Nitrogen is the basis of bioorganic compounds (part of proteins and), therefore knowledge about its cycle in nature and the impact of human activities on the processes that implement the nitrogen cycle is very important for preventing the environmental crisis and disasters associated with it.
The nitrogen content is relatively constant and amounts to 78% by volume. Molecular nitrogen is a very inert substance that practically does not react with any substances under normal conditions. And only during lightning discharges, nitrogen oxides are formed from nitrogen and molecular oxygen (first nitrogen oxide (II), and then nitrogen oxide (IV)), which, reacting with oxygen, form nitric acid, and it, falling into the soil with rain, forms nitrates used by plants for the synthesis of organic nitrogen-containing substances.
This is one of the ways to naturally include molecular nitrogen in the cycle. Some of the molecular nitrogen is fixed by nitrogen-fixing bacteria. Certain amounts of nitrogen oxides are produced during eruptions. Inorganic nitrogen compounds, in particular ammonia, are obtained from the decomposition of animal waste products (decomposition of urea), as well as from the action of putrefactive bacteria on the remains of animals, plants and other organisms. When organic substances burn, molecular nitrogen is formed, which replenishes atmospheric nitrogen. Part of the organic substances containing nitrogen goes into the composition of rocks and is removed from the processes for a long time. The interconnection of processes that ensure the transition of nitrogen from one unit to another is realized through the processes of decay and the formation of inorganic compounds (ammonia, ammonium salts and organic matter - urea), which can be absorbed by plants for the synthesis of complex bioorganic compounds. The transfer of nitrogen compounds from one territory to another is carried out due to the movements of animals, winds, water, rivers, etc.
The natural processes occurring in the implementation of the nitrogen cycle in nature are greatly influenced by human activity. Humans receive agricultural products in large volumes and, as a result, significantly deplete the soil of nitrogen compounds. To increase soil fertility, it is necessary to apply fertilizers containing nitrogen, for which both organic and inorganic fertilizers are used, as well as organomineral mixtures. It is known that all nitrogen compounds (except complex organic ones) are soluble and cannot be fixed in the soil - if they are not absorbed by plants, they are washed out and transported by water to different areas, ending up in groundwater. As shown above, nitrogen compounds have a harmful effect on warm-blooded animals and humans and pollute natural waters, making them unsuitable for use. In addition, plants, accumulating inorganic nitrogen compounds in the form of nitrates in their bodies, become unsuitable for consumption by both humans and animals.
To obtain fertilizers, a technology has been developed for fixing molecular nitrogen (it is first converted into ammonia, from which ammonium nitrate or other nitrates used as fertilizers can ultimately be obtained).
A large amount of nitrogen-containing substances is obtained as animal waste. Inorganic nitrogen compounds (nitrogen and its oxides) are formed during the combustion of fuel, during production and in other industries. This disrupts the natural processes of the cycle of substances and damages the ecological state of the planet.
Isotopes
Natural nitrogen consists of two stable isotopes 14 N - 99.635% and 15 N - 0.365%.
Radioactive isotopes of nitrogen are known with mass numbers 11,12,13,16 and 17. All of them are very short-lived isotopes. The most stable of them, 13 N, has a half-life of 10 minutes.
Magnetic moment of isotope nuclei I N 14 =1,I N 15 =1/2.
Prevalence
Outside the Earth, nitrogen (its compounds and radicals - CN", NH", NH` 2, NH 3) is found in gas nebulae, the solar atmosphere, on Uranus, Neptune, and interstellar space. About 2% nitrogen has been recorded in the atmosphere of Venus, but this figure still requires confirmation. Nitrogen is the fourth most abundant element in the solar system (after hydrogen, helium and oxygen). Life owes a lot to nitrogen, but nitrogen, at least atmospheric nitrogen, owes its origin not so much to the Sun as to life processes.
Most nitrogen is found in a free state in nature. Nitrogen, in the form of diatomic N2 molecules, makes up most of the atmosphere, where its content is 75.6% (by mass) or 78.084% (by volume), that is, about 3.87 * 10 15 tons. In general, we live in a nitrogen atmosphere moderately enriched with oxygen.
The mass of nitrogen dissolved in the hydrosphere, taking into account that the processes of dissolving atmospheric nitrogen in water and releasing it into the atmosphere simultaneously occur, is about 2 * 10 13 tons, in addition, approximately 7 * 10 11 tons of nitrogen are contained in the hydrosphere in the form of compounds.
