First, the main source of nitrogen in plants
The nitrogen forms that plants can use are mainly ammonium nitrogen and nitrate nitrogen, and they can also absorb some simple organic nitrogen-containing compounds such as amino acids and amides (such as urea). Air contains nearly 79% nitrogen, which can only be used by some microorganisms (including nitrogen-fixing microorganisms born in higher plants), and most plants do not have this ability. The nitrogen absorbed by plants mainly comes from the medium in which they live-soil. There is not much nitrogen in the soil itself, and the nitrogen in the soil can not be fully utilized by plants. Plants can only use a small part of them, that is, ammonium nitrogen and nitrate nitrogen existing in the soil, while some organic nitrogen, such as simple amino acids and amides, can also be absorbed and utilized by crops, but its amount is small, and it will be transformed into other forms by microorganisms, so it is difficult to remain in the soil for a long time. It is not as easy for plants to absorb it as inorganic nitrogen, which can only make plants survive, but not make them rich.
Second, different forms will have different effects.
Plants absorb and metabolize two forms of nitrogen differently. First of all, ammonium nitrogen must combine with organic acids as soon as possible after entering plant cells to form amino acids or amides. The accumulation of ammonium in plants is very toxic to plants. After nitrate nitrogen enters plants, part of it is reduced to ammonium nitrogen, which is metabolized in cytoplasm, and the rest can be "stored" in vacuoles of cells, sometimes reaching a higher concentration will not have adverse effects on plants. Therefore, the simple application of nitrate nitrogen fertilizer will generally not have adverse effects, while the simple application of ammonium nitrogen will cause ammonium salt poisoning, which is more likely to occur under hydroponic conditions.
Why don't plants absorb nitrate in a planned way according to their needs, but they should luxuriously absorb nitrate and "store" it in vacuoles? ! Studies have shown that the accumulation of nitrate nitrogen during the growth of vegetative organs is the nature of all plants. With the continuous growth of plants, the content of nitrate nitrogen in the body is less and less. It is understood that plants absorb a lot of nutrients in the vegetative growth stage, on the one hand, to meet the needs of current growth, on the other hand, to supply the needs of later growth. The accumulation of nitrate nitrogen in plants is a "storage" measure of plants and a manifestation of adaptation to adversity. During the vegetative growth period, a large amount of nitrate nitrogen is accumulated, and plants can still grow and develop well even if the supply of soil nutrients is insufficient in the later period; The more nitrate nitrogen accumulated, the better the later growth and development. In addition, NO3- is an important osmotic adjustment substance in vacuoles. When carbohydrate synthesis in plants decreases and the content of organic matter in vacuoles decreases, NO3- can replace them for osmotic adjustment, and the required energy is low.
Although ammonium nitrogen and nitrate nitrogen are the main inorganic nitrogen absorbed by plant roots, they have different effects on plants due to different forms.
Nitrate nitrogen promotes the absorption of cations and the synthesis of organic anions in plants. Ammonium nitrogen promotes the absorption of anions and consumes organic acids.