Abstract: We investigated the effects of dicyandiamide (DCD) on the growth and Cd concentrations in pakchoi cultivated under different instant soluble N fertilizers [ammonium sulfate, ammonium sulfate and sodium nitrate (1:1, ammonium/nitrate), and urea] in Cd-contaminated soils. The results showed that the fresh weight of the edible parts of Cd-stressed pakchoi were increased by 583.3%, 41.5%, and 206.8% under ammonium, ammonium/nitrate, and urea treatments in the presence of DCD, respectively compared with control, and the tolerance index and photosynthetic rate significantly increased, whereas no significant changes were observed under nitrate supply. Under all N treatments with DCD, the MDA and H₂O₂ contents and the superoxide radical production rate in the leaves of pakchoi were decreased, with the highest reduction occurred in ammonium and urea treatments. Cd concentrations in the leaves of pakchoi fertilized with ammonium, ammonium/nitrate, and urea were lowered by 58.3%, 34.0%, and 44.5% and those in the petioles were lowered by 61.8%, 29.4%, and 55.6%, respectively. Cd concentration in the leaves and petioles of pakchoi in the nitrate treatment did not differ significantly from control. These changes could be attributable to the reduction in the acidification of rhizosphere soil in response to the combined application of N fertilizer and DCD. Accordingly, in Cd-contaminated soils with a low buffering capacity, the application of DCD combined with ammonium, ammonium/nitrate, or urea N fertili-zers could alleviate Cd-induced growth stress and inhibit photosynthesis in pakchoi plants and effectively minimize the Cd accumulation.

Key words: cadmium, soil pH, tolerance index, nitrification inhibitor, oxidative stress, nitrogen fertilizer