Rice. Nitrogen cycle in the biosphere
The nitrogen cycle covers all areas of the biosphere. Its absorption by plants is limited, since they absorb nitrogen only in the form of combining it with hydrogen and oxygen (N0 3- and NH 4). And this despite the fact that the reserves of nitrogen in the atmosphere are inexhaustible (78% of its volume). Decomposers(destructors), or rather soil bacteria, gradually decompose the protein substances of dead organisms and convert them into ammonium compounds, nitrates and nitrites. Some of the nitrates enter the groundwater during the cycle and pollute them.
Nitrogen returns to the atmosphere again with gases released during decay. True, part of it oxidizes in the air - during thunderstorms - and enters the soil with rainwater, but in this way it is fixed 10 times less than with bacteria.
Human intervention The nitrogen cycle is as follows:
at burning fossil fuels Large amounts of nitrogen oxide (NO) are released into the atmosphere. Nitric oxide then combines with oxygen in the atmosphere to form nitrogen dioxide (NO2), which when reacting with water vapor can form nitric acid (HNO 3 ) . This acid becomes component of acid precipitation.
use of fertilizers leads to the release into the atmosphere " greenhouse gas» nitrous oxide (N 2 O)
increase in the amount of nitrates and ammonium ions in aquatic ecosystems when fertilizer runoff from fields. Excess nutrients lead to rapid algae growth, during the decomposition of which dissolved oxygen is consumed, which leads to massive pestilence of fish.
The biochemical cycles of phosphorus and sulfur are much less perfect, because The bulk of them are contained in the reserve fund of the earth's crust, in the “inaccessible” reserve.
The cycle of sulfur and phosphorus is a typical sedimentary biogeochemical cycle. Such cycles easily damaged by various types of influences and part of the exchanged material leaves the cycle. It can return again to the cycle only as a result of geological processes or through the extraction of biophilic components by living matter.
Phosphorus cycle
Rice. Phosphorus cycle in the biosphere
Phosphorus, mainly in the form of phosphate ions (PO 3- and HPO 4 2-), is an important nutritional element for both plants and animals. It is part of the molecules DNA, carrying genetic information; molecules ATP and ADP, which store what organisms need chemical energy, used in cellular respiration; molecules fat, forming cell membranes in plant and animal cells; as well as substances included in the composition bones and teeth.
Phosphorus is found in rocks formed in past geological epochs. It can enter the biogeochemical cycle if these rocks rise from the depths of the earth’s crust to the land surface, into the weathering zone. Erosion processes he is being taken out in the sea in the form of a well-known mineral - apatite.
The general phosphorus cycle can be divided into two parts - water and land.
In terrestrial ecosystems, phosphorus is released slowly destruction(or weathering) phosphate ores, is dissolved by soil moisture and absorbed by plant roots.
Animals get the phosphorus they need, eating plants or other herbivorous animals. A significant portion of this phosphorus is in the form animal excrement and decomposition products dead animals and plants are returned to soil, with erosion - in rivers, and, in the end, to the bottom of the ocean in the form of insoluble phosphate sedimentary rocks.
Phosphorus is absorbed in aquatic ecosystems phytoplankton and is transmitted along the trophic chain up to third-order consumers - seabirds. Their excrement ( guano) fall into again sea and enter into a cycle, or accumulate on the shore And washed out to sea. Thus, part of the phosphorus returns to the surface of the land in the form of guano - organic mass enriched with phosphorus in the excrement of fish-eating birds (pelicans, gannets, cormorants, etc.). However, an incomparably larger amount of phosphates is washed annually from the land surface into the ocean as a result of natural processes and anthropogenic activities.
Human intervention in the phosphorus cycle comes down mainly to two options:
production large quantities phosphate ores for the production of mineral fertilizers and detergents;
increase in excess of phosphate ions in aquatic ecosystems when you hit them contaminated runoff from livestock farms, washed away from the fields phosphate fertilizers, as well as treated and untreated municipal waste drains. An excess of these elements contributes to the “explosive” growth of blue-green algae and other aquatic plants, which upsets the vital balance in aquatic ecosystems.
Sulfur cycle
Sera also has a main reserve fund in sediments And soil, but unlike phosphorus it has a reserve fund and in atmosphere.
In an exchange fund the main role belongs to microorganisms. One of them reducing agents, other - oxidizing agents.
In rocks sulfur occurs in the form of sulfides (FeS2, etc.), in solutions– in the form of an ion (SO 4 -2), in the gaseous phase– in the form of hydrogen sulfide (H2S) or sulfur dioxide (SO2). Sulfur accumulates in some organisms in its purest form(S2) and when they die off, deposits of native sulfur are formed on the bottom of the seas.