Biological role
Nitrogen is an element necessary for the existence of animals and plants. It is part of proteins (16-18% by weight), amino acids, nucleic acids, nucleoproteins, chlorophyll, hemoglobin, etc. in the composition of living cells by the number of nitrogen atoms - about 2%, by mass fraction - about 2.5% (fourth place after hydrogen, carbon and oxygen). In this regard, a significant amount of fixed nitrogen is contained in living organisms, “dead organic matter” and dispersed matter of the seas and oceans. This quantity is estimated at approximately 1.9 * 10 11 tons. As a result of the processes of decay and decomposition of nitrogen-containing organic matter, subject to favorable environmental factors, natural deposits of minerals containing nitrogen can be formed, for example, “Chilean saltpeter” (sodium nitrate with admixtures of other compounds), Norwegian, Indian saltpeter.
Nitrogen cycle in nature
Nitrogen is a colorless, odorless gas and is slightly soluble in water. It is slightly lighter than air: the mass of one liter of nitrogen is 1.25 g. Molecular nitrogen is a chemically inactive substance. At room temperature it interacts only with lithium. The low activity of nitrogen is explained by the high strength of its molecules, which determines the high activation energy of reactions occurring with the participation of nitrogen.
The total nitrogen content in the earth's crust is 0.04% (mass.). Nitrogen makes up about 79% of the atmosphere, but a huge number of living things are unable to directly use this supply of nitrogen. It must first be fixed by specialized organisms or humans - in this latter case the fixation is carried out using specially designed industrial processes.
Despite the greatest complexity, this cycle occurs quickly and smoothly. The air, containing 78% nitrogen, simultaneously serves as both a huge container and a safety valve for the system. It continuously and in different forms feeds the nitrogen cycle.
The nitrogen cycle is as follows. Its main role is that it is part of the vital structures of the body - protein amino acids, as well as nucleic acids. Living organisms contain approximately 3% of the total active nitrogen fund. Plants consume approximately 1% nitrogen; its cycle time is 100 years.
From producer plants, nitrogen-containing compounds pass to consumers, from which, after the elimination of amines from organic compounds, nitrogen is released in the form of ammonia or urea, and urea is then also converted into ammonia (due to hydrolysis).
Rice. 1. Transformation and use of CO 2 in nature
Subsequently, in the processes of oxidation of ammonia nitrogen (nitrification), nitrates are formed that can be assimilated by plant roots. During denitrification, some of the nitrites and nitrates are reduced to molecular nitrogen entering the atmosphere. All these chemical transformations are possible as a result of the vital activity of soil microorganisms. These amazing bacteria - nitrogen fixers - are able to use the energy of their respiration to directly absorb atmospheric nitrogen and synthesize proteins. In this way, about 25 kg of nitrogen per 1 hectare is annually introduced into the soil.
But the most effective bacteria live in symbiosis with legumes in nodules developing on the roots of plants. In the presence of molybdenum, which serves as a catalyst, and a special form of hemoglobin (unique in plants), these bacteria ( Rhizobium) assimilate enormous amounts of nitrogen. The resulting (fixed) nitrogen continually diffuses into the rhizosphere (part of the soil) as the nodules disintegrate. But nitrogen also enters the above-ground part of plants. This makes legumes exceptionally rich in proteins and highly nutritious for herbivores. The annual reserve, thus accumulated in clover and alfalfa crops, is 150-140 kg/ha.
In addition to legumes, such bacteria live on the leaves of plants (in the tropics) from the family Rublaceae, as well as actinomycetes - on alder roots, fixing nitrogen. In the aquatic environment, these are blue algae.
On the other hand, denitrifying bacteria decompose nitrates and release N2, which evaporates into the atmosphere. But this process is not very dangerous, since it decomposes approximately 20% of the total nitrogen, and then only on soils highly fertilized with manure (approximately 50-60 kg of nitrogen per hectare).
Although people and land animals live at the bottom of an ocean of air, mainly consisting of nitrogen, it is this element that most determines the food supply for the inhabitants of this ocean. We all depend on available fixed nitrogen resources. “Fixed” is nitrogen that is included in a chemical compound that can be used by plants and animals. Nitrogen is not active in the atmosphere, but some organisms can still bind it. A smaller amount of atmospheric nitrogen is fixed in natural ionization processes. The atmosphere is ionized by cosmic rays, burning meteorites, and electrical discharges (lightning) in a short time, releasing a large amount of energy necessary for nitrogen to react with oxygen or hydrogen in water. Even some marine organisms fix nitrogen, but apparently the largest suppliers of fixed nitrogen in nature are soil microorganisms and symbiotic associations between such organisms and plants.
