Nitrogen fixation

Under nitrogen fixation is generally understood as any conversion of the chemically inert elemental, molecular nitrogen (N2). It is an important part of the nitrogen cycle.

We distinguish:

• Biotic nitrogen fixation ( by microorganisms )
• Abiotic nitrogen fixation ( formation of nitrogen oxides by burns or lightning ) and
• Technical nitrogen fixation (see Haber- Bosch process ).

Nitrogen fixation should be distinguished from the " ammonium fixing " the binding of the positively charged ammonium ion to negatively charged clay minerals in the soil (see nutrient ( plant ), and cation exchange capacity).

Biotic nitrogen fixation

By some microorganisms elemental, molecular nitrogen (N2 ) is reduced to give compounds which are more reactive, and in particular bio-available. The process is due to the very stable triple bond molecular, elemental nitrogen with 946 kilojoules per mole ( kJ / mol ) is very energy-intensive.

Microorganisms can fix nitrogen ( nitrogen fixers ), are either free- living or live in symbiosis with plants. Well-known free-living representatives are the genera Azotobacter, Azomonas and cyanobacteria, the latter often fix nitrogen in specialized cells called heterocysts. The best-known symbiotic nitrogen fixers are nodule bacteria (for example, legumes) and Frankia ( in woody plants such as alder ).

Since nitrogen fixation is very energy-intensive for sentient beings, it is strictly regulated and is only used when the living entity has no other way to nitrogen supply.

Abiotic nitrogen fixation

By lightning during thunderstorms, combustion and volcanoes of nitrogen and oxygen in the air produces nitrogen oxides that react with water droplets in the atmosphere to form nitrous acid and nitric acid and go as acid rain in the soil.

Technical nitrogen fixation

After the Haber- Bosch process, N2 can be reduced. The process requires a temperature of 500 ° C, a pressure of 450 bar and catalysts. The reduction is similar to that in (