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 20 days after N fertilizer application.