Nitrogen makes up almost 78% of the atmosphere's mass. The main part of it forms N2 molecules from two atoms. Most organisms are not able to use this nitrogen due to the strong bond of the atoms. They require nitrogen in chemical forms such as ammonia, ammonium ions, nitrate and nitrite ions, which participate in chemical reactions with oxygen. Therefore, fixed nitrogen is important for this biogeochemical cycle.
The natural biogeochemical cycle of nitrogen is shown in Fig. 16. The total nitrogen flow into the biosphere is about 14·10 10 t/year. The main supplier of fixed nitrogen is nitrogen-fixing bacteria. The most famous of them are found in the nodules of legume plants. The traditional method of increasing fertility is based on their activities. Legumes are grown in the field, then they are plowed, and the nitrogen accumulated in the nodules passes through the soil. The following year, the field is sown with other crops that use this nitrogen. Some nitrogen binds during thunderstorms. An electrical discharge heats the air to a temperature at which various nitrogen oxides are formed. As in the case of carbon, a certain amount of nitrogen compounds comes from the depths of the Earth.
The reverse process - the reduction of nitrate ions is carried out by a chain of bacteria:
· ammonifying bacteria decompose nitrogenous organic compounds, forming ammonia (NH 3) or ammonium ions (NH 4 +);
· Nitrifying bacteria oxidize ammonia into nitrous acid – NO 2 –. (nitrites);
· nitrate bacteria convert nitrous acid into nitric acid – NO 3 – (nitrates) and the cycle begins again.
Rice. 15. Biogeochemical nitrogen cycle
Anthropogenic nitrogen flow into the biosphere approximately equal to natural. The greatest contribution comes from the use of nitrogen fertilizers (8·10 10 t/g). The consequence may be an increase in the content of nitrites, nitrates and nitrosamines in products with a wide range of toxic effects.
The source of nitrogen oxides (2·10 10 t/g) are many metallurgical processes, transport and fuel combustion in the production of heat and electricity. Nitrogen oxides are involved in the formation of acid rain and photochemical smog.
Ecosystems absorb a certain amount of nitrogen. Its excess is washed out and accumulates in water bodies. The process of increasing biogenic elements (not just nitrogen compounds) in water is called eutrophication. Its main causes are the discharge of industrial and municipal wastewater into water bodies, the chemicalization of agriculture and the concentration of livestock farming. Currently, this phenomenon has affected 90% of all lakes in the world. The process sometimes causes irreversible damage to aquatic ecosystems and deteriorates water quality (see section 6.2.3.). The main measures to reduce eutrophication are wastewater treatment and control over the use of fertilizers.
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
Carbon cycle.
The most intense biogeochemical cycle is the carbon cycle. IN
In nature, carbon exists in two main forms - in carbonates
(limestones) and carbon dioxide. The content of the latter is 50 times greater than
in the atmosphere. Carbon is involved in the formation of carbohydrates, fats, proteins and
nucleic acids.
The bulk is accumulated in carbonates on the ocean floor (1016 tons), in
crystalline rocks (1016 tons), coal and oil (1016 tons) and
participates in a large circulation cycle.
The main link of the large carbon cycle is the interconnection of processes
photosynthesis and aerobic respiration (Fig. 1).
Another link in the large carbon cycle is
anaerobic respiration (without oxygen); various types of anaerobic
bacteria convert organic compounds into methane and other substances
(for example, in swamp ecosystems, waste dumps).
The small cycle involves the carbon contained in
plant tissues (about 1011 tons) and animal tissues (about 109 tons).
Oxygen cycle.
In quantitative terms, the main component of living matter is
oxygen, the circulation of which is complicated by its ability to enter into
various chemical reactions, mainly oxidation reactions. IN
As a result, many local cycles occur between
atmosphere, hydrosphere and lithosphere.
(sedimentary calcites, iron ores), is of biogenic origin and should
considered as a product of photosynthesis. This process is the opposite
the process of oxygen consumption during breathing, which is accompanied by
destruction of organic molecules, interaction of oxygen with hydrogen
(split off from the substrate) and the formation of water. In some ways
The oxygen cycle resembles the reverse carbon dioxide cycle. IN
It mainly occurs between the atmosphere and living organisms.
