Effects of combined application of different nitrogen sources and magnesium fertilizers on cabbage yield, quality and nutrient uptake

Vegetable crops have preferences for nitrate-nitrogen (N). Nitrate accumulation in vegetable crops is closely related with the form of N and application rate of N fertilizer, especially under over fertilization of N. The purpose of this research was to investigate the effects of combined application of different N sources and magnesium (Mg) fertilizers on yield, quality and nutrient uptake of cabbage (Brassica oleracea L.) under field conditions. The experiment was conducted in a randomized complete block design (RCBD) with three replicates of each treatment. Four N sources (no N, sole nitrate-N, sole ammonium-N, and mixed nitrate/ammonium-N) and four rates of magnesium sulfate fertilizer (0, 75 kg·hm^-2, 150 kg·hm^-2 and 300 kg·hm^-2) were used in the study. Results showed that under the same N conditions, combined application of different N sources and Mg fertilizers had significant effects on yield, quality and nutrient uptake of cabbage. The highest cabbage yield was obtained under combined application of sole nitrate-N and medium rate (150 kg·hm^-2) of Mg fertilizer. Cabbage yield under treatment of sole nitrate-N and medium rate Mg fertilizer increased by 56.9%, 14.7% and 5.2%, respectively, compared with those under no fertilizer, sole ammonium-N, and mixed nitrate/ammonium-N applications with medium rate of Mg fertilizer. N resources had significant influence on cabbage yield. Sole nitrate-N treatment increased cabbage yield by 13.0% over sole ammonium-N treatment. No significant yield difference was noted between sole nitrate-N and mixed nitrate/ammonium-N treatments. Compared with treatments without Mg fertilizer, 75 kg·hm^-2 Mg fertilizer increased cabbage yield by 9.3%. Nevertheless, no significant increase was noted with further increase in Mg fertilizer rate. The highest cabbage nitrate content was about 2 759.0 mg·kg^-1, obtained under combined application of sole nitrate-N with high rate (300 kg·hm^-2) of Mg fertilizer. Compared with no fertilizer treatment, cabbage nitrate content under sole nitrate-N with high Mg fertilizer rate increased by 101.4%, 82.3% and 14.1%, respectively, over those under no fertilizer, sole ammonium-N and mixed nitrate/ammonium-N treatments with high Mg fertilizer rate. While cabbage nitrate content was not influenced by low and medium Mg fertilizer rates, significant increase (11.2%) was noted in nitrate content at high Mg fertilizer rate compared with no fertilizer. Cabbage contents of vitamin C, reducing sugar, total amino acids significantly increased with increasing proportion of nitrate-N. Uptakes of P, K and Ca significantly increased with increasing proportion of nitrate-N. Moreover, uptakes of P, K and Mg also increased with increasing Mg fertilizer rate. Analysis revealed that interactions among different N sources and Mg had significant effects on cabbage vitamin C content and cabbage N, P, K, Ca and Mg uptake. This research indicated that mixed nitrate/ammonium-N application with proper Mg application rates not only enhanced cabbage yield and vitamin C, reducing sugar and total amino acids contents, but also reduced nitrate content and improved nutritional quality of cabbage.