Nitrogen cycle

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Graphic representation of the nitrogen cycle

The nitrogen cycle or nitrogen cycle is the constant migration and biogeochemical conversion of the bio-element nitrogen in the earth's atmosphere , in water , in soils and in biomass .


There are 10 15 tons of nitrogen in the earth's atmosphere , almost exclusively as molecular air nitrogen (N 2 ). Other nitrogenous gases in the atmosphere, such as In contrast, nitrogen oxides and ammonia do not play a major role in terms of quantity, but they do play a role as air pollutants. These gases enter the atmosphere as by-products of oxidative processes in fires and explosions (lightning strikes, volcanism, combustion engines) or in reductive degradation processes (excretions, putrefaction) as air pollutants. As water-soluble compounds, these gases are washed out by the rain, then take part in the nitrogen cycle, but by no means alone can cover the nitrogen requirements of plants and other living beings .

Nitrogen has to be absorbed by all living beings, because the chemical element nitrogen is a component of amino acids in proteins , nucleic acids , special lipids such as sphingolipids and other essential substances. For growth and livelihood, all living things have to absorb nitrogen-containing compounds with food from the environment (nitrogen assimilation ). Through the excretions of living beings and after their death, the nitrogen is released again from the dead biomass , ultimately also in the form of nitrogen-containing compounds. Living beings are at the center of the nitrogen cycle in the surface layers of the earth.

Material balances and estimates show that the nitrogen cycle works despite its bottlenecks. Thus, the available nitrogen during the Earth's average been 900 to 1000 times of organisms in their body built and excreted, but while some 900,000 times was inhaled and exhaled without him was there chemically modified. For comparison: the earth's air and oceanic oxygen has been used an average of around 60 times by “Factory Life”, incorporated into biomass and then excreted again.

The molecules of the element nitrogen N 2 consist of two nitrogen atoms covalently linked by a triple bond . Since the triple bond can only be broken with a high expenditure of energy, the N 2 molecule is very inert and cannot be used either by plants or animals directly for biosynthesis e.g. B. be used by proteins and the other essential products. Only special bacteria , in particular cyanobacteria , nodule bacteria , and some plants living in symbiosis with such bacteria on or in their roots ( legumes , such as peas, beans, lentils, alfalfa, lupins) can absorb air N 2 nitrogen use (see diazotrophy and nitrogen fixation ) and in complex nitrogen-containing compounds such. B. convert amino acids and proteins. Other plants rely on simple, water-soluble nitrogen compounds, such as B. ammonium (NH 4 + ), nitrate (NO 3 - ) ions or urea , as a nitrogen source and these compounds must be supplied via fertilizers or slurry .

Steps of the nitrogen cycle

Nitrogen fixation

Main article: Nitrogen fixation

In connection with the course of the natural nitrogen cycle, nitrogen fixation means the conversion of the gaseous, molecular , chemically inert air nitrogen (N 2 ) into water-soluble nitrogen compounds that can be absorbed by plants and living beings. This makes nitrogen fixation the first and fundamental process in the nitrogen cycle. The amount of nitrogen fixed annually is estimated at over 120 million tons.

Nitrogen fixation can be biotic or abiotic :

  • Abiotic:
    • in the oxidative formation of water-soluble nitrogen oxides ( nitrite and nitrate ), which arise as by-products in burns, explosions (car engines) or lightning strikes
    • technical ( Haber-Bosch process ) in the reductive formation of ammonia or water-soluble ammonium compounds

These products are components of nitrogen fertilizers .


Main article: nitrification

In a two-stage, aerobic process, two groups of bacteria, nitrite bacteria (e.g. Nitrosomonas ) and nitrate bacteria (e.g. Nitrobacter ) can oxidize ammonia via the intermediate stage nitrite to nitrate with energy gain :

Nitrogen assimilation

Plants can assimilate ammonium (NH 4 + ), but mostly prefer nitrate (NO 3 - ), whereby the soil is not acidified. The inorganic nitrogen compounds ammonium and nitrate are absorbed by plants and microorganisms and used to build nitrogen-containing organic compounds such as proteins and nucleic acids .


Main article: ammonification

By primary producers , as well as primary and secondary consumer (s. Food chain ) is constantly organic material in the form of nitrogen-containing excrement produced or dead organic matter. The nitrogen contained in it can release destructors (decomposers) such as fungi and bacteria ) as ammonia (NH 3 ) or as ammonium ions (NH 4 + ). These are then available to the ecosystem as inorganic minerals and can be used by autotrophic organisms (plants, etc.).

Nitrate reduction to nitrite

If no oxygen is available under anoxic conditions, certain bacteria can use nitrate instead of oxygen (O 2 ) as an oxidizing agent for the oxidation of organic substances as an energy-producing reaction. Nitrate is reduced to nitrite (NO 2 - ), which is toxic to some organisms.


Main article: Denitrification

Denitrifying, facultative anaerobic bacteria such. B. Species of the genera Pseudomonas , Paracoccus , Flavobacterium , can - if no oxygen is available under anoxic conditions - use nitrate and nitrite as an oxidizing agent for the oxidation of organic substances or of H 2 and in this way gain energy, with nitrate and nitrite be reduced over several intermediate stages to elemental N 2 , which finally escapes for the most part back into the atmosphere. Since these processes are, in the broadest sense, forms of anaerobic respiration , they are sometimes referred to collectively as nitrate respiration .