Consumption of atmospheric oxygen and its replacement by plants in
The process of photosynthesis occurs quite quickly. Calculations show
that it takes about
two thousand years. On the other hand, in order for all water molecules
hydrospheres were subjected to photolysis and re-synthesized by living
organisms, it takes two million years. Most of the oxygen
produced during geological epochs, did not remain in the atmosphere, but
was fixed by the lithosphere in the form of carbonates, sulfates, iron oxides, and its
mass is 5.9 * 1016 tons. The mass of oxygen circulating in the biosphere in
the form of gas or sulfates dissolved in oceanic and continental
waters, several times less (0.4*1016 t).
Note that, starting from a certain concentration, oxygen is very
toxic to cells and tissues (even in aerobic organisms). And alive
an anaerobic organism cannot withstand (this was proven in the past
century L. Pasteur) oxygen concentration exceeding atmospheric by 1%.
Nitrogen cycle
Nitrogen gas results from the oxidation of ammonia,
formed during volcanic eruptions and decomposition of biological waste:
4NH3 + 3O2 (2N2 + 6H2O.
The nitrogen cycle is one of the most complex, but at the same time the most
ideal cycles. Despite the fact that nitrogen is about 80%
atmospheric air, in most cases it cannot be
directly used by plants, because they do not metabolize gaseous
nitrogen. The intervention of living beings in the nitrogen cycle is subject to strict
hierarchies: only certain categories of organisms can exert influence
into individual phases of this cycle. Nitrogen gas is continuously supplied to
atmosphere as a result of the work of some bacteria, while other bacteria
– fixatives (together with blue-green algae) constantly absorb it,
converting to nitrates. Nitrates are also formed inorganically in the atmosphere
as a result of electrical discharges during thunderstorms.
The most active consumers of nitrogen are bacteria on the root system
plants of the legume family. Each type of these plants has its own special
bacteria that convert nitrogen into nitrates. In the process of biological
cycle, nitrate ions (NO3-) and ammonium ions (NH4+) are absorbed by plants from
soil moisture are converted into proteins, nucleic acids, etc. Further
waste is generated in the form of dead organisms that are objects
vital activity of other bacteria and fungi, converting them into ammonia. So
a new cycle arises. There are organisms that can
convert ammonia into nitrites, nitrates and nitrogen gas. Basic links
The nitrogen cycle in the biosphere is represented by the diagram in Fig. 3.
The biological activity of organisms is supplemented by industrial
methods for producing nitrogen-containing organic and inorganic substances,
many of which are used as fertilizers to increase
productivity and plant growth.
The anthropogenic impact on the nitrogen cycle is determined by the following
processes:
1. combustion of fuel leads to the formation of nitrogen oxide, and then
reactions:
2. 2NO + O2 (2NO2,
3. 4NO2 + 2H2O.+ O2 (4HNO3,
4. contributing to acid rain;
5. as a result of the action of certain bacteria on fertilizers and wastes
livestock production produces nitrous oxide - one of the components
creating a greenhouse effect;
6. mining of minerals containing nitrate ions and ammonium ions,
for the production of mineral fertilizers;
7. When harvesting, nitrate ions and ammonium ions are removed from the soil;
8. runoff from fields, farms and sewers increases the amount of nitrates
ions and ammonium ions in aquatic ecosystems, which accelerates growth
algae and other plants; during the decomposition of the latter it is consumed
oxygen, which ultimately leads to the death of fish.
Phosphorus cycle
Phosphorus is one of the main components (mainly in the form and
) living matter and is part of nucleic acids (DNA and RNA),
cell membranes, adenosine triphosphate (ATP) and adenosine diphosphate (ADP),
fats, bones and teeth. The phosphorus cycle, like other nutrients,
elements, occurs in large and small cycles.
Phosphorus reserves available to living beings are entirely concentrated in
lithosphere. The main sources of inorganic phosphorus are igneous or
sedimentary rocks. In the earth's crust, the phosphorus content does not exceed 1%, which
limits the productivity of ecosystems. Inorganic from rocks of the earth's crust
phosphorus is involved in circulation by continental waters. It's absorbed
plants that, with its participation, synthesize various organic
compounds and are thus included in food chains. Then
organic phosphates together with corpses, waste and secretions of living beings
return to the ground, where they are again exposed to microorganisms and
converted into mineral forms used by green plants.
In the ocean ecosystem, phosphorus is carried by flowing waters, which
promotes the development of phytoplankton and living organisms.
In terrestrial systems, the phosphorus cycle occurs at optimal levels.
natural conditions with a minimum of losses. In the ocean, things are different. This
associated with constant settling (sedimentation) of organic substances.
