Nitrogen Cycle

Nitrogen Cycle

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Plants require a number of elements other than those they obtain directly from the atmosphere (carbon and oxygen in the form of carbon dioxide) and from groundwater (hydrogen and oxygen).

All but one of these elements comes from the disintegration of rocks and is captured by plants from the ground. The exception is nitrogen, which represents 78% of the earth's atmosphere.

At earth surface rocks They are also the primary source of nitrogen, which penetrates the soil, indirectly through the atmosphere, and through the soil, penetrates the plants that grow on it.

Most living things are unable to use atmospheric nitrogen to synthesize proteins and other organic substances. Unlike carbon and oxygen, nitrogen is very chemically unreactive, and only certain bacteria and blue algae They have the highly specialized ability to assimilate nitrogen from the atmosphere and convert it into a form that can be used by cells. Usable nitrogen deficiency is often the main limiting factor for plant growth.

The process by which nitrogen circulates through plants and soil through the action of living organisms is known as the nitrogen cycle.


Much of the nitrogen found in soil comes from dead organic materials, which exist in the form of complex organic compounds such as proteins, amino acids, nucleic acids and nucleotides. However, these nitrogenous compounds are generally rapidly decomposed into simpler substances by soil-living organisms.

At saprophytic bacteria and various fungal species are primarily responsible for the decomposition of dead organic materials. These microorganisms use proteins and amino acids as the source for their own proteins and release excess nitrogen in the form of ammonium (NH4+). This process is called ammonification. Nitrogen may be supplied as ammonia gas (NH3), but this process usually occurs only when decomposing large amounts of nitrogen-rich materials, such as in a large portion of fertilizer or fertilizer. In general, ammonia produced ammonia is dissolved in soil water, where it combines with protons to form the ammonium ion.


Several species of bacteria commonly found in soils are capable of oxidizing ammonia or ammonium. Ammonia oxidation, known as nitrificationis a process that produces energy and the released energy is used by these bacteria to reduce carbon dioxide, just as autotrophic plants use light energy to reduce carbon dioxide. Such organisms are known as chemosynthetic autotrophic agents (different from photosynthetic autotrophs such as plants and algae). At nitrifying bacteria chemosynthetic Nitrosomonas and Nitrosococcus oxidize ammonia to nitrite (NO2-):

2 NH 3 + 302 --------> 2 NO2- + 2 H+ + 2 H2O

(ammonia gas) (nitrite)

Nitrite is toxic to higher plants, but rarely accumulates in soil. Nitrobacter, another genus of bacteria oxidizes nitrite to form nitrate (NO3-), again with energy release:

2 NO2- + O2 ---------> 2 NO3-

(nitrite) (nitrate)

Nitrate is the form in which almost all nitrogen moves from the soil into the roots.

Few plant species are able to use animal protein as a source of nitrogen. These species, which comprise the carnivorous plants, have special adaptations used to attract and capture small animals. They digest by absorbing nitrogenous compounds and other organic and mineral compounds such as potassium and phosphate. Most carnivorous plants are found in swamps, which are generally strongly acidic and therefore unfavorable to the growth of nitrifying bacteria.

Nitrogen Loss

As we have observed, the nitrogen compounds of chlorophyllate plants return to the soil upon their death (or the animals that fed on them), being reprocessed by the soil organisms and microorganisms, absorbed by the roots in the form of nitrate dissolved in the soil water. converted to organic compounds. During this cycle there is always a "loss" of a certain amount of nitrogen, making it unusable for the plant.

One of the main causes of this nitrogen loss is soil removal plants. Cultivated soils often exhibit a steady decline in nitrogen content. Nitrogen can also be lost when the topsoil is beheaded by erosion or when its surface is destroyed by fire. Nitrogen is also removed by leaching; nitrates and nitrites, which are anions, are particularly susceptible to water leaching through the soil. In some soils, denitrifying bacteria break down nitrates and release nitrogen into the air. This process that supplies the bacteria with the oxygen needed for respiration is costly in terms of energy needs (ie.2 can be reduced faster than NO3-) and occurs extensively only in oxygen-deficient soils, that is, in soils that are poorly drained and therefore poorly ventilated.