In the marine environment, sulfate ion ranks second in content after chlorine and is the main available form of sulfur, which is reduced by autotrophs and included in amino acids.
Sulfur cycle, although organisms require it in small quantities, is key in the overall process of production and decomposition.
In terrestrial ecosystems, sulfur returns to soil at dying off of plants, is captured microorganisms, which restore it to H2S. Other organisms and exposure to oxygen itself cause these products to oxidize. Formed sulfates dissolve and absorb plants from the pore solutions of the soil - this is how the cycle continues.
The sulfur cycle, as well as the nitrogen cycle, can be disrupted human intervention . This is primarily due to burning fossil fuels, and especially coal. Sulfur dioxide (SO2) disrupts photosynthesis processes and leads to death of vegetation.
When biogeochemical cycles are disrupted by humans, the circulation of substances becomes impossible cyclical, A acyclic. The protection of natural resources should be particularly aimed at turning acyclic biogeochemical processes into cyclic ones.
Nitrogen in nature.
One of the most important elements is nitrogen. It is part of proteins and nucleic acids. Some of the nitrogen is absorbed during lightning, combining with oxygen to form nitrogen oxides. But the bulk of nitrogen passes into soil and water as a result of the fixation of atmospheric nitrogen by living organisms (Fig. 77).
Thus, in the process of biogenic migration, as a result of the interaction of living (biotic) and nonliving (abiotic) nature, the transition of inorganic matter into living organisms and their transformation with a return to the abiotic state occurs. This nitrogen cycle occurs continuously in nature, and is accomplished through four sequential processes: nitrogen fixation, ammonification, nitrification And denitrification.
Nitrogen fixation is the process of converting elemental atmospheric nitrogen into nitrogenous compounds by various microorganisms.
Nitrogen-fixing bacteria living in the soil enrich the soil with nitrogen as a result of their mineralization after death. Thus, about 25 kg of nitrogen accumulates annually on each hectare of land.
The most effective nitrogen fixing agents are tuber bacteria, living in the root system of leguminous plants, and freely living in the soil azotobacteria.
Ammonia is partially absorbed by plants, and partially by bacteria and turns into nitrates. This process is called nitrification.
Nitrates, like ammonium salts, are used by plants and microorganisms. Some of the nitrates are broken down by individual bacteria into elemental nitrogen and released into the atmosphere. This process is called denitrification.
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12.2. Cycle of nitrogen, oxygen, carbon
The nitrogen cycle (Figure 12.2) is one of the most complex cycles in nature. Covers the entire biosphere, as well as the atmosphere, lithosphere, and hydrosphere. Microorganisms play a very important role in the nitrogen cycle. The following stages are distinguished in the nitrogen cycle:
Stage 1 (nitrogen fixation): a) nitrogen-fixing bacteria bind (fix) gaseous nitrogen to form the ammonium form (NH and ammonium salts) - this is biological fixation; b) as a result of lightning discharges and photochemical oxidation, nitrogen oxides are formed; when interacting with water, they form nitric acid, which in the soil turns into nitrate nitrogen.
Stage 2 – conversion into vegetable protein. Both forms (ammonium and nitrate) of fixed nitrogen are absorbed by plants and converted into complex protein compounds.
Stage 3 – transformation into animal protein. Animals eat plants, and in their bodies plant proteins are converted into animal proteins.
Stage 4 – protein decomposition, rotting. Metabolic products of plants and animals, as well as tissues of dead organisms, under the influence of microorganisms, decompose to form ammonium (ammonification process).
Stage 5 – nitrification process. Ammonia nitrogen is oxidized to nitrite and nitrate nitrogen.
Stage 6 – denitrification process. Under the influence of denitrifying bacteria, nitrate nitrogen is reduced to molecular nitrogen, which enters the atmosphere. The circle closes.
Figure 12.2 – Structural diagram of the nitrogen cycle
(according to N.I. Nikolaikin, 2004)
Anthropogenic impacts on the nitrogen cycle are as follows:
1 Industrial use of nitrogen to produce ammonia increases the total amount of nitrogen fixed naturally by approximately 10%.
2 The widespread use of nitrogen fertilizers, exceeding the needs of plants, leads to environmental pollution, while part of the excess nitrogen is washed into water bodies, causing the dangerous phenomenon of “eutrophication”. It causes secondary pollution of water bodies, disruption of the cycle of substances, and changes in their trophic status.