Organic phosphorus settled at a shallow depth returns to the cycle.
Phosphates deposited at great sea depths do not participate in shallow
cycle. However, tectonic movements contribute to the rise of sedimentary
rocks to the surface.
Thus, phosphorus is slowly moved out of phosphate deposits
on land and shallow ocean sediments to living organisms and back
Considering the phosphorus cycle on the scale of the biosphere for a relatively
short period, we can conclude that it is not completely closed. Reserves
phosphorus on earth is small. Therefore, it is believed that phosphorus is the main factor
limiting the growth of primary production of the biosphere. It is even believed that phosphorus is
the main regulator of all other biogeochemical cycles, it is the most
a weak link in the chain of life that ensures human existence.
The anthropogenic influence on the phosphorus cycle is as follows:
1. mining of large quantities of phosphate ores for mineral fertilizers and
detergents leads to a decrease in the amount of phosphorus in
biotic cycle;
2. runoff from fields, farms and municipal waste leads to an increase
phosphate ions in water bodies, to the sharp growth of aquatic plants and
imbalance in aquatic ecosystems.
Sulfur cycle
From natural sources, sulfur enters the atmosphere in the form of hydrogen sulfide,
sulfur dioxide and particles of sulfate salts (Fig. 5).
About one third of sulfur compounds and 99% of sulfur dioxide are anthropogenic
origin. Reactions occur in the atmosphere that lead to acidic
2SO2 + O2 (2SO3,
SO3 + H2O (H2SO4 .
The water cycle
Water, like air, is the main component necessary for life. IN
quantitatively it is the most common inorganic
component of living matter. Seeds of plants in which the water content is not
exceeds 10%, belong to forms of slow life. Same phenomenon
(anhydrobiosis) is observed in some animal species that, when
unfavorable external conditions can lose most of the water in their
Water in three states of aggregation is present in all components
parts of the biosphere: atmosphere, hydrosphere and lithosphere. If the water located
in various hydrogeological forms, evenly distributed over
corresponding regions of the globe, then layers of the following are formed
thickness: for the World Ocean 2700 m, for glaciers 100 m, for groundwater
15 m, for surface fresh water 0.4 m, for atmospheric moisture 0.03 m.
The main role in the circulation and biogeochemical cycle of water is played by
atmospheric moisture, despite the relatively small thickness of its layer.
Atmospheric moisture is distributed unevenly across the Earth, which causes
large differences in the amount of precipitation in different regions of the biosphere. Average
geographical latitude. For example, at the North Pole it is 2.5 mm (in
column of air with a cross section of 1 cm2), at the equator - 45 mm.
The mechanism of the hydrogeological cycle was discussed above - in the section
concerning the description of the features of the hydrosphere. The water that fell onto land then
used for percolation (or infiltration), evaporation and runoff.
Seepage is especially important for terrestrial ecosystems as it contributes to
supplying the soil with water. During the process of infiltration, water enters aquifers
horizons and underground rivers. Evaporation from the soil surface also plays a role
important role in the water regime of the area, but a more significant amount
water is released by the plants themselves with their foliage. Moreover, the amount of water
secreted by plants, the more, the better they are supplied with it. Plants,
producing one ton of plant matter consume at least 100 tons
The main role in the water cycle on continents is played by the total
evaporation (trees and soil).
The last component of the water cycle on land is runoff. Surface
flow and resources of underground aquifers provide water supply
streams. At the same time, with a decrease in the density of vegetation cover, the runoff
becomes the main cause of soil erosion.
As already noted, water also participates in the biological cycle, being
source of oxygen and hydrogen. However, its photolysis during photosynthesis does not
plays a significant role in the cycle process.
Biogeochemical cycles
Unlike energy, which once used by the body,
turns into heat and is lost to the ecosystem, substances circulate in
biosphere, which is called biogeochemical cycles. Out of more than 90
About 40 elements found in nature are needed by living organisms.
The most important for them and required in large quantities: carbon,
hydrogen, oxygen, nitrogen. Oxygen enters the atmosphere as a result
photosynthesis and is consumed by organisms during respiration. Nitrogen is extracted from
atmosphere due to the activity of nitrogen-fixing bacteria and returns to
it with other bacteria.
The cycles of elements and substances are carried out due to
self-regulating processes in which all components participate
ecosystems These processes are waste-free. There is nothing in nature
useless or harmful, even volcanic eruptions have benefits,
since the necessary elements enter the air with volcanic gases,
for example nitrogen.
