Why does over-applying nitrogen fertilizer reduce crop yields, and how much nitrogen is actually needed?

Nitrogen is one of the essential nutrient elements for crops, and supplying nitrogen fertilizer in appropriate amounts can increase crop yields and improve quality. But if nitrogen fertilizer is over-applied, crops will instead reduce yields. What is the reason for this? How much nitrogen do crops actually need?

Over-applying nitrogen fertilizer causes the carbohydrates produced by crop photosynthesis to be used in large amounts for synthesizing proteins, chlorophyll and other nitrogen-containing compounds, with only a small amount of carbohydrates used as raw material for building cell walls——cellulose, hemicellulose, pectin and the like. This results in large cells with thin walls, making the tissues soft, the leaves drooping and shading one another, and poor ventilation and light penetration within the leaf canopy, causing slender basal internodes in rice, excessive vegetative growth in cotton plants, delayed maturity with prolonged greenness, reduced drought resistance, cold resistance and disease resistance, and even lodging and yield reduction. Therefore, the amount of nitrogen fertilizer applied must be appropriate. 

So how much nitrogen do crops actually need? According to analysis and statistics from a large amount of data, for example, to harvest 100 kg of rice grain (including the corresponding nutrients in stems and leaves, the same below), about 1.6-3.2 kg of nitrogen must be absorbed; to produce 100 kg of ginned cotton, 12-15 kg of nitrogen must be absorbed; to produce 100 kg of rapeseed (Brassica napus), about 6.8-7.8 kg of nitrogen must be absorbed; to harvest 100 kg of corn grain, about 2.5-4.0 kg of nitrogen must be absorbed; to harvest 100 kg of fresh tea leaves, 1.2-1.4 kg of nitrogen must be absorbed; to produce 100 kg of apples and citrus, 0.3 kg and 0.6 kg of nitrogen are absorbed respectively; to produce 100 kg of sweet potato, 0.35-0.42 kg of nitrogen must be absorbed. It can be seen that the nitrogen required varies greatly among different crops, and when applying nitrogen fertilizer, one cannot apply the same formula across the board.