4 %, and 59.4 %-113.6 %, respectively, and enhanced the biomass of B. juncea by 7.7 %-38.8 %, 10.9 %-29.5 %, and 19.9 %-46.8 %, respectively. Consequently, the bioconcentration factor increased by 3.4 %-30.9 % and the phytoextraction rate increased by 18.5 %-98.7 %. Treatment with NO3--N showed the most significant effect. In structural equation modeling, R. qingshengii inoculation showed greater path coefficients with soil pH and ABA and indoleacetic acid concentrations of B. juncea than N fertilization, indicating that R. qingshengii contributed more to HM extraction efficiency than N fertilizers. Additionally, differences in the extraction rates of Cd, Zn, and Pb from B. juncea were reduced following N fertilization. In summary, synergistic R. qingshengii inoculation and N fertilization have substantial potential to enhance phytoremediation efficiency. Combined application of R. qingshengii and NO3--N fertilizers is recommended.