There is a law of global closure of the biogeochemical cycle in
biosphere, operating at all stages of its development, as well as the rule of increase
closedness of the biogeochemical cycle during succession. In progress
evolution of the biosphere, the role of the biological component in the closure increases
biogeochemical cycle. An even greater role for biogeochemical
the cycle is exerted by man. But his role is the opposite
direction. Man disrupts the existing cycles of substances, and in this
its geological power manifests itself, destructive to the biosphere
to date.
When life appeared on Earth 2 billion years ago, the atmosphere
consisted of volcanic gases. It contained a lot of carbon dioxide and little
oxygen (if there was any at all), and the first organisms were anaerobic. Because
production on average exceeded respiration, over geological time in
oxygen accumulated in the atmosphere and the carbon dioxide content decreased.
burning large quantities of fossil fuels and reducing absorption
green belt abilities. The latter is the result of a decrease
the number of green plants themselves, and is also due to the fact that dust and
Pollutant particles in the atmosphere reflect the rays entering the atmosphere.
As a result of anthropogenic activity, the degree of isolation
biogeochemical cycles decrease. Although it is quite high (for
different elements and substances it is not the same), but nevertheless not
absolute, as shown by the example of the emergence of an oxygen atmosphere.
Otherwise, evolution would be impossible (the highest degree of closedness
biogeochemical cycles observed in tropical ecosystems –
the most ancient and conservative).
Thus, we should not talk about a person changing what is not
should change, but rather about the human influence on speed and direction
changes and expansion of their boundaries, violating the rule of transformation measures
nature. The latter is formulated as follows: during operation
natural systems cannot exceed certain limits that allow these
systems to maintain self-sustaining properties. Violation of measures as to the side
increasing as well as decreasing leads to negative
results. For example, excess fertilizer application is as harmful as
flaw. This sense of proportion has been lost by modern man, who believes that
in the biosphere everything is allowed to him.
Hopes for overcoming environmental difficulties are pinned on
in particular, with the development and commissioning of closed
technological cycles. Human-created material transformation cycles
it is considered desirable to arrange them so that they are similar to natural
cycles of substance circulation. Then the problems would be solved at the same time
providing humanity with irreplaceable resources and the problem of protecting
natural environment from pollution, since today only 1 - 2% of the weight of natural
resources are utilized in the final product.
Theoretically, closed cycles of substance transformation are possible. However
complete and final restructuring of the industry according to the principle of circulation
substances in nature is not real. At least a temporary break in isolation
technological cycle is almost inevitable, for example, when creating
synthetic material with new properties unknown to nature. This
the substance is first thoroughly tested in practice, and only then can
methods of its decomposition must be developed in order to introduce its constituent parts
into natural cycles.
Related information.
Nitrogen and its compounds play the same important and irreplaceable role in the life of the biosphere as carbon. The biophilicity of nitrogen is comparable to the biophilicity of carbon. The index of biogenic enrichment of soils in relation to the earth's crust, and of plants in relation to soils, is 1000 and 10000 for nitrogen, respectively (Kovda, 1985).
The main reservoir of nitrogen in the biosphere is also the air envelope. About 80% of all nitrogen reserves are concentrated in the planet’s atmosphere, which is associated with the direction of biogeochemical flows of nitrogen compounds formed during denitrification. The main form in which nitrogen is contained in the atmosphere is molecular - N 2. As minor impurities, the atmosphere contains various nitrogen oxide compounds NOx, as well as ammonia NH3. The latter is the most unstable under the conditions of the earth's atmosphere and is easily oxidized. At the same time, the value of the redox potential in the atmosphere is not sufficient for the stable existence of oxide forms of nitrogen, which is why its free molecular form is the main one.
Primary nitrogen in the atmosphere probably appeared as a result of degassing processes in the upper mantle and from volcanic exudates. Photochemical reactions in high layers of the atmosphere lead to the formation of nitrogen compounds and their noticeable entry onto land and into the ocean with precipitation (3-8 kg/ha of ammonium nitrogen per year and 1.5-6 kg/ha of nitrate nitrogen). This nitrogen is also included in the general biogeochemical flow of dissolved compounds migrating with water masses and participates in soil-forming processes and in the formation of plant biomass.