Fixation of atmospheric nitrogen in nature occurs in two main directions - abiogenic and biogenic. The first pathway involves mainly reactions of nitrogen with oxygen. Since nitrogen is chemically very inert, large amounts of energy (high temperatures) are required for oxidation. These conditions are achieved during lightning discharges when the temperature reaches 25,000 o C or more. In this case, the formation of various nitrogen oxides occurs. There is also the possibility that abiotic fixation occurs as a result of photocatalytic reactions on the surface of semiconductors or broadband dielectrics (desert sand).
However, the main part of molecular nitrogen (about 1.4 * 10 8 t/year) is fixed biogenically. For a long time it was believed that only a small number of species of microorganisms (albeit widespread on the Earth’s surface) could bind molecular nitrogen: bacteria Azotobacter And Clostridium, nodule bacteria of leguminous plants Rhizobium, cyanobacteria Anabaena, Nostoc etc. It is now known that many other organisms in water and soil have this ability, for example, actinomycetes in the tubers of alder and other trees (160 species in total). All of them convert molecular nitrogen into ammonium compounds (NH 4 +). This process requires significant energy expenditure (to fix 1g of atmospheric nitrogen, bacteria in legume nodules consume about 167.5 kJ, that is, they oxidize approximately 10g of glucose). Thus, the mutual benefit from the symbiosis of plants and nitrogen-fixing bacteria is visible - the former provide the latter with a “place to live” and supply the “fuel” obtained as a result of photosynthesis - glucose, the latter provide the nitrogen necessary for plants in a form that they can absorb.
Of all the types of human intervention in the natural cycle of substances, industrial nitrogen fixation is the largest in scale. In earlier times, when there was no mass production of artificial fertilizers, when nitrogen-fixing legumes were not yet grown over large areas, the amount of nitrogen removed from the atmosphere in the process of natural fixation was completely balanced by its return to the atmosphere as a result of the activity of organisms converting organic nitrates into gaseous nitrogen . Nitrogen in the form of ammonia and ammonium compounds, resulting from the process of biogenic nitrogen fixation, is quickly oxidized to nitrates and nitrites (this process is called nitrification). The latter, not connected by plant tissues (and further along the food chain by herbivores and predators), do not remain in the soil for long. Most nitrates and nitrites are highly soluble, so they are washed away by water and ultimately end up in the world's oceans (this flow is estimated at 2.5 - 8 * 10 7 tons / year).
Excessive removal of nitrogenous compounds into rivers can cause algal blooms and, as a result of increased biological activity, the water can be deprived of oxygen, which will cause the death of fish and other organisms that require oxygen. The most famous example of this is the rapid eutrophication of Lake Erie.
In the absence of human activity, the processes of nitrogen fixation and nitrification are almost completely balanced by the opposite reactions of denitrification. Some nitrogen enters the atmosphere from the mantle with volcanic eruptions, some is firmly fixed in soils and clay minerals, and nitrogen leaks from the upper layers of the atmosphere into interplanetary space.
Nitrogen included in the tissues of plants and animals, after their death, undergoes ammonification (decomposition of nitrogen-containing complex compounds with the release of ammonia and ammonium ions) and denitrification, that is, the release of atomic nitrogen, as well as its oxides. These processes occur entirely due to the activity of microorganisms under aerobic and anaerobic conditions.
To get an idea of the complexly branched pathways along which nitrogen moves in the biosphere, let us trace the path of nitrogen atoms from the atmosphere into the cells of microorganisms, then into the soil as fixed nitrogen, and from the soil into higher plants, from where fixed nitrogen can enter organisms animals. Plants and animals, when they die, return fixed nitrogen to the soil, from where it either enters new generations of plants and animals, or passes into the atmosphere in the form of elemental nitrogen.
Some organisms find it beneficial to oxidize nitrogen compounds, while other organisms living in the same environment survive only due to their ability to reduce these compounds. In addition to photosynthetic organisms that use light energy, all living beings obtain energy through chemical transformations. Usually this is the oxidation of one compound with the simultaneous reduction of another, although sometimes different molecules of the same substance or even different fragments of the same molecule can be oxidized and reduced. The nitrogen cycle in living nature is possible because the oxidation of reduced inorganic nitrogen compounds by atmospheric oxygen releases energy in a biologically effective form. Under anaerobic conditions, oxidized nitrogen compounds can serve as oxidizers of organic compounds, releasing useful energy.
