Nitrogen cycle in nature
Nitrogen cycle includes many transformations, including nitrogen in the air is fixed into ammonia nitrogen by microorganisms and microorganisms and plants and converted into organic nitrogen; Nitrides existing in plants and microorganisms are eaten by animals and converted into animal proteins in animals; Organic nitrides, such as the carcasses and excrements of animals, plants and microorganisms, are released in the form of ammonia when they are decomposed by various microorganisms; Ammonia is nitrated and oxidized into nitric acid under aerobic conditions, and the ammonium salt and nitrate produced can be absorbed and utilized by plants and microorganisms; Under anaerobic conditions, nitrate can be reduced to molecular nitrogen and returned to the atmosphere, thus completing the nitrogen cycle. Nitrogen cycle includes nitrogen fixation, ammonification, nitrification, denitrification and assimilation of plants and microorganisms.
Biological nitrogen
The role of microorganisms in nitrogen cycle
1. Nitrogen fixation
The process in which molecular nitrogen is reduced to ammonia or other nitrides is called nitrogen fixation. There are two ways to fix nitrogen in nature. One is abiotic nitrogen fixation, that is, nitrogen produced by lightning, volcanic eruption and ionizing radiation. In addition, it also includes chemical nitrogen fixation with iron as catalyst at high temperature (500℃) and high pressure (30.3975MPa) invented by human beings, and there are few nitrides formed by abiotic nitrogen fixation. The second is biological nitrogen fixation, that is, nitrogen fixation by microorganisms, 90% of which is in the atmosphere.
Molecular nitrogen in the world can only be fixed into nitride by microorganisms. Microorganisms that can fix nitrogen are prokaryotes, mainly including bacteria, actinomycetes and cyanobacteria. Among the nitrogen-fixing organisms, the most important contribution is the genus leptospira which infects leguminous plants, followed by the actinomycetes Frankia which coexists with non-leguminous plants, followed by various cyanobacteria, and finally some autotrophic nitrogen-fixing bacteria. Chemical nitrogen fixation has made great contributions to agricultural production, but its production needs high temperature conditions and high pressure equipment, and the material and energy consumption are too high, and the product price is high and rising. Biological nitrogen fixation is of decisive significance to the role of nitrogen in natural nitrogen cycle.
2. Add ammonia (function)
The process of decomposing nitrogen-containing organic matter by microorganisms to produce ammonia is called ammoniation. There are many kinds of nitrogenous organic compounds, mainly urea, uric acid and chitin in protein.
Ammonification is very important in agricultural production. All kinds of animal and plant residues and organic fertilizers applied to the soil, including green manure, compost and manure, are rich in nitrogen-containing organic matter, which can only be absorbed and utilized by plants through the action of various microorganisms, especially through ammoniation.
Biological nitrogen 3. Nitrification The process by which microorganisms oxidize ammonia into nitrate is called nitrification. Nitrification is divided into two stages. In the first stage, ammonia is oxidized into nitrite, which is completed by nitrifying bacteria, mainly including nitrosomonas and nitrosomonas. The second stage is that nitrite is oxidized into nitrate, which is completed by nitrifying bacteria, mainly including some species of nitrifying bacteria, nitrifying cellulose bacteria and nitrifying cocci. Nitrification is an indispensable part of nitrogen cycle in nature, but it is of little benefit to agricultural production.
4. Assimilation
Ammonium salts and nitrates are good inorganic nitrogen nutrients for plants and microorganisms, which can be absorbed and utilized by plants and microorganisms to synthesize nitrogen-containing organic substances such as amino acids, protein and nucleic acids.
Biological nitrogen 5. Denitrification The process in which microorganisms reduce nitrate and release molecular nitrogen and nitrous oxide is called denitrification. Denitrification is generally only carried out under anaerobic conditions.
Denitrification is one of the important reasons for soil nitrogen loss. In agriculture, intertillage and scarification are often used to inhibit denitrification. But from the whole nitrogen cycle, denitrification is still beneficial, otherwise the natural nitrogen cycle will be interrupted and nitrate will accumulate in water, which will pose a great threat to human health and the survival of aquatic organisms.
