Very good question. Earlier also ,we debated this issue at length. Usually nitrates must be the preferred choice over ammonium . However, some prefer to use ammonium salts with biuret ..
Both high nitrogen and cytokinin contents are associated with plant juveniiity. When high cytokinin and nitrogen combine it will result in delayed senescence.
Nitrogen leads to senescence in the leaves. Because of its effect in increasing the soluble protein in the vector vessels in the vegetative total, leading to delayed degradation of Chloroplasts protein. Plant growth depends on the absence of nitrogen in the plant.