The specific role of nitrogen in biological processes is due to an unusually large number of oxidation states, that is, valences. Valence- this is the property of an atom of a given element to attach or replace a certain number of atoms of another element. In the body of animals and plants, most of the nitrogen is present either in the form of ammonium ion or in the form of amino compounds. In both forms, nitrogen is highly reduced: having combined with three other atoms, it has accepted three electrons from them, that is, it has an oxidation state of -3. In another highly oxidized form (nitrate ion (NO 3 +5), the five outer electrons of the nitrogen atom participate in the formation of bonds with the oxygen atom, thereby acquiring an oxidation state of +5. Nitrate ion is the main form in which nitrogen is present in the soil. When an ammonium ion or amino acids passes into soil nitrates, the valency of nitrogen must change by 8 units, that is, the atom loses 8 electrons.When nitrate nitrogen passes into amino nitrogen, the atom gains 8 electrons.
Inorganic nitrogen compounds do not occur in nature in large quantities, with the exception of sodium nitrate NaNO 3, which forms thick layers on the Pacific coast of Chile. The soil contains small amounts of nitrogen, mainly in the form of nitric acid salts. But in the form of complex organic compounds - proteins - nitrogen is part of all living organisms. The transformations that proteins undergo in plant and animal cells form the basis of all life processes. Without protein there is no life, and since nitrogen is an essential component of protein, it is clear what an important role this element plays in living nature.
In general, reactions in soil that reduce nitrogen provide significantly more energy than oxidative reactions that remove electrons from nitrogen atoms. To summarize, we can say that in nature, any reaction in which at least 15 kcal/mol is formed when converting one compound into another serves as a source of energy for a particular organism or group of organisms.
Nitrogen fixation requires energy. First, nitrogen must be “activated,” that is, the nitrogen molecule must be broken into two atoms. This will take at least 160 kcal/mol. Fixation itself, that is, the combination of two nitrogen atoms with three hydrogen molecules to form two ammonia molecules, gives about 13 kcal. This means that in total, at least 147 kcal are spent on the reaction. But it is not known whether nitrogen-fixing organisms actually have to expend this amount of energy. Indeed, in reactions catalyzed by enzymes, there is not just an exchange of energy between the reactants and the final products, but a decrease in the activation energy.
Animals consume plant proteins, amino acids and other nitrogen-containing substances with food. Thus, plants make organic nitrogen available to other organisms - consumers.
All living organisms supply nitrogen to the environment. On the one hand, they all release nitrogen metabolism products during their life: ammonia (NH 3), urea (CO(NH 2) 2) and uric acid. The last two compounds decompose in the soil to form ammonia (which, when dissolved in water, produces ammonium ions).
Uric acid secreted by birds and reptiles is also quickly mineralized by special groups of microorganisms to form NH 3 and CO 2. On the other hand, nitrogen included in the composition of living beings, after their death, undergoes ammonification (decomposition of nitrogen-containing complex compounds with the release of ammonia and ammonium ions) and nitrification.
Ammonia, or the ammonium ion, produced in the soil can be absorbed by plant roots. Nitrogen is then included in amino acids and becomes part of the protein. If the plant is then eaten by an animal, the nitrogen is incorporated into other proteins. In either case, the protein eventually returns to the soil, where it is broken down into its constituent amino acids. Under aerobic conditions, soil contains many microorganisms that can oxidize amino acids to carbon dioxide, water and ammonia. When decomposing, for example, glycine releases 176 kcal/mol.
Some microorganisms of the genus Nitrosomonas use nitrification of ammonium ion as the only source of energy. In the presence of oxygen, ammonia produces nitrite ion and water; The energy yield in this reaction is 65 kcal/mol, and this is quite enough for a “decent” existence. Nitrosomonas belongs to the group of so-called autotrophs - organisms that do not consume energy stored in organic matter. Photoautotrophs use light energy, and chemoautotrophs like Nitrosomonas, obtain energy from inorganic compounds.
Another specialized group of microorganisms, of which Nitrobacter, is capable of extracting energy from nitrites, which was neglected Nitrosomonas. At the oxidation of a nitrite ion into a nitrate ion releases about 17 kcal/mol - not much, but quite enough to support existence Nitrobacter .
There are many different types of bacteria in the soil - denitrifiers, which, once in anaerobic conditions, can use nitrate and nitrite ions as electron acceptors during the oxidation of organic compounds.
Nitrification products - NO 3 - and (NO 2 -) are subsequently subject to denitrification. This process occurs entirely due to the activity of denitrifying bacteria, which have the ability to reduce nitrate through nitrite to gaseous nitrous oxide (N 2 O) and nitrogen (N 2). These gases freely pass into the atmosphere.
