Effects of nitrogen forms on the growth and Cd concentration of pakchoi (Brassica chinensis L.) under Cd stress

Objectives Effects of common nitrogen (N) supply forms (ammonium, nitrate, ammonium/nitrate and urea) on the growth of Chinese cabbage pakchoi (Brassica chinensis L.) grown in the cadmium (Cd)-contaminated soil were studied to develop proper nitrogen management strategies for Cd-contaminated soils.Methods A pot experiment was conducted using soils from vegetable garden in Hangzhou. The tested soil was adjusted to four Cd contamination levels (0, 1, 3 and 5 mg/kg) with CdCl2 solution in advance. Four N supply formsammonium, nitrate, ammonium/nitrate (1:1) and urea were supplied for the pakchoi respectively, with the addition amount of N 400 mg/kg soil. After the harvest, the biomass, photosynthesis, Cd-induced oxidative stress and Cd concentration of the plants in different treatments were investigated.Results 1) Compared with the Cd 0, the fresh weights of the edible parts of pakchoi were reduced by 31%, 16%, 21% and 26% in Cd 1, by 58%, 28%, 35% and 39% in Cd 3, and by 83%, 38%, 52% and 69% in Cd 5 in the presence of ammonium, nitrate, ammonium/nitrate and urea, respectively. There were no significant differences in the tolerance index (TICd) between the nitrate and ammonium/nitrate treatments, and the tolerance indices of the nitrate and ammonium/nitrate treatments were higher than those of the ammonium and urea treatments. 2) Compared with Cd 0, the photosynthetic rates in the plants exposed to ammonium, nitrate, ammonium/nitrate, and urea were significantly decreased by 14%, 10%, 12% and 13% in Cd 1, by 33%, 22%, 25% and 40% in Cd 3, and by 53%, 42%, 41% and 56% in Cd 5; the levels of MDA and O2 production rates were increased by 11%, 4%, 9% and 11%, and 5%, 1%, 2% and 4% in Cd 1, respectively. The Cd induced-photosynthesis inhibition and oxidative stress were relatively lower in the pakchoi with nitrate supply. 3) The content of Cd in pakchoi were in the order of:nitrate ammonium/nitrate urea and ammonium treatments.Conclusions In the Cd-contaminated soils, nitrate N was more effective than ammoniam N in alleviating the Cd induced photosynthesis inhibition and oxidative stress and ultimately decreased the Cd contents in plants.