Nitrogen is a chemical element, an inert gas without color or odor, discovered by French chemist Lavoisier in the second half of the 18th century, and is the main component of atmospheric air (78.08%). The name means “non-living,” as it does not support combustion or respiration. However, further research has shown the enormous role of nitrogen in plant life and the entire organic world.
Nitrogen is part of:
- proteins, peptides, and amino acids, which are an integral part of the protoplasm and nucleus of plant cells;
- nucleic acids (DNA and RNA), carriers of hereditary properties of living organisms and involved in metabolism;
- chlorophyll molecules;
- most vitamins.
Nitrogen nutrition of plants
All enzymes are protein substances, so when plants are insufficiently supplied with nitrogen, enzyme synthesis slows down, which leads to disturbances in the processes of biosynthesis, metabolism, eventually reducing yields.
Regulating nitrogen nutrition of plants, you can influence the yield of crops, taking into account other factors of life. Maximum yield is achieved when plants are sufficiently provided with all the conditions of their growth. Academician D.N. Pryanishnikov wrote that the whole history of farming in Western Europe shows that the main condition determining the average yield height at different epochs was the degree of provision of agricultural plants with nitrogen.
Optimal nitrogen nutrition promotes the synthesis of protein substances, plants form powerful stems and leaves with intense green coloration. Powerful assimilation apparatus allows plants to accumulate more photosynthesis products, increasing yield and, as a rule, its quality.
One-sided excessive nitrogen feeding, especially in the second half of the growing season, leads to delayed maturation of plants; a large vegetative mass is formed, but the yield of reproductive organs does not have time to form.
Lack of nitrogen leads to a strong slowdown in plant growth. First of all, it affects the development of vegetative mass: leaves become small, light green, turn yellow earlier, stems are thin, weakly branching. The formation of reproductive organs decreases, and the yield decreases sharply. Nitrogen starvation in cereal crops leads to weakening of tillering, the number of grains in an ear decreases, and grain protein content decreases.
Nitrogen content in plants
In terms of chemical composition, nitrogen in plants accounts for 0.5-5.0% of the air-dry weight, the main amount is in the seeds. Protein content clearly correlates with the amount of nitrogen in plants. Nitrogen content is lower in vegetative organs: in legume straw 1.0-1.4%, in cereal straw 0.45-0.65%. Even less nitrogen is accumulated in root and tuber crops and vegetable crops: potatoes (tubers) 0.32%, sugar beets (roots) 0.24%, cabbage 0.33% raw matter.
Nitrogen content in plants depends on age, soil and climatic conditions, nutritional regime, in particular the provision with nutrients.
Table. Content of protein and nitrogen in the seeds of various crops, % Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
The nitrogen content in young vegetative organs is higher. As they age, the nitrogenous substances migrate to the emerging leaves and shoots.
Table. Nitrogen content in the vegetative mass of cereal crops by phases of development, % on air-dry substanceYagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry/ Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
Input and transformation of nitrogen into protein substances
The rate of accumulation of organic matter by plants outstrips the intake of nitrogen and other nutrients. There is a “growth dilution” of the content of nutrients. At ripening, there is a pronounced movement of nitrogen to reproductive organs, where it is accumulated in the form of spare proteins.
Nitrogen enters plants mainly in nitrate and ammonium forms, but some soluble organic compounds, such as urea, amino acids, and asparagine, are also able to assimilate.
Of the nitrogen compounds from the soil to plants, only ammonia is directly used for amino acid synthesis. Nitrates and nitrites are included in amino acid synthesis only after reduction in plant tissues.
The reduction of nitrates to ammonia begins already in the roots with the help of flavin metalloenzymes:
In excess, some of the nitrates enter the leaves unchanged, where they are reduced according to the same scheme.
The formation of amino acids (amination) occurs as a result of the interaction of ammonia with keto acids: pyruvic acid, oxalic acid, ketoglutaric acid, etc., formed in the process of carbohydrate oxidation. Amination is regulated by enzymes. Thus, when pyruvic acid interacts with ammonia, alanine is formed:
CH3—CO—COOH + NH3 + 2H → CH3—CHNH2—COOH + H2O
Similarly, the interaction of ammonia with oxalic acetic acid leads to the formation of asparagic acid (СООН-СН2-СНNН2-СООH), with ketoglutaric acid – glutamic acid (СООН-СН2-СН2-СНNН2-СООH).