10 [H] + 2H+ +2NO 3 - = N 2 + 6H 2 O
In the absence of oxygen, nitrate serves as the final hydrogen acceptor. The ability to obtain energy by using nitrate as the final hydrogen acceptor to form a nitrogen molecule is widespread in bacteria. Temporary losses of nitrogen in limited areas of the soil are undoubtedly associated with the activity of denitrifying bacteria. Thus, the nitrogen cycle is impossible without the participation of soil microflora.
The comparative value of ammonium and nitrite ions as sources of nitrogen for plants has been the subject of much research. It would seem that the ammonium ion is clearly preferable: the oxidation state of nitrogen in it is -3, that is, the same as that of nitrogen in amino acids; The oxidation state of nitrate nitrogen is +5. This means that in order to use nitrogen from the nitrate ion, the plant must expend energy on the reduction of pentavalent nitrogen to trivalent. In reality, everything is more complicated: which form of nitrogen is preferable depends, as it turns out, on completely different factors. Since the ammonium ion is positively charged, almost immediately after its formation in the soil it is captured by sludge particles, on which it remains until oxidation. The negative nitrate ion, on the contrary, moves freely in the soil, which means it more easily enters the root zone.
Soil nitrogen-fixing organisms remained poorly understood until the end of the 19th century. Scientists even feared that denitrifying bacteria, discovered at that time, would gradually exhaust the supply of fixed nitrogen in the soil and reduce fertility. In his speech to the Royal Society in London, Sir W. Crookes sketched a grim picture of the famine that awaits humanity in the near future unless artificial methods of nitrogen fixation are developed. At that time, the main source of nitrate for both the production of fertilizers and the production of explosives were deposits in Chile. It is the need for
After the nitrogen cycle was studied in general terms, the role of denitrifying bacteria became clear. Without such bacteria returning nitrogen to the atmosphere, most of the atmospheric nitrogen would now be in bound form in the ocean and sediments. There is currently not enough oxygen in the atmosphere to convert all the free nitrogen into nitrates. But it is likely that a one-sided process in the absence of denitrifiers led to the acidification of ocean water with nitrates. Carbon dioxide would begin to be released from carbonate rocks. Plants would constantly extract carbon dioxide from the air, the carbon would be deposited over time in the form of coal or other hydrocarbons, and free oxygen would saturate the atmosphere and combine with nitrogen. Due to the diversity and complexity of all these processes, it is difficult to say what the world of the denitrification reaction would look like, but it would certainly be an unusual world for us.
The process of biological nitrogen fixation is not known in detail. I would like to know how the activating enzyme used by nitrogen-fixing bacteria can, at normal temperature and normal pressure, do what happens in a chemical reactor at hundreds of degrees and atmospheres. All over the world, several kilograms of this amazing enzyme will accumulate.
Nitrogen-fixing organisms are divided into two large groups: those living independently and those living in symbiosis with higher plants. The boundary between these groups is not so sharp. The degree of interdependence of plants and microorganisms may vary. Symbiotic microorganisms directly depend on the plant as a source of energy and possibly some nutrients. Free-living nitrogen fixers obtain energy from the plant indirectly, and some of them use light energy directly.
The main suppliers of fixed nitrogen in soils occupied by cereals and in other ecosystems where there are no plants with nitrogen-fixing symbionts are various bacteria. Under suitable conditions, blue-green algae can be an important source of fixed nitrogen. Their contribution to nitrogen fixation is especially noticeable in rice fields and other places where conditions are favorable for their development. But for the Earth as a whole, leguminous plants are the most important natural source of fixed nitrogen. They are more important than other nitrogen-fixing plants from an economic point of view and therefore are better studied.
The nitrogen cycle is currently highly impacted by humans. On the one hand, mass production of nitrogen fertilizers and their use leads to excessive accumulation of nitrates. Nitrogen supplied to fields in the form of fertilizers is lost through crop waste, leaching and denitrification.
On the other hand, when the rate of conversion of ammonia into nitrates decreases, ammonium fertilizers accumulate in the soil. It is possible to suppress the activity of microorganisms as a result of soil contamination with industrial waste. However, these processes are local in nature. Much more important is the entry of nitrogen oxides into the atmosphere during fuel combustion at thermal power plants, transport, and factories (“fox tail” (NO 2)). In industrial areas, their concentration in the air becomes very dangerous. Under the influence of radiation, reactions of organic matter (hydrocarbons) with nitrogen oxides occur with the formation of highly toxic and carcinogenic compounds.