Unlike carbon, atmospheric nitrogen cannot be directly used by higher plants. Therefore, fixative organisms play a key role in the biological nitrogen cycle. These are microorganisms of several different groups that have the ability, through direct fixation, to directly extract nitrogen from the atmosphere and, ultimately, fix it in the soil. These include:
· some free-living soil bacteria;
· symbiont nodule bacteria (existing in symbiosis with legumes);
· cyanobionts, which are also symbionts of fungi, mosses, ferns, and sometimes higher plants.
As a result of the activity of nitrogen fixing organisms, it is bound in soils in nitrite form (compounds based on NH 3).
Nitrite nitrogen compounds are capable of migrating in aqueous solutions. At the same time, they are oxidized and converted into nitrates - salts of nitric acid HNO 3. In this form, nitrogen compounds can be effectively absorbed by higher plants and used for the synthesis of protein molecules based on peptide C-N bonds. Further, through trophic chains, nitrogen enters the organisms of animals. It returns to the environment (in aqueous solutions and soil) in the processes of excretory activity of animals or decomposition of organic matter.
The return of free nitrogen to the atmosphere, as well as its extraction, is carried out as a result of microbiological processes. This link in the cycle functions thanks to the activity of soil denitrifying bacteria, which again convert nitrogen into molecular form.
In the lithosphere, as part of sedimentary deposits, a very small part of nitrogen is bound. The reason for this is that mineral nitrogen compounds, unlike carbonates, are very soluble. The loss of a certain amount of nitrogen from the biological cycle is also compensated by volcanic processes. Thanks to volcanic activity, various gaseous nitrogen compounds enter the atmosphere, which, under the conditions of the Earth's geographic envelope, inevitably turns into a free molecular form.
Thus, the main specific features of the nitrogen cycle in the biosphere can be considered the following:
· predominant concentration in the atmosphere, which plays the exclusive role of a reservoir from which living organisms draw the reserves of nitrogen they need;
· a leading role in the nitrogen cycle of soils and, in particular, soil microorganisms, whose activity ensures the transition of nitrogen in the biosphere from one form to another (Fig. 3.5.3).
Rice. 3.5.3. Scheme of the biogeochemical nitrogen cycle
Therefore, the biosphere contains a huge amount of nitrogen in bound form: in organic matter of the soil cover (1.5x10 11 t), in plant biomass (1.1x10 9 t), in animal biomass (6.1x10 7 t). Nitrogen is also found in large quantities in some biogenic minerals (saltpeter).
At the same time, there is a paradox - with a huge nitrogen content in the atmosphere due to the extremely high solubility of nitric acid salts and ammonium salts, there is little nitrogen in the soil and almost always not enough to feed plants. Therefore, the need of cultivated plants for nitrogen fertilizers is always high. Therefore, according to various estimates, from 30 to 35 million tons of nitrogen are added to the soil annually in the form of mineral fertilizers. Thus, inputs from nitrogen fertilizers account for 30% of the total nitrogen inputs to land and oceans. This often leads to significant environmental pollution and serious illnesses for humans and animals. The losses of nitrate forms of nitrogen are especially large, since it is not sorbed by the soil, is easily washed out by natural waters, is reduced to gaseous forms, and up to 20-40% of it is lost for plant nutrition. A significant disruption of the nitrogen cycle is the ever-increasing amount of animal waste, industrial waste and wastewater from large cities, the release of ammonium and nitrogen oxides into the atmosphere when burning coal, oil, fuel oil, etc. The penetration of nitrogen oxides into the stratosphere (exhausts from supersonic aircraft, rockets, nuclear explosions) is dangerous, as this can cause the destruction of the ozone layer. All this naturally affects the biogeochemical nitrogen cycle.
The history of the development of biogeochemical nitrogen cycles on the planet is complex and contradictory. Nitrogen entered the composition of the terrestrial planet as a result of the condensation of interstellar cosmic protoplanetary matter, which included nitrogen and its various compounds (NO, NH 3, HC 3 N, etc.).
Radioactive heating of the planet and the formation of a molten mantle were accompanied by the release of gaseous nitrogen compounds and its accumulation in the primary atmosphere, in which N 2 dominates (n · 10 15 t) even today. Cooling lava and gas fumaroles of volcanoes continue to supply nitrogen, its oxides, ammonium chloride and ammonium carbonate to the biosphere.
Electrochemical discharges, photochemical reactions, ultra-high temperatures and pressure contributed to the emergence of non-cellular molecular forms of organic nitrogen compounds on the planet.