Oxygen cycle accompanied by its inflow and outflow.
The arrival of oxygen includes: 1) secretion during photosynthesis; 2) formation in the ozone layer under the influence of UV radiation (in small quantities); 3) dissociation of water molecules in the upper layers of the atmosphere under the influence of UV radiation; 4) formation of ozone - O 3.
Consumption oxygen includes: 1) consumption by animals during respiration; 2) oxidative processes in the earth’s crust; 3) oxidation of carbon monoxide (CO) released during volcanic eruptions.
The oxygen cycle is closely related to the carbon cycle.
Carbon cycle(Figure 12.3). The mass of carbon dioxide (CO 2) in the atmosphere is estimated at 10 12 tons.
The arrival of carbon dioxide includes: 1) respiration of living organisms; 2) decomposition of dead organisms of plants and animals by microorganisms, the fermentation process; 3) anthropogenic emissions from fuel combustion; 4) deforestation.
Carbon dioxide consumption includes: 1) fixation of carbon dioxide from the atmosphere during photosynthesis with the release of oxygen; 2) consumption of part of the carbon by animals eating plant foods; 3) fixation of carbon in the lithosphere (formation of organic rocks - coal, peat, oil shale, as well as soil components such as humus); 4) fixation of carbon in the hydrosphere (formation of limestones, dolomites).
The gradual increase in carbon dioxide content in the atmosphere, in combination with other reasons, has led to the “greenhouse effect”, which affects the heat balance and the climate of our planet.
In addition to the elements considered, phosphorus, sulfur, and iron also play a major role in the general cycle of substances in nature.
Figure 12.3 – Structural diagram of the carbon cycle
(according to N.I. Nikolaikin, 2004)
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The circulation of substances in the biosphere is the “journey” of certain chemical elements along the food chain of living organisms, thanks to the energy of the Sun. During the “journey”, some elements, for various reasons, fall out and remain, as a rule, in the ground. Their place is taken by the same ones that usually come from the atmosphere. This is the most simplified description of what guarantees life on planet Earth. If such a journey is interrupted for some reason, then the existence of all living things will cease.
To briefly describe the cycle of substances in the biosphere, it is necessary to put several starting points. Firstly, of the more than ninety chemical elements known and found in nature, about forty are needed for living organisms. Secondly, the quantity of these substances is limited. Thirdly, we are talking only about the biosphere, that is, about the life-containing shell of the earth, and, therefore, about the interactions between living organisms. Fourthly, the energy that contributes to the cycle is the energy coming from the Sun. The energy generated in the bowels of the Earth as a result of various reactions does not take part in the process under consideration. And one last thing. It is necessary to get ahead of the starting point of this “journey”. It is conditional, since there cannot be an end and a beginning to a circle, but this is necessary in order to start somewhere to describe the process. Let's start with the lowest link of the trophic chain - with decomposers or gravediggers.
Crustaceans, worms, larvae, microorganisms, bacteria and other gravediggers, consuming oxygen and using energy, process inorganic chemical elements into an organic substance suitable for feeding living organisms and its further movement along the food chain. Further, these already organic substances are eaten by consumers or consumers, which include not only animals, birds, fish and the like, but also plants. The latter are producers or producers. They, using these nutrients and energy, produce oxygen, which is the main element suitable for breathing by all living things on the planet. Consumers, producers and even decomposers die. Their remains, along with the organic substances contained in them, “fall” at the disposal of the gravediggers.
And everything repeats itself again. For example, all the oxygen that exists in the biosphere completes its turnover in 2000 years, and carbon dioxide in 300. Such a cycle is usually called the biogeochemical cycle.
Some organic substances during their “journey” enter into reactions and interactions with other substances. As a result, mixtures are formed that, in the form in which they exist, cannot be processed by decomposers. Such mixtures remain “stored” in the ground. Not all organic substances that fall on the “table” of gravediggers cannot be processed by them. Not everything can rot with the help of bacteria. Such unrotted remains go into storage. Everything that remains in storage or in reserve is removed from the process and is not included in the cycle of substances in the biosphere.
Thus, in the biosphere, the cycle of substances, the driving force of which is the activity of living organisms, can be divided into two components. One - the reserve fund - is a part of the substance that is not associated with the activities of living organisms and does not participate in circulation for the time being. And the second is the revolving fund. It represents only a small part of the substance that is actively used by living organisms.
Atoms of which basic chemical elements are so necessary for life on Earth? These are: oxygen, carbon, nitrogen, phosphorus and some others. Of the compounds, the main one in the circulation is water.
