Effects of form, rate and time of N fertilizer application on yield and nitrate content of greengrocery

The effects of different types and application methods of N fertilizer on greengrocery (Brassica campestris L. ssp. Chinnesis) yield and nitrate content were investigated in a field experiment. Results showed that: ① At the same N fertilizer application rate, yield was highest with the Calcium nitrate and urea + micronutrient treatments. The yield difference among urea, organic-inorganic fertilizer mixture and ammonium bicarbonate treatments was not significant, and the DMPP （3，4-dimethylpyrazole phosphate）treatment had the lowest yield among the six treatments. Top-dressing of micronutrients could enhance yield. The nitrate content declined with harvesting time, except the organic-inorganic fertilizer mixture treatment. At harvesting, underground-plant samples from the six treatments showed the following order of nitrate accumulation ability: calcium nitrate DMPP urea organic-inorganic fertilizer mixture urea + micronutrients ammonium bicarbonate. The synergistic absorption of N with P and K could reduce nitrate content in plants. Considering the yield and quality together, the organic-inorganic fertilizer mixture treatment would be the best; ② Greengrocery yield and Vc content increased with N fertilizer rate, so was nitrate content; ③ Greengrocery yield and Vc content declined as the fertilization time was delayed. The accumulation peak of nitrate content in plants is 20days after N fertilizer application.