The appearance of free-living nitrogen-fixing bacteria and heterotrophic bacteria probably marked the beginning of the biogenic enrichment of the primary biosphere with nitrogen compounds, the formation of amino acids, proteins, and mineral nitrogen compounds (ammonium, nitrate salts). It is possible that biogenic nitrogen fixation preceded the emergence of photosynthesis, occurred in an oxygen-free anaerobic environment of the distant past and was carried out by microorganisms of the Clostridium type. Bacteria of this genus are still the most important agents of nitrogen fixation under anaerobic conditions.
Biological fixation of nitrogen by microorganisms is much more widespread in nature than was thought 20-30 years ago. In addition to bacteria of the Rhizobium group, which fix nitrogen in nodule formations on the roots of legumes, non-symbiotic (associative) nitrogen fixation by numerous heterotrophic bacteria and fungi is widely developed (Umarov, 1983). This type of nitrogen fixation is carried out by hundreds of species of various microorganisms living in the rhizosphere of plants, in the soil and on the surface of stems and leaves (phyllosphere).
On average, associative (non-symbiotic) nitrogen fixation in ecosystems is 40-50 kg/ha per year; but in the world literature there are indications that non-symbiotic nitrogen fixation in tropical conditions reaches 200-600 kg/ha per year (Umarov, 1983). In this case, most (> 90%) of the nitrogen mass is fixed in the rhizosphere using the energy of root exudates and dying small roots. Therefore, in the presence of vegetation cover, soils always fix several times more nitrogen than soils of pure fallow.
As established by the studies of Umarov (1983), associative nitrogen fixation is characteristic of most species of herbaceous and many woody plants, including their cultivated forms. Meadow, chernozem and chestnut soils (90-330 kg/ha), as well as mountain forest soils of the Caucasus (up to 180 kg/ha) have a high potential for nitrogen fixation in the rhizosphere. During the growing season in the fields alone, this type of fixation can give the soil 30-40 kg/ha of additional nitrogen. This is not surprising, since nitrogen-fixing microorganisms can make up from 20 to 80% of their total number.
There is a clear positive relationship between the processes of nitrogen fixation by microorganisms and plant photosynthesis in ecosystems. The higher the photosynthetic productivity of plants, the more nitrogen is fixed in soils. This is the most important mechanism of nitrogen biogeochemistry in the biosphere and in agriculture.
The role of blue-green algae in the biogeochemistry of nitrogen is great, numerous species of which also have the ability to fix nitrogen simultaneously with the process of photosynthesis. Blue-green algae (Cyanophyta) enrich soils with nitrogen, especially irrigated rice fields, river, lake and swamp waters and sediments. But they also live on the surface of bare rocks or desert soils.
The development of vegetation cover and plant-associated microorganisms has significantly increased the involvement of atmospheric nitrogen into the composition of biomass. The increasing complexity of life forms on the planet caused the lengthening of food chains, the accumulation of living and dead organic matter on land and in the ocean. This created the possibility of long-term existence of organic nitrogen compounds in the biosphere and lithosphere. The role of herbaceous plants is especially great in this. The above-ground and underground parts of herbaceous vegetation annually consume from 20-25 to 600-700 kg/ha of nitrogen (usually the roots contain 2-6 times more nitrogen than the above-ground part). In this case, the total biomass, as a rule, contains 10-50 times more carbon than nitrogen. All this confirms the enormous overall role of carbon and nitrogen in the creation of phytomass (Titlyanova, 1979). But nitrogen compounds are easily leached from plant tissues by rain moisture. When they enter the soil, they are re-consumed by plants.
How complex and little studied the biogenic nitrogen cycles are is evidenced by the established facts of the transfer of nitrogen compounds from plant to plant (of the same or different species) through root secretions into the soil, and possibly by direct contact of roots. This amazing mechanism shows how “economical” plants are in nitrogen nutrition. This phenomenon probably also exists in the biogeochemistry of other elements.
As is known, the protein content of wheat grains and the nitrogen content in them increases with a decrease in atmospheric precipitation in the steppes of the Russian Plain. This has already been established for the content of total nitrogen in the biomass of herbaceous plants. Under steppe conditions, the nitrogen content in dry grass biomass reaches 2-2.6%; with increasing humidity it decreases to 1-1.5%.
All these facts indicate the enormous role of vegetation (especially grasses) and microorganisms in the biogeochemistry of nitrogen on land. The development of vegetation cover, the emergence of the soil-forming process (300-400 million years ago), the formation of a humus shell and soil fine earth, its demolition and accumulation in the form of sedimentary rocks expanded the process of transferring atmospheric nitrogen into the biosphere, raising its content in the latter to the level n 10 15 t.