Mineral nutrition and nitrogen nutrition of plants
Plants not only absorb water from soil, but also absorb various mineral elements and nitrogen elements to maintain normal life activities. Some of these elements absorbed by plants are plant components, some participate in regulating life activities, and some have both functions. Usually, the absorption, transportation and assimilation of minerals and nitrogen by plants and the role of minerals and oxygen in life activities are called mineral and nitrogen nutrition of plants. People's understanding of plant minerals and nitrogen nutrition was basically determined in the middle of19th century after long-term practice and exploration. The first person who explored the source of plant nutrition by experimental method was Dutch Van Helmont (see introduction). Later, Glauber (1650) found that adding nitrate to soil can increase plant yield, so he thought that water and nitrate were the basis of plant growth. 1699, Woodward of England cultivated mint with water extracts from rain, river, mountain spring, tap water and garden soil. It was found that plants grew better in river than in rain, but they grew best in soil extracts. Based on this, he came to the conclusion that plants are not only composed of water, but also some special substances in the soil. Saussure of Switzerland (1804) reported that if the seeds are planted in distilled water, the growing plants will soon die and their ash content will not increase. If the ash and nitrate of plants are added to distilled water, plants can grow normally. This proves the necessity of ash element for plant growth. 1840, Justus von J.Liebig established the theory of mineral nutrition, and established the viewpoint that soil provides inorganic nutrition for plants. J Boussingault further cultivated plants by adding inorganic chemicals into quartz sand and charcoal, and quantitatively analyzed the gas around plants, which proved that carbon, hydrogen and oxygen came from air and water, while mineral elements came from soil. 1860, Knop and Sachs successfully cultivated plants with inorganic salt solutions with known components, and from then on, they discovered the fundamental nature of plant nutrition, that is, autotrophic (inorganic nutrition type).
Minerals and nitrogen nutrition are very important for plant growth and development. Understanding the physiological functions, absorption and transport of minerals and nitrogen, and the assimilation law of nitrogen can be used to guide rational fertilization, increase crop yield and improve quality.
Biological nitrogen metabolism The assimilation, alienation and excretion of nitrogen and nitrogen-containing biomass are called nitrogen metabolism.
Plants generally absorb inorganic nitrogen compounds such as ammonium salt or nitrate. Once nitrate is reduced to ammonium salt or reaches the stage related to ammonium salt, it is used for the synthesis of amino acids and protein. On the contrary, animals can only use amino acids or organic nitrogen compounds such as protein as nitrogen sources, otherwise they cannot use them. Animals use amino acids absorbed in the body as raw materials to synthesize their own protein. This process of transforming external nitrogen components into biological components is called nitrogen assimilation. However, for plants, as seen by spraying urea on their leaves, they are not unable to use organic nitrogen. Most microorganisms such as bacteria can also use bound nitrogen, but some can fix free nitrogen. The initial stage of the process of reducing nitrate to ammonia salt in plants depends on the role of nitrate reductase. A. Nason, H. J. Evans and others have clarified that this enzyme contains Mo and FAD. This enzyme is also found in fungi (Alternaria). However, there is another mechanism in nitrate reduction of these fungi. The physiological significance of nitric acid reduction is not only the synthetic route of protein, but also plays the role of anaerobic respiration (nitric acid respiration, that is, nitric acid replaces oxygen to form terminal electron acceptor). Some bacteria do not reduce nitric acid to ammonia and release it in the form of nitrogen, but show denitrification. In addition, in the soil, some bacteria can also oxidize ammonium salt or nitrite into nitrate for nitrification (nitrifying bacteria). The synthetic route of ammonia to amino acids is that ammonia and α -ketoglutaric acid are reduced by glutamate dehydrogenase to produce glutamic acid. It is generally believed that this is the main way for ammonia to produce amino acids, and the way for glutamine synthetase and glutamic acid synthetase to synthesize glutamic acid from ammonia has also been clarified. If amino groups are further transferred between glutamic acid and pyruvic acid, various amino acids can be produced. On the other hand, due to the deamination reaction of hydrolysis and redox, amino acids are also decomposed in organisms. Some anaerobic bacteria can carry out a viscous reaction between two amino acids. Bacteria, especially spoilage bacteria, can decarboxylate amino acids to produce amines. Ammonia produced by deamination decomposition of amino acids accumulates in plants in the form of glutamine or asparagine, while animals excrete ammonia or convert it into uric acid and urea.