Oxygen
The oxygen cycle in the biosphere should begin with the process of photosynthesis, as a result of which it appeared billions of years ago. It is released by plants from water molecules under the influence of solar energy. Oxygen is also formed in the upper layers of the atmosphere during chemical reactions in water vapor, where chemical compounds decompose under the influence of electromagnetic radiation. But this is a minor source of oxygen. The main one is photosynthesis. Oxygen is also found in water. Although there is 21 times less of it than in the atmosphere.
The resulting oxygen is used by living organisms for respiration. It is also an oxidizing agent for various mineral salts.
And a person is a consumer of oxygen. But with the beginning of the scientific and technological revolution, this consumption has increased many times over, since oxygen is burned or bound during the operation of numerous industrial production, transport, to satisfy household and other needs in the course of human life. The previously existing so-called exchange fund of oxygen in the atmosphere amounted to 5% of its total volume, that is, as much oxygen was produced in the process of photosynthesis as it was consumed. Now this volume is becoming catastrophically small. Oxygen is consumed, so to speak, from the emergency reserve. From there, where there is no one to add it.
This problem is slightly mitigated by the fact that some of the organic waste is not processed and does not fall under the influence of putrefactive bacteria, but remains in sedimentary rocks, forming peat, coal and similar minerals.
If the result of photosynthesis is oxygen, then its raw material is carbon.
Nitrogen
The nitrogen cycle in the biosphere is associated with the formation of such important organic compounds as proteins, nucleic acids, lipoproteins, ATP, chlorophyll and others. Nitrogen, in molecular form, is found in the atmosphere. Together with living organisms, this is only about 2% of all nitrogen on Earth. In this form, it can only be consumed by bacteria and blue-green algae. For the rest of the plant world, nitrogen in molecular form cannot serve as food, but can only be processed in the form of inorganic compounds. Some types of such compounds are formed during thunderstorms and fall into water and soil with rainfall.
The most active “recyclers” of nitrogen or nitrogen fixers are nodule bacteria. They settle in the cells of legume roots and convert molecular nitrogen into its compounds suitable for plants. After they die, the soil is also enriched with nitrogen.
Putrefactive bacteria break down nitrogen-containing organic compounds into ammonia. Some of it goes into the atmosphere, and the rest is oxidized by other types of bacteria to nitrites and nitrates. These, in turn, are supplied as food to plants and are reduced to oxides and molecular nitrogen by nitrifying bacteria. Which re-enter the atmosphere.
Thus, it is clear that various types of bacteria play the main role in the nitrogen cycle. And if you destroy at least 20 of these species, then life on the planet will cease.
And again the established circuit was broken by man. In order to increase crop yields, he began to actively use nitrogen-containing fertilizers.
Carbon
The carbon cycle in the biosphere is inextricably linked with the circulation of oxygen and nitrogen.
In the biosphere, the carbon cycle scheme is based on the life activity of green plants and their ability to convert carbon dioxide into oxygen, that is, photosynthesis.
Carbon interacts with other elements in a variety of ways and is part of almost all classes of organic compounds. For example, it is part of carbon dioxide and methane. It is dissolved in water, where its content is much higher than in the atmosphere.
Although carbon is not among the top ten in terms of prevalence, in living organisms it makes up from 18 to 45% of dry mass.
The oceans serve as a regulator of carbon dioxide levels. As soon as its share in the air increases, the water levels out the positions by absorbing carbon dioxide. Another consumer of carbon in the ocean is marine organisms, which use it to build shells.
The carbon cycle in the biosphere is based on the presence of carbon dioxide in the atmosphere and hydrosphere, which is a kind of exchange fund. It is replenished by the respiration of living organisms. Bacteria, fungi and other microorganisms that take part in the process of decomposition of organic residues in the soil also participate in the replenishment of carbon dioxide in the atmosphere. Carbon is “conserved” in mineralized, unrotten organic residues. In coal and brown coal, peat, oil shale and similar deposits. But the main carbon reserve fund is limestone and dolomite. The carbon they contain is “safely hidden” in the depths of the planet and is released only during tectonic shifts and emissions of volcanic gases during eruptions.
Due to the fact that the process of respiration with the release of carbon and the process of photosynthesis with its absorption passes through living organisms very quickly, only a small fraction of the planet’s total carbon participates in the cycle. If this process were nonreciprocal, then sushi plants alone would use up all the carbon in just 4-5 years.