Amino acids contain nitrogen in the form of an amino group (-NH2). The processes of amino acid formation occur in the roots and in the above-ground parts of plants.
Experiments using labeled atoms show that just a few minutes after feeding plants with ammonia fertilizers, in tissues amino acids synthesized from the ammonia added to the dressing can be detected. The first amino acid formed is alanine, followed by asparagine and glutamic acids.
Nitrate nitrogen can accumulate in plants in large quantities without harming them. Ammonia in free form in tissues is contained in insignificant amounts. Its accumulation, especially with a lack of carbohydrates, leads to ammonia poisoning of plants.
However, plants have the ability to bind excess free ammonia: its part interacts with synthesized asparagine and glutamine amino acids, forming the corresponding amides – asparagine and glutamine:
СООН-СН2-СНNН2-СООH + NH3 → CONH2-CH2-CHNH2-COOH + H2O
СООН-СН2-СН2-СНNН2-СООH + NH3 → CONH2-CH2-CH2-CHNH2-COOH + H2O
The formation of asparagine and glutamine allows plants to protect themselves from ammonia poisoning and create a reserve of ammonia; in addition, amides participate in protein synthesis.
In 1937, biochemists A.E. Braunstein and M.G. Kritzman discovered the reamination reaction, which consists in the transfer of an amino group from an amino acid to a keto acid to form other amino and keto acids. The reaction is catalyzed by the enzymes transaminases or aminoferases.
For example, the addition of an amine group from glutamic acid to pyruvic acid (СН3-СO-СООH) leads to the formation of alanine (CH3-CHNH2-COOH) and ketoglutaric acid (COOH-CO-CH2-CH2-COOH):
СН3-СO-СООH + COOH-CH2-CH2-CHNH2-COOH → COOH-CO-CH2-CH2-COOH + CH3-CHNH2-COOH
A significant number of amino acids are synthesized through overamination. In plants, glutamic and asparagic acids are the most easily overamined.
Amino acids are the components of polypeptides and proteins. 20 amino acids, asparagine and glutamine participate in the construction of protein molecules in different ratios and spatial orientations, which leads to a huge variety of proteins. More than 90 amino acids are currently known, about 70 of them are present in plants in a free form and are not included in proteins.
Plants synthesize amino acids that cannot be formed in humans and higher animals, but are essential for their lives. These include lysine, histidine, phenylalanine, tryptophan, valine, leucine, isoleucine, threonine, and methionine.
Non-protein organic nitrogen in plants accounts for 20-26% of the total amount. Under unfavorable conditions, such as potassium deficiency or insufficient light, the amount of non-protein nitrogenous compounds increases.
In plant tissues, proteins are in dynamic equilibrium with non-protein nitrogenous compounds. Simultaneously with the synthesis of proteins and amino acids, there is a process of their decomposition: the detachment of the amino group from the amino acid to form keto acids and ammonia. This process is called deamination. The keto acids released are used by plants to synthesize carbohydrates, fats, and other substances; ammonia reenters the amination reaction of other keto acids to form new amino acids, with an excess of asparagine and glutamine.
Thus, the entire cycle of nitrogenous compound transformations in plants begins (amination) and ends (deamination) with ammonia.
“Ammonia is the alpha and omega in the metabolism of nitrogenous substances in plants.”
A large number of protein compounds are synthesized during the entire growing season of a plant, and the exchange of nitrogenous substances occurs differently during different periods of growth.
During germination of seeds, tubers, and bulbs, there is a breakdown of stored proteins. Decomposition products are used to synthesize amino acids, amides, and proteins in the tissues of seedlings before they reach the soil surface. Then, as the root system and leaf apparatus form, protein synthesis proceeds at the expense of mineral nitrogen absorbed from the soil.
In young plants, protein synthesis predominates. As plants age, protein breakdown begins to predominate. Decomposition products from aging organs migrate to young, intensively growing organs, where they are used to synthesize new proteins at growth points. As plants mature and reproductive organs form, proteins decompose in vegetative parts, and the decomposition products migrate to reproductive organs, where they are used to form spare proteins. By this point, the nitrogen supply to plants from the soil slows down significantly or stops completely.
Peculiarities of ammonium and nitrate nutrition of plants
At the end of the 19th century, the leading role in agronomic science was played by the theory of nitrate nutrition of plants, and the role of ammonia as a source of mineral nutrition was denied.