Sometimes a high proportion of nitrogen in the soil is not available to plants. This immobilization occurs when there is excess carbon. When carbon-rich but nitrogen-poor organic substances, straw is a good example, if they are in abundance in the soil, the microorganisms that attack these substances will need more nitrogen than they contain in order to fully utilize the carbon present. As a result, they will not only use the nitrogen present in the straw or similar material, but also all available nitrogen salts in the soil. Consequently, this imbalance tends to normalize as carbon is supplied as carbon dioxide by microbial respiration, and as the ratio of nitrogen to carbon in the soil increases.

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Nitrogen fixation

As we can see, if all the nitrogen that is removed from the ground were not constantly replenished, practically life-giving on this planet would finally disappear. Nitrogen is replenished in the soil by nitrogen fixation. Nitrogen fixation is the process by which nitrogen gas in the air is incorporated into nitrogenous organic compounds and thus introduced into the nitrogen cycle. Fixation of this gas, which can be done to a considerable degree by only a few bacteria and blue algae, is a process on which all living organisms today depend, just as they all ultimately depend on photosynthesis for obtaining energy.

One to two hundred million metric tons of nitrogen is added to the earth's surface each year by biological systems. Man produces 28 million metric tons, most of which are used as fertilizers; However, this process is carried out with high energy cost in terms of fossil fuels. The total amount of energy required for ammonium fertilizer production is currently estimated to be equivalent to 2 million barrels of oil per day. Indeed, it is estimated that the costs of nitrogen fertilization are reaching the point of diminishing profits. Traditional crops in areas such as India do not achieve significantly increased yields using nitrogen fertilizers but have low nitrogen requirements, but are now being replaced by “miracle cereals” and other crops that no longer produce with nitrogen fertilization. - precisely at a time when such treatment is becoming prohibitively expensive.

Of the various classes of nitrogen-fixing organisms, symbiotic bacteria are by far the most important in terms of the total amounts of nitrogen fixed. The most common nitrogen-fixing bacteria is Rhizobium, which is a type of bacteria that invades the roots of legumes (angiosperms of the family Fabaceae or Leguminosae) such as clover, pea, beans, vetches and alfalfa.

The beneficial effects of legumes on the soil are so obvious that they were recognized hundreds of years ago. Theophrastus, who lived in the third century BC wrote that the Greeks used bean crops to enrich the soil. Where legumes grow, a certain amount of “extra” nitrogen can be released into the soil, where it becomes available to other plants. In modern agriculture it is common practice to alternate a non-legume crop such as maize with a legume such as alfalfa. The legumes are then harvested for hay leaving the nitrogen-rich roots, or even better, plowed back in the field. A good crop of alfalfa, which is relocated to the ground, can provide 450 kilograms of nitrogen per hectare. Application of the trace elements cobalt and molybdenum required by symbiotic bacteria greatly increases nitrogen production if these elements are present in limited quantities, as in much of Australia.

Free-living nitrogen-fixing microorganisms

Non-symbiotic bacteria of genera Azotobacter and Clostridium are able to fix nitrogen. Azotobacter is aerobic, whereas Clostridium is anaerobic; Both are common saprophytic bacteria found in the soil. They are estimated to probably supply about 7 kilograms of nitrogen per hectare of soil per year. Another important group includes many photosynthetic bacteria. Free-living blue algae also play an important role in nitrogen fixation. They are crucial for rice cultivation, which is the main diet of more than half of the world's population. Blue algae can also play an important ecological role in nitrogen fixation in the oceans.

The distinction between nitrogen fixation by free-living and symbiotic organisms may not be as strict as traditionally thought. Some microbes occur regularly in the soil around the roots of certain carbohydrate-depleting plants by consuming these compounds and at the same time indirectly providing nitrogen to the plants. Symbiotic associations between normally free-living bacteria such as Azotobacter, and higher plant cells in tissue cultures induced their growth in a nitrogen-deprived artificial medium.

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