Nitrogen enhances cadmium phytoremediation in poplar via physiological, molecular, and rhizobacterial mechanisms

Nitrogen (N) plays a crucial role in enhancing plant growth and stress tolerance, but the physiological mechanisms and multi-omics evidence underlying its effect on cadmium (Cd) accumulation and detoxification in woody plants have not been fully understood. In this study, by integrating physiological, transcriptomic, metabolomic, and rhizobacterial analyses, the effects of NH4HCO3-based N fertiliser on 3-month-old poplars subjected to Cd stress were investigated to determine its potential for bioaccumulation and detoxification. Exogenous N significantly enhanced the Cd uptake efficiency and Cd content in whole plants by 93.79 and 160%, respectively, compared to the Cd-only group. N selectively recruited Bacillus, Fictibacillus, and Nitrospira, which are associated with a reduced soil pH, increased Cd bioavailability, and phytohormones (brassinolide and zeatin) biosynthesis, facilitating plant growth and Cd absorption. Concurrently, multi-omics analyses revealed the upregulation of genes involved in reduced glutathione (GSH) and phytohormones biosynthesis, antioxidant defence, and Cd transport and chelation (e.g., PyGCLC, PyGSS, PyDWF, PyIPT, PyAPX, PyCAT, PyNRAMP, PyMT, PyHIPP). Consistently, the accumulation of GSH, key amino acids (cysteine, glutamate, glutamine), phytohormones, flavonoid derivatives (eriodictyol, dihydrokaempferol, glyceollin II), and osmoprotectants (proline, soluble sugars) and the activities of antioxidant enzymes (catalase, superoxide dismutase, peroxidase, ascorbate peroxidase) were enhanced. Thus, Cd-induced reactive oxygen species and lipid peroxidation were reduced, and Cd accumulation and detoxification-related responses were enhanced. These findings suggest that N improves the phytoremediation efficiency of poplar by affecting the rhizosphere environment and Cd bioavailability and by modulating physiological and metabolic processes in plant cells.