Abstract: 【Objective】Cultivation media have important influence on the citrus absorbing different forms of nitrogen. Calcareous yellow soil is the main soil type in Karst area, southwest of China, and citrus are widely distributed in this region. Research on the citrus absorption and utilization of different forms of nitrogen will provide a reasonable scientific method of selecting nitrogen fertilizer for citrus in calcareous yellow soil in this region. 【Method】 A soil culture experiment was conducted to determine the total leaf area, plant height, base diameter, fresh weight, dry biomass, root/cap ratio, nitrogen absorption and utilization efficiency of young Newhall navel orange with different N treatments: single NO₃^--N, single NH₄⁺-N and different proportions mixture of both forms, which were grafted on Poncirus trifoliata and cultivated in the calcareous yellow soil. The conventional method of depletion was carried out to study the kinetics parameters of NO₃^--N and NH₄⁺-N absorption in both spring and summer.【Result】Results indicated that plant growth was obviously inhibited, and the total leaf area, height, base diameter, fresh weight and dry weight biomass, root-shoot ratio, nitrogen absorption amount and nitrogen use efficiency were brought down when using NO₃^--N, NH₄⁺-N and CO(NH₂)₂ alone. All these indexes were the lowest when applying CO(NH₂)₂ alone. Under mixed supply of sodium nitrate and NH₄⁺-N, the plant growth, nitrogen absorption amount and nitrogen use efficiency were better than using them alone. When the proportion of sodium nitrate﹕NH4⁺-N reached 75﹕25, growth indexes, nitrogen absorption amount and nitrogen use efficiency were the highest, the total leaf area was 0.44 m², height was 73.95 cm, base diameter was 1.36 cm, fresh weight was 232.95 g/plant, dry weight biomass was 130.27 g/plant, the ratio of fresh weight and root-shoot was 1.02 and the ratio of dry weight and root-shoot was 1.06, nitrogen absorption amount and nitrogen use efficiency were 3.80 g/plant and 0.0292 g·mg^-1, respectively. Under mixed supply of sodium nitrate and NH₄⁺-N, the plant growth, nitrogen absorption amount and nitrogen use efficiency were decreased when the ratio of sodium nitrate increased, the ammonia poisoning phenomena happened intheroot system of navel orange young trees when using NH₄⁺-N or CO(NH₂)₂ alone. No matter in spring or summer, there was no obvious difference in maximum absorption rate (I_max) between fertilizing NO₃^--N alone and mixed different ratios of NO₃^--N and NH₄⁺-N, however, the absorption of NO₃^- was more stable than the NH₄⁺. The I_max of NO₃^- in spring was smaller than in summer, but the affinity of root to NO₃^- was stronger than in summer, and the K_m of root to NO₃^- was no significant difference in summer. When the ratio of NO₃-N and NH₄⁺-N was up to 50%-75%, the affinity of root to NO₃^- could be enhanced in spring, and the flow rate of NO₃^- (α) could also be increased in both spring and summer. In spring, the maximum absorption rate and the flow rate of root to NH₄⁺ were both increased with the increasing ratio of NH₄⁺-N, but the affinity of the root to NH₄⁺ decreased. In summer, the maximum absorption rate, affinity and the flow rate of root to NH₄⁺ were all reduced with the raising of the ratio of NH₄⁺-N, and the lowest values of them were observed when using NH₄⁺-N alone. Mixed fertilizer of NO₃^--N and NH₄⁺-N, the affinity of root to NO₃^- was stronger than to NH₄⁺, as well as the α of NO₃^- was faster than NH₄⁺ in both spring and summer. 【Conclusion】The root system of young navel orange trees grafted on Poncirus trifoliata prefered absorption NO₃^- in calcareous yellow soil. The proportion of sodium nitrate and NH₄⁺-N application reached 75﹕25 was able to promote the plant growth and nitrogen absorption and increase the use efficiency for navel orange.

Key words: nitrogen fertilizer, calcareous yellow soil; navel orange grafted on Poncirus trifoliata, absorptive amount, use efficiency, maximum absorption rate, ion affinity