Currently, thanks to human activity, the plant world has no shortage of carbon dioxide. It is replenished immediately and simultaneously from two sources. By burning oxygen during the operation of industry, production and transport, as well as in connection with the use of those “canned goods” - coal, peat, shale, and so on - for the work of these types of human activities. Why did the carbon dioxide content in the atmosphere increase by 25%.
Phosphorus
The phosphorus cycle in the biosphere is inextricably linked with the synthesis of organic substances such as ATP, DNA, RNA and others.
The phosphorus content in soil and water is very low. Its main reserves are in rocks formed in the distant past. With the weathering of these rocks, the phosphorus cycle begins.
Phosphorus is absorbed by plants only in the form of orthophosphoric acid ions. This is mainly a product of the processing of organic remains by gravediggers. But if the soils have a high alkaline or acidic factor, then phosphates practically do not dissolve in them.
Phosphorus is an excellent nutrient for various types of bacteria. Especially blue-green algae, which develops rapidly with increased phosphorus content.
However, most of the phosphorus is carried away with river and other waters into the ocean. There it is actively eaten by phytoplankton, and with it by seabirds and other species of animals. Subsequently, phosphorus falls to the ocean floor and forms sedimentary rocks. That is, it returns to the ground, only under a layer of sea water.
As you can see, the phosphorus cycle is specific. It is difficult to call it a circuit, since it is not closed.
Sulfur
In the biosphere, the sulfur cycle is necessary for the formation of amino acids. It creates the three-dimensional structure of proteins. It involves bacteria and organisms that consume oxygen to synthesize energy. They oxidize sulfur to sulfates, and single-celled prenuclear living organisms reduce sulfates to hydrogen sulfide. In addition to them, entire groups of sulfur bacteria oxidize hydrogen sulfide to sulfur and then to sulfates. Plants can only consume sulfur ion from the soil - SO 2-4. Thus, some microorganisms are oxidizing agents, while others are reducing agents.
The places where sulfur and its derivatives accumulate in the biosphere are the ocean and atmosphere. Sulfur enters the atmosphere with the release of hydrogen sulfide from water. In addition, sulfur enters the atmosphere in the form of dioxide when fossil fuels are burned in production and for domestic purposes. Primarily coal. There it oxidizes and, turning into sulfuric acid in rainwater, falls to the ground with it. Acid rain itself causes significant harm to the entire plant and animal world, and in addition, with storm and melt water, it enters rivers. Rivers carry sulfur sulfate ions into the ocean.
Sulfur is also contained in rocks in the form of sulfides, and in gaseous form - hydrogen sulfide and sulfur dioxide. At the bottom of the seas there are deposits of native sulfur. But this is all “reserve”.
Water
There is no more widespread substance in the biosphere. Its reserves are mainly in the salty-bitter form of the waters of the seas and oceans - about 97%. The rest is fresh water, glaciers and underground and groundwater.
The water cycle in the biosphere conventionally begins with its evaporation from the surface of reservoirs and plant leaves and amounts to approximately 500,000 cubic meters. km. It returns back in the form of precipitation, which falls either directly back into water bodies, or by passing through the soil and groundwater.
The role of water in the biosphere and the history of its evolution is such that all life from the moment of its appearance was completely dependent on water. In the biosphere, water has gone through cycles of decomposition and birth many times through living organisms.
The water cycle is largely a physical process. However, the animal and, especially, plant world takes an important part in this. The evaporation of water from the surface areas of tree leaves is such that, for example, a hectare of forest evaporates up to 50 tons of water per day.
If the evaporation of water from the surfaces of reservoirs is natural for its circulation, then for continents with their forest zones, such a process is the only and main way to preserve it. Here the circulation occurs as if in a closed cycle. Precipitation is formed from evaporation from soil and plant surfaces.
During photosynthesis, plants use the hydrogen contained in a water molecule to create a new organic compound and release oxygen. And, conversely, in the process of breathing, living organisms undergo an oxidation process and water is formed again.
Describing the circulation of various types of chemicals, we are faced with a more active human influence on these processes. Currently, nature, due to its multi-billion-year history of survival, is coping with the regulation and restoration of disturbed balances. But the first symptoms of the “disease” are already there. And this is the “greenhouse effect”. When two energies: solar and reflected by the Earth, do not protect living organisms, but, on the contrary, strengthen each other. As a result, the ambient temperature rises. What consequences of such an increase could there be, besides the accelerated melting of glaciers and the evaporation of water from the surfaces of the ocean, land and plants?
Video - Cycle of substances in the biosphere
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, during the combustion of 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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