The response process of citrus to NH₄⁺-N was elaborated by observing the impact of NH₄⁺-N level on different pH soil solutions, combined with a study on the impact of orange rootstock seedling growth and physiological indicators, which provided theoretical support for the best citrus nitrogen management in acid soil.

The experiment was designed as a two-factor experiment, with 2 soils as the main treatment and 5 N levels as the secondary treatment. Using acidic yellow soil and alkaline purple soil as the test soil, the seedlings of Xiangcheng (junos Sieb. ex Tanaka) were selected as the test material, and five NH₄⁺-N levels of 0 (A0), 50 (A50), 100 (A100), 200 (A200), and 400 mg·kg^-1 (A400) were set. The effects of ammonium application level on the concentration of ammonium nitrogen and nitrate nitrogen in soil solution and on the biomass, root morphology, nitrogen absorption, antioxidant system, nitrogen metabolism, MDA content and root activity of citrus were studied.

Compared with calcareous soil, the nitrification process in acid soil was slowed down. The NH₄⁺-N concentration and NH₄⁺/ NO₃^- ratio in the soil solution of acid soil remained at a higher level at 30 days of the test. The root length of the citrus under A400 treatment was reduced by 13% when compared with the A0 treatment, and the root vitality was significantly negatively correlated with the ammonium application level. The MDA content of leaves and roots was positively correlated with the ammonium level, which stimulated the oxidative stress response of the roots, especially increased the POD enzyme activity of the leaves. Compared with calcareous soil, the total nitrogen accumulation of citrus on acid soil decreased by 17.6%, while the ratio of ammonium to nitrate in leaves and roots increased by 27.2% and 61.1%, respectively. Cluster analysis showed that citrus in acidic soil was toxic when N application rate exceeded 100 mg kg^-1, while citrus in alkaline soil was not significantly stressed.

In acidic soil, the excessive application of ammonium nitrogen caused the accumulation of ammonium nitrogen in soil solution for a long time, resulting in the increase of MDA content, cell membrane damage and nitrogen metabolism disorders as well as other ammonium toxic phenomena, indicating that ammonium toxicity in citrus was closely related to soil nitrification.