Factors influencing the nitrogen cycle
In the absence of human activity, the processes of nitrogen fixation and nitrification are almost completely balanced by the opposite reactions of denitrification. Part of the nitrogen enters the atmosphere from the mantle with volcanic eruptions, part is firmly fixed in soils and clay minerals, in addition, nitrogen is constantly leaking from the upper layers of the atmosphere into interplanetary space. But currently, the nitrogen cycle is influenced by many human-caused factors. The first is acid rain, a phenomenon in which there is a decrease in the pH of rainfall and snow due to air pollution from acidic oxides (for example, nitrogen oxides). The chemistry of this phenomenon is as follows. To burn fossil fuels, air or a mixture of fuel and air is supplied to internal combustion engines and boilers. Almost 4/5 of the air consists of nitrogen gas and 1/5 of oxygen. At high temperatures created inside installations, a reaction of nitrogen with oxygen inevitably occurs and nitrogen oxide is formed:
N 2 + O 2 = 2NO - Q
This reaction is endothermic and occurs under natural conditions during lightning discharges, and also accompanies other similar magnetic phenomena in the atmosphere. Nowadays, as a result of our activities, humans greatly increase the accumulation of nitric oxide (II) on the planet. Nitric oxide (II) is easily oxidized to nitrogen oxide (IV) already under normal conditions:
2NO 2 + H 2 O = HNO 3 + HNO 2
nitric and nitrous acids are formed. In droplets of atmospheric water, these acids dissociate to form nitrate and nitrite ions, respectively, and the ions enter the soil with acid rain. The second group of anthropogenic factors affecting soil nitrogen metabolism are technological emissions. Nitrogen oxides are one of the most common air pollutants. And the steady increase in the production of ammonia, sulfuric and nitric acid is directly related to the increase in the volume of waste gases, and consequently, to the increase in the amount of nitrogen oxides emitted into the atmosphere. The third group of factors is overfertilization of soils with nitrites, nitrates (sodium nitrate (NaNO 3), potassium nitrate (KNO 3), calcium nitrate (Ca(NO 3) 2), ammonium nitrate NH 4 NO 3) and organic fertilizers. Finally, soil nitrogen metabolism is negatively affected by increased levels of biological pollution. Possible causes: wastewater discharge, non-compliance with sanitary standards (dog walking, uncontrolled organic waste dumps, poor functioning of sewer systems, etc.). As a result, the soil becomes contaminated with ammonia, ammonium salts, urea, indole, mercaptans and other products of organic decomposition. Additional ammonia is formed in the soil, which is then processed by bacteria into nitrates.
Relevance of studying the nitrogen cycle
There is a constant exchange of chemical elements between the lithosphere, hydrosphere, atmosphere and living organisms of the Earth. This process is cyclical: having moved from one sphere to another, the elements return to their original state.
Anthropogenic biocenoses are special natural communities formed under the direct influence of man, who himself can create new landscapes and seriously change the ecological balance. In addition, human activity has a huge impact on the cycle of elements. It has become especially noticeable in the last century because there have been major changes in natural cycles due to the addition or removal of chemicals present in them as a result of human-induced impacts. Nitrogen is an element necessary for the existence of animals and plants; it is part of proteins, amino acids, nucleic acids, chlorophyll, genes, etc. In this regard, a significant amount of bound nitrogen is found in living organisms, “dead organic matter” and dispersed matter of the seas and oceans.
To study the characteristics of the nitrogen cycle, you can use a comprehensive methodology to study the content of nitrite (NO 2 -), nitrate (NO 3 -) and ammonium (NH 4 +) ions in the soil and its microbiological parameters.
It is very important to study and control the nitrogen cycle, especially in anthropogenic biocenoses, because a small failure in any part of the cycle can lead to serious consequences: severe chemical pollution of soils, overgrowing of water bodies and their contamination with decomposition products of dead organic matter (ammonia, amines, etc. ), high content of soluble nitrogen compounds in drinking water.
Toxicology of nitrogen and its compounds
Atmospheric nitrogen itself is inert enough to have a direct effect on the human body and mammals. However, with high blood pressure, it causes narcosis, intoxication or suffocation (due to lack of oxygen); When pressure decreases rapidly, nitrogen causes decompression sickness. Animals placed in a nitrogen atmosphere quickly die, but not due to the toxicity of nitrogen, but due to the lack of oxygen.
