Effects of different nitrogen rates on open-field vegetable growth and nitrogen utilization in the north China plain

Advanced nitrogen (N) advisory systems require target values of N supply for the crop and the results of soil testing for inorganic nitrogen (N-min) before cultivation and estimates of N release from mineralization. Two field experiments with different N supply levels were conducted in the Beijing region to determine the target values of N supply and N mineralization rates for optimization of N fertilization of Chinese cabbage (Brassica campestris L. ssp. Pekinensis) and carrot (Daucus carota L.). Crop yield, N uptake, and soil inorganic N were investigated during the crop growth periods. Marketable yields of the two crops increased significantly with N application rate, and were described as linear-plateau functions (Chinese cabbage: Y = 0.16N + 80.5, when 0 < N < 319; Y = 112.7, when N greater than or equal to 319; and carrot: Y = 0.12 N + 50.5, when 0 < N less than or equal to 247; when Y = 67.1, N > 247; where: Y was marketable yield, t FW ha(-1); N was N supply level, the sum of soil inorganic N at preplanting and applied fertilizer N, kg N ha(-1)). Similar trends were also observed in crop N uptake. The agronomically effective N supply levels for Chinese cabbage and carrot growth in Beijing region were 349 and 277 kg N ha(-1), respectively, for target marketable yields of 120 t FW ha(-1) for Chinese cabbage and 65 t FW ha(-1) for carrot considering extra 30 kg N ha(-1) of fertilizer to avoid the risk of crop yield reduction. During the crop growth period net N mineralization in the root zone was 38 kg N ha(-1) for Chinese cabbage and 46 kg N ha(-1) for carrot. Weekly N mineralization rate was about 3.2 kg N ha(-1). Nitrogen loss coefficient, defined as the ratio of average weekly apparent N loss to N supply level, ranged from -0.011 to 0.053 for the different N supply levels in the two experiments. The N net mineralization rates determined, N loss rates and target values were found to be useful for prediction of optimum N supply based on N balance calculations for sustainable vegetable production.