Importance of conversions

The conversions in the nitrogen cycle move a total of 250-300 million tonnes of nitrogen annually, which is only one millionth of the nitrogen in the atmosphere. The considerable emissions of harmful nitrogen oxides as by-products of combustion (motor vehicles) and the emissions of ammonia from fertilizer production and animal husbandry can lead to environmental problems. The various NO and NH compounds can cause eutrophication (over-fertilization) of soils and waters through nitrogen deposition. The groundwater is polluted by nitrate leaching from over-fertilized soils . In addition, nitrogen oxides act as acid generators (“ acid rain ”).

Nitrogen cycle in lakes

The nitrogen bound in organic substances, for example in dead biomass , is converted into ammonia (NH 3 ) by destructors in the tropholytic layer . Under aerobic conditions, aerobic bacteria oxidize the released ammonia during nitrification to nitrite (NO 2 - ) and further to nitrate (NO 3 - ).

In water, ammonia reacts with water to form ammonium ions (NH 4 + ), which creates OH ions and therefore increases the pH value:

If there are anaerobic conditions, for example through the consumption of oxygen by aerobic and facultative anaerobic microorganisms, certain anaerobic bacteria can reduce nitrate via nitrite to ammonium. This process is known as nitrate ammonification . Other bacteria convert nitrate to nitrogen (N 2 ) during denitrification by using it as an oxidant for their oxidative energy metabolism . The resulting N 2 is released and is released into the atmosphere.

In the trophogenic layer , phytoplankton extracts nitrogen from the nitrate and ammonium still present for the synthesis of endogenous substances, for example proteins and nucleic acids . This means that new biomass is produced. This biomass is now entering the food chain. First and second order consumers release the ammonia formed during the breakdown of organic substances back into the nitrogen cycle.

In addition, some bacteria, for example some types of cyanobacteria , bind elemental nitrogen N 2 by reducing it to NH 3 ( nitrogen fixation ). When these bacteria die, additional nitrogen enters the circulation.

The nitrogen cycle is now closed.

Importance of the nitrogen cycle in fish ponds

  1. Too many fish , food , plant residues and leaves enrich the pond water with organic material that contains nitrogen compounds. Rainwater from cisterns , pollen and garden fertilizer used for refilling also contribute to the overfertilization of the pond.
  2. Microorganisms decompose the biomass while consuming oxygen and thereby release the nitrogen it contains as ammonium or toxic ammonia . From pH 8.5 there is so much of it as ammonia that it is dangerous for fish; (the pH optimum is 7–8).
  3. The nitrifying bacteria, e.g. B. Bacteria of the genera Nitrosomonas and Nitrobacter, oxidize both under oxic conditions to nitrate (nitrification). This end product of protein degradation is an important mineral in all plants and is harmless to fish.
  4. Dead organic material enters the pond through plant remains, thereby closing the cycle.

Effects of interference

  1. The pond plants can usually only partially use up the nitrate. The excess amount increases with each cycle and over-fertilizes the water. Algae are prevalent and cloud the pond.
  2. When the excess is used up, most of the algae die. Their decomposition by microorganisms consumes a lot of oxygen, especially at night. If the fish gasp on the surface , this is a sure sign of a lack of oxygen.
  3. Many bacteria reduce nitrate to nitrite, which is toxic for fish, under anoxic conditions, which can prevail in the sediment (sludge) or - in the case of high oxygen consumption due to heavy pollution with organic substances - also in the water body.

Elimination of the faults

  1. Oxygen deficiency can be remedied technically by bringing in oxygen from the air, e.g. B. by pumping the water, water features , streams and spring stones.
  2. Nevertheless, the water remains cloudy because the excess minerals are still in the water and lead to the next algal bloom . That is why the excess nitrate must be removed - for example by bacterial denitrification.

On the situation in Germany

In Germany, an average of 20 to 40 kilograms of nitrogen per hectare are entered by air as nitrogen deposition every year ; in approximately equal parts in reduced and oxidized form. This oversupply of nitrogen is a major threat to biodiversity , as species adapted to nitrogen deficiency are being displaced by nitrophilic species. Over 70 percent of the vascular plants on the Red List in Germany are indicators of nitrogen deficiency. Added to this is the input from nitrogen fertilizers in agriculture.

On April 28, 2016, the EU Commission submitted a lawsuit against Germany that had been in preparation for years for non-compliance with the Nitrate Directive to the European Court of Justice . The limit values ​​for nitrate in water bodies have been exceeded in Germany for years. According to the EU Commission, however, the German government is not doing enough. The fertilizer ordinance discussed in 2016 is an indication of this.

See also


Web links

Nitrogen: Strategies for solving an urgent environmental problem

Individual evidence

  1. Clark, David P. 1952-, Jahn, Dieter 1959-, Jahn, Martina 1963-, Martinko, John M., Stahl, David A .: Brock microbiology compact . [1. Edition, translation of the 13th edition of the original]. Pearson Studium, Hallbergmoos 2015, ISBN 3-86894-260-2 .
  2. Robert Guderian, Günter Gunkel (Hrsg.): Handbook of environmental changes and ecotoxicology , Vol. 3: Robert Guderian: Aquatic systems , Vol. 3A: Basics, physical stress factors, inorganic substance inputs . Springer Verlag, Heidelberg et al. O. 2000, ISBN 978-3-540-66187-0 , p. 19ff.
  3. a b VDI 3959 sheet 1: 2008-12 Vegetation as an indicator for nitrogen inputs; Evaluation of nitrogen availability by Ellenberg indicator values ​​of forest ground vegetation (Vegetation as an indicator of nitrogen input; Assessment of nitrogen availability by Ellenberg indicator values ​​of forest ground vegetation). Beuth Verlag, Berlin, p. 2.
  5. August 8, 2016: Article on the effects of nitrogen