Many nitrogen compounds are very active and often toxic
Up to 13% of the nitrogen contained in mineral fertilizers goes into groundwater. The World Health Organization (WHO) has adopted a maximum permissible concentration of nitrates in drinking water: 45 mg/l for temperate latitudes and 10 mg/l for the tropics.
Rice. 100. Nodule bacteria on the roots of a legume plant
When organic matter rots, a significant part of the nitrogen contained in them is converted into ammonia, which, under the influence of nitrifying bacteria living in the soil, is then oxidized into nitric acid. The latter, reacting with carbonic acid salts in the soil, for example CaCO 3, forms nitrate: 2HNO 3 + CaCO 3 = Ca(NO 3) 2 + CO 2 + H 2 O
Some part of organic nitrogen is always released when rotting freely into the atmosphere. Free nitrogen is also released during the combustion of organic substances, when burning wood, coal, peat, etc. In addition, there are bacteria that, with insufficient access to oxygen, can take away nitric acid salts, destroying them with the release of free nitrogen. The activity of these denitrifying bacteria leads to the fact that part of the bound nitrogen from a form accessible to green plants (nitrates) becomes inaccessible (free).
Thus, not all that was part of the dead plants returns back to the soil; part of it is constantly released in free form and, therefore, is lost to plants. The continuous loss of mineral nitrogen compounds should have long ago led to the complete cessation of life on earth if processes did not exist in nature that compensate for the loss of nitrogen. Such processes include, first of all, electrical discharges occurring in the atmosphere, during which a certain amount of nitrogen oxides is always formed; the latter produce nitric acid with water, which turns into nitrate in the soil. Another source of replenishment of soil nitrogen compounds is the vital activity of the so-calledazotobacteria capable of assimilating atmospheric nitrogen. Some of these bacteria settle on the roots of plants from the legume family, causing the formation of characteristic swellings - “nodules”, which is why they are called nodule bacteria (Fig. 100). Assimilating atmosphericnitrogen, nodule bacteria process it into nitrogen compounds, and plants, in turn, convert the latter into proteins and other complex compounds. Therefore, legumes areSthenia, unlike the others, can develop well on soils that contain almost no nitrogen compounds.
Rice. 101. Scheme of the nitrogen cycle in nature
The activity of bacteria that assimilate atmospheric nitrogen is the main reason why the amount of fixed nitrogen in the soil remains more or less constant, despite the losses that occur during the decomposition of nitrogen compounds. This decomposition is compensated by the new formation of nitrogen compounds, and thus a continuous nitrogen cycle occurs in nature (Fig. 101).
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Cycle of substances in nature
The activity of living organisms is accompanied by the extraction of large quantities of minerals from the surrounding inanimate nature.
After the death of organisms, their constituent chemical elements are returned to the environment.
This is how the biogenic cycle of substances arises in nature, i.e. circulation of substances between the atmosphere, hydrosphere, lithosphere and living organisms.
Nitrogen cycle in nature
Nitrogen continuously circulates in the earth's biosphere through a network of closed interconnected pathways. Artificial nitrogen fixation in the production of mineral fertilizers has been added to natural processes.
Nitrogen is one of the most abundant substances in the biosphere, the narrow shell of the Earth where life is supported. So, almost 80% of the air we breathe consists of this element. Most atmospheric nitrogen is in free form, in which two nitrogen atoms are joined together to form a nitrogen molecule, N2. Due to the fact that the bonds between two atoms are very strong, living organisms are not able to directly use molecular nitrogen - it must first be transferred to a “bound” state. During the binding process, nitrogen molecules are split, allowing individual nitrogen atoms to participate in chemical reactions with other atoms, and thus preventing them from recombining into a nitrogen molecule. The bond between nitrogen atoms and other atoms is weak enough to allow living organisms to utilize nitrogen atoms. Therefore, nitrogen fixation is an extremely important part of life processes on our planet.
The nitrogen cycle is a series of closed, interconnected pathways through which nitrogen circulates in the earth's biosphere. Let us first consider the process of decomposition of organic matter in the soil.
Various microorganisms extract nitrogen from decomposing materials and convert it into the molecules they need for metabolism. In this case, the remaining nitrogen is released in the form of ammonia (NH3) or ammonium ions (NH4+). Other microorganisms then fix this nitrogen, usually converting it into the form of nitrates (NO3–). Entering plants, this nitrogen participates in the formation of biological molecules. After the organism dies, nitrogen is returned to the soil and the cycle begins again. During this cycle, both nitrogen losses and compensation for these losses due to volcanic eruptions and other types of geological activity are possible.
