[Characteristics of ammonium N and nitrate N accumulation in dryland soil in relation with wheat yield]

A total of eleven field trials in Yongshou, Shaanxi Province and seven in Luoyang, Henan Province were conducted with two treatments (without N as control and with 150 kg N x hm(-2) addition). The wheat biomass and seed yield were determined, and so were the nitrate and ammonium N concentrations in five layers (0-20, 20-40, 40-60, 60-80, 80-100 cm) at each trial site. Results showed that soil ammonium concentrations in the two provinces were very low while nitrate N concentrations were relatively high. Soil nitrate N occupied 91% of the total mineral N, and exhibited the same trend as the total mineral N in reflecting soil N supplying capacity. Without N application, the cumulative nitrate N amounts in the 0-40, 0-60, 0-80 and 0-100 cm layers in Yongshou were significantly correlated with wheat biomass and seed yield while no such correlation existed in Luoyang. With N addition, the relations of cumulative nitrate N of the different layers to wheat biomass and yield were greatly declined in Yongshou whereas those of Luoyang were changed to be negative. The yield increases were significantly correlated with the amount of nitrate N accumulated in the 0-80 and 0-100 cm layers of the two sites under N addition. Wheat mainly depended on nitrate N from the 0-20 cm layer at seedling stage, 0-40 cm at reviving, and 0-60 cm at elongation stages, and could utilize 0-100 cm nitrate N at maturity. After wheat harvest, the concentration of ammonium N was not significantly different from the initial value while that of nitrate N greatly decreased.