At the same time, it must be emphasized that the return of nitrogen to the atmosphere through denitrification is as universal a process as fixation and nitrification. This process ensures the global nitrogen cycle on the planet.
Redox conditions within soils are very heterogeneous. Even in aerated soils there are areas where oxygen is deficient where denitrification can occur. The abundance of fresh mobile organic matter and the supersaturation of soils with moisture always sharply intensify the processes of denitrification after rains, during waterlogging, and during irrigation. Denitrification is even more pronounced in aquatic landscapes (swamps, lakes, estuaries, etc.).
This directed planetary biogeochemical process is polycyclic in nature. The predominant part of nitrogen fixed in nature, through microcyclic repeated transformations, nitrification and denitrification, is ultimately returned in the form of molecular gaseous nitrogen (N 2) to the atmosphere. But as the biosphere developed, the duration of existence and the size of the mass of organic and mineral biogenic nitrogen compounds on the planet increased. The amount of buried organic sediments increased. The duration of individual microcycles of the general terrestrial biogeochemical nitrogen cycle fluctuates in the present era from short (days, weeks, months) in the tissues of microorganisms to significant (years) in ecosystems of herbaceous vegetation and to long (decades, centuries, millennia) in woody ecosystems and in soil humus. Complete earth cycles of nitrogen found in sediments of rivers, lakes, seas, and in combustible fossils of the earth's crust span a time of the order of tens of thousands of years, hundreds of thousands and millions of years.
Natural biogeochemical cycles of nitrogen (as well as carbon) in the biosphere were “almost closed,” but had the character of a directed, expanded reproduction of reserves in the biosphere. The biosphere not only did not give up the completely captured masses of nitrogen and carbon, but progressively increased their total reserves in a fixed form (in humus, peat, in the mass of fossil coals, oil, shale, bitumen, etc.).
The anthropogenic era has brought significant changes to the established natural nitrogen cycles. The main thing that has happened and is happening is (besides agriculture) the emergence in the biosphere of a new anthropogenic industrial mechanism for fixing masses of nitrogen in the form of tens of millions of tons of nitrogen fertilizers, as well as the entry into the environment of nitrogen oxides from large masses of burned fossil fuels (heating plants, transport, aviation , rockets). Technogenic sources of nitrogen compounds in the biosphere are growing rapidly, doubling every 6-7 years. Already in the 70-80s of the XX century. Every year the world produces (in terms of nitrogen) 50-60 million tons/year of nitrogen fertilizers. At the beginning of the 21st century. this value can reach 100-150 million tons/year. It is likely that by this time the technogenic influx of nitrogen into the biosphere may be equal to or exceed all biogenic forms of its supply.
In the anthropogenic era, especially in the modern period, the process of enriching the environment with nitrogen compounds has noticeably intensified. As we noted earlier, there is a process of technogenic nitrogenization of the environment, accompanied by a complex set of positive (increased yields, increased proportion of proteins in the diet) and negative (cancer, methohemoglobinemia, increased acidity of soils and precipitation) consequences. The destruction of forests, steppes (and mycorrhizae), the replacement of legumes with cereals, the destruction of humus horizons of soils rich in microflora, and the reduction of soil surface also caused additional changes in the biogeochemistry of nitrogen in the biosphere. All these changes, often of an opposite nature, have not been studied or quantified. Apparently, there is still a tendency to reduce the role of biogenic nitrogen fixation in the general cycle of nitrogen on the planet.
It was against this background of disturbances in the normal nitrogen cycle in nature that mineral fertilizers of soils introduced the above-mentioned changes in the input items of the nitrogen balance and in the geography of its distribution, and also increased the overall level of concentration of nitrates and ammonium salts in soils and waters. But an even more serious factor in the imbalance, level of concentration and forms of nitrogen compounds in the atmosphere and especially in the hydrosphere and soils turned out to be the modern fuel, energy and transport economy.
According to approximate data, the emission of ammonia and various nitrogen oxides from the combustion of coal, oil, fuel oil, gasoline, peat, shale, etc. together amounts annually to about 200-350 million tons in the form of gases and aerosols. The oxidation of ammonia and nitrogen oxides leads to the formation of mainly nitric acid and partly ammonium salts, which fall on land and the surface of the ocean. If these figures are exaggerated even by a factor of two, we still have to admit that the emission of nitrogen compounds into the atmosphere has already become a noticeable component in the revenues of the nitrogen cycle on our planet.