Imagine that the biosphere consists of two connected reservoirs of nitrogen - a huge one (nitrogen contained in the atmosphere and oceans) and a very small one (nitrogen contained in living things). Between these reservoirs there is a narrow passage in which nitrogen is bound in one way or another. Under normal conditions, nitrogen from the environment enters biological systems through this passage and returns to the environment after the death of biological systems.
Let's give some numbers. The atmosphere contains about 4 quadrillion (4 1015) tons of nitrogen, and the oceans contain about 20 trillion (20 1012) tons. A small part of this amount - about 100 million tons - is annually bound and included in living organisms. Of these 100 million tons of fixed nitrogen, only 4 million tons are found in plant and animal tissues—the rest accumulates in decomposing microorganisms and is returned to the atmosphere.
The main supplier of fixed nitrogen in nature is bacteria: thanks to them, approximately 90 to 140 million tons of nitrogen are fixed. The most famous nitrogen-fixing bacteria are found in the nodules of legume plants. The traditional method of increasing soil fertility is based on their use: first, peas or other legumes are grown in the field, then they are plowed into the ground, and the bound nitrogen accumulated in their nodules passes into the soil. Then the field is sown with other crops, which can already use this nitrogen for their growth.
Some nitrogen is converted into a bound state during thunderstorms. You will be surprised, but lightning flashes occur much more often than you think - about a hundred lightning strikes every second. While you were reading this paragraph, approximately 500 lightning flashed around the world. The electrical discharge heats the atmosphere around it, nitrogen combines with oxygen (combustion reaction) to form various nitrogen oxides. And while this is a pretty spectacular form of sequestration, it only covers 10 million tons of nitrogen per year.
Thus, as a result of natural processes, from 100 to 150 million tons of nitrogen are bound per year. In the course of human activity, nitrogen is also fixed and transferred to the biosphere (for example, sowing fields with legumes leads to the formation of 40 million tons of fixed nitrogen annually). Moreover, when fossil fuels are burned in electric generators and internal combustion engines, the air heats up, as is the case with a lightning discharge. Every time you drive a car, additional amounts of fixed nitrogen enter the biosphere. Approximately 20 million tons of nitrogen per year are bound when burning fossil fuels.
But humans produce the most fixed nitrogen in the form of mineral fertilizers. As often happens with the achievements of technological progress, we owe the technology of nitrogen fixation on an industrial scale to the military. In Germany, before the First World War, a method was developed for producing ammonia (one of the forms of fixed nitrogen) for the needs of the military industry. A lack of nitrogen often inhibits plant growth, and farmers buy artificially fixed nitrogen in the form of mineral fertilizers to increase yields. Currently, just over 80 million tons of fixed nitrogen are produced each year for agriculture. Summing up the entire human contribution to the nitrogen cycle, we get a figure of about 140 million tons per year. Approximately the same amount of nitrogen is naturally bound in nature. Thus, in a relatively short period of time, humans began to have a significant impact on the nitrogen cycle in nature. What will be the consequences? Each ecosystem is able to absorb a certain amount of nitrogen, and the consequences of this are generally favorable - plants will grow faster. However, when the ecosystem becomes saturated, nitrogen will begin to wash into rivers. Lake algae pollution is the most vexing nitrogen-related environmental problem. Nitrogen fertilizes the lake algae, and they grow, crowding out all other forms of life.
Nitrogen is involved in the formation of protein structures. Most of the biospheric nitrogen is found in the atmosphere. It is the main source of nitrogen for organic compounds.
Its transition into forms accessible to living organisms can be carried out in different ways. For example, electrical discharges during thunderstorms contribute to the synthesis of nitrogen oxides, which then enter the soil with rainwater in the form of nitrate or nitric acid and are absorbed by plants.
Only certain types of organisms are capable of directly assimilating atmospheric nitrogen: blue-green algae and bacteria. As they die, they enrich the soil with organic nitrogen, which quickly mineralizes. The most effective fixation is carried out by bacteria that form a symbiosis with legumes. They are able to fix atmospheric nitrogen and make it available to plant roots.
The nitrogen cycle in the biosphere is a slow process; according to some estimates, its speed is 108 years. Human impacts on the nitrogen cycle are primarily related to the production of nitrogen fertilizers.
Their impact on ecosystems is similar to the effect of phosphorus: a large amount of nitrogen in water bodies promotes increased growth of blue-green algae and subsequent waterlogging of the reservoir.
Secondly, as a result of the combustion of organic fuel, a large amount of nitrogen oxides appears in the combustion products. The latter, entering the atmosphere, interact with water and other substances, forming
