Microstructural and physiological responses to cadmium stress under different nitrogen forms in two contrasting Populus clones

•Nitrogen forms differently affected Cd response in Populus clones with different sensitivity to nitrogen deficiency.•Nanlin 1388 had stronger Cd tolerance than Nanlin 895 under the different nitrogen forms and status.•NH4+ supply promoted Cd accumulation and toxicity relative to NO3− supply, especially in Nanlin 895.•NO3− supply enhanced Cd tolerance by improving ion homeostasis, detoxification and defense preparedness in Nanlin 1388 relative to Nanlin 895.