In light of these facts, it is necessary to better understand the future needs of agriculture for nitrogen fertilizers, the paths of global, air and water migration of nitrogen compounds on the planet, and to find out the areas where the accumulation of nitrate and ammonium compounds predominantly occurs. This is all the more necessary because emissions of nitrogen oxides into the atmosphere will continue and even increase. The facts of precipitation of acidified atmospheric waters in Canada, Scandinavia, and the USA have already been established, which is accompanied by a decrease in the pH of soils and local waters (usually under the influence of combined precipitation with dilute solutions of sulfuric acid). Acidification of the environment will increase the weathering of minerals, the removal of calcium, magnesium and other plant nutrients from the soil, which will increase the need for liming of fields.
One more factor that disrupts the normal level of concentration and nitrogen cycle in nature should be pointed out. This is waste from industrial livestock and poultry farming, as well as waste and sewage from modern large cities. The waste and effluents of this origin are very large. There are more than 3 billion livestock in the world, producing huge amounts of waste. Modern poultry farms, industrial livestock enterprises, and cities create numerous pockets of abnormally high levels of nitrogen and phosphorus in the form of organic and mineral compounds, which locally saturate soils, streams, rivers, lakes, estuaries and estuaries. Sometimes in such soils the N-NO 3 content reaches 400 parts per million (ppm), and N-NH 4 - up to 2200 ppm. According to scientists, urban runoff, livestock waste, and soil erosion play an equally important, and sometimes even greater, role in polluting soils and waters with nitrogen compounds to toxic levels (Cooke and Williams, 1970).
An increase in the concentration of nitrogen compounds in natural waters is an alarming fact. In river waters of forest areas of temperate climates, the nitrate content reaches 0.3-0.5 mg/l, and in arid climates - 1.2-1.7 mg/l. In drainage waters of irrigation systems, the concentration of NO 3 is usually about 5-6 mg/l, but it can also be 10-15 mg/l. In soil solutions of saline irrigated soils, NO 3 concentrations of up to 100-300 mg/l were observed. In groundwater there is sometimes a concentration of nitrates of the order of 10-15 and even 50-100 mg/l. Over 25 years (1945-1970) of regular observations in the state of Illinois, the content of nitrate nitrogen in surface runoff waters, according to average and maximum data, increased two, three and even four times.
Not only surface waters are enriched with excess concentrations of nitrates, but also groundwater - the main source of drinking water supply to the population. Nitrates penetrate into groundwater to depths of 10-15 m and even more, causing their concentration to increase to 10-15 mg/l N, which is clearly dangerous for people (in terms of NO 3 this is 45-60 mg/l).
The total nitrogen balance for the United States was calculated (Accumulation of Nitrate, 1972). The total input of nitrogen into US soils is expressed as 21.0 million tons of N per year (including 5.6 million tons with precipitation, 7.5 million tons with mineral fertilizers and biogenic fixation 4.8 million tons) . Of this amount, about 17 million tons are used for food production and textile raw materials, and 4 million tons are not used.
All types of denitrification (including more than 10 million tons in the aquatic environment) amount to about 18.5 million tons, and about 1.5 million tons remain in soils and waters annually. The denitrification data here is clearly exaggerated. The nitrogen balance in waters and soils is at least two to three times higher. As a result of considering the elements of the modern biogeochemical nitrogen cycle on land, the following are outlined: main forms receipts of its connections:
- biogenic nitrogen fixation in soils by microorganisms of symbiotic and non-symbiotic types;
- entry into solutions with metabolites of food chains, with dead organic matter, with mineralization products of soil organic matter;
- intake of nitrogen oxides from fossil fuel combustion products;
- introducing nitrogen compounds into soils in the form of organic and mineral fertilizers;
- transfer and accumulation of nitrates during evaporation of groundwater.
Expense items The nitrogen balance on land consists of the following main forms:
- absorption of mineral nitrogen compounds by higher and lower plants and their entry into the food chains of ecosystems;
- transition of nitrogen compounds into organic forms with the formation of humus;
- denitrification and eventual return to the atmosphere of most of the nitrogen in gaseous molecular form N 2 and partly in the form of oxides and ammonia;
- flushing, removal and alienation of nitrogen compounds from biological cycles into geological ones; burial for a geologically long time in sedimentary rocks, fossil fuels or salt deposits.
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