The role of ammonia as a source of mineral nutrition was denied:
- experiments in water crops: good plant development was observed against the background of nitrate salts, against the background of ammonium salts the development was poor;
- discovery of nitrification in the soil; this was the basis for believing that when ammonium fertilizers are put into the soil they are converted into the nitrate form that is assimilated by plants;
- application of Chilean nitrate (NaNO3) significantly increased crop yields.
However, at the end of the century, P.S. Kossovich showed in experiments with sterile crops that plants could also assimilate ammonia nitrogen without oxidation into the nitrate form. The same conclusion was reached by French researcher Mazet in 1900. After that, the conditions and peculiarities of feeding on ammonium and nitrate forms of nitrogen were studied. Fundamental research on this issue was carried out by D.N. Pryanishnikov. He showed that the efficiency of using various forms of nitrogen depends on the reaction of the environment: in neutral reaction ammonium nitrogen is better absorbed, in acid reaction – nitrate nitrogen.
During the initial phases of growth, biological features are of significant importance. When germinating seeds with little carbohydrate reserves, such as in sugar beets, and thus organic keto acids, the excessive supply of ammonium to plants has a negative effect. Ammonium nitrogen does not have time to be used for amino acid synthesis, it accumulates in plant tissues and causes plant poisoning. In this case, nitrate forms of nitrogen fertilizers are used, as they also accumulate in plant tissues, but do no harm. Seeds and crops with large carbohydrate reserves, such as potatoes, use ammonium nitrogen for amino acid synthesis without restriction. Therefore, for these crops, the ammonium and nitrate forms are of equal value in the initial stages of growth.
The absorption of nitrate and ammonium nitrogen is influenced by the supply of other nutritional elements. The increased content of potassium, calcium and magnesium in the soil promotes the absorption of ammonium. With nitrate nutrition, the provision of plants with phosphorus and molybdenum is important. Molybdenum deficiency leads to a delay in the reduction of nitrates to ammonia and contributes to the accumulation of nitrates in plant tissues.
Considering that the ammonium form of nitrogen when entering plants can be used immediately for the synthesis of amino acids, while the nitrate form is used only after reduction to ammonia, ammonium is more energy-saving form.
Nitrogen cycling and balance in agriculture
The mineral nitrogen compounds in the soil are mobile. Their stock and transformation are the result of numerous, often interdependent physical, physical-chemical and biological processes of nitrogen cycle.
In virgin soils of natural biocenoses there is a closed cycle of nitrogen and other biogenic elements cycle. It includes incoming items:
- nitrogen inputs with plant fallout and root residues;
- excrement and animal remains;
- biological fixation of atmospheric nitrogen by microorganisms;
inputs with atmospheric precipitation (formation of NO3– and NH4+ by lightning discharges and industrial emissions).
- nitrogen intake by plants;
- losses from water and wind erosion;
- nitrogen immobilization – conversion of mineral nitrogen into organic nitrogen (in this case nitrogen is not lost, but passes into a form inaccessible to plants).
The ratio of inputs and outputs in the nitrogen cycle constitutes the balance of this element. If consumption exceeds accumulation, it is negative; if intake exceeds expenditure, it is positive; if balance parts are equal, it is balanced, or zero.
Natural biocenoses are usually characterized by balanced balance of nitrogen and other biogenic elements. Its losses from leaching and denitrification are compensated by its inflow with precipitation and biological nitrogen fixation.
As a result of plowing soils, the nitrogen regime undergoes changes: the expenditure parts of the balance increase sharply. A large amount of nitrogen is alienated with agricultural products. Intensive tillage leads to increased mineralization of organic matter, increased losses due to infiltration, denitrification and erosion.
Under conditions of sufficient and excessive moisture, irrigation, especially on light soils, nitrate consumption increases for leaching together from the root-containing layer into drainage water. Loamy and clayey soils rich in humus retain water better, so nitrate leaching is insignificant – up to 3-5 kg/ha of nitrate nitrogen per year. On light soils of granulometric composition, especially in the fallow field, nitrate nitrogen losses from leaching reach 20-30 kg/ha per year.
Losses of gaseous forms of nitrogen from denitrification are also significant. The process of denitrification takes place under anaerobic conditions. It is logical to assume that plowing eliminates these conditions, however, loosened soil consists of separate structural aggregates (clumps) of different sizes. Anaerobic conditions are created within each soil aggregate, and the more intense the more nitrates not absorbed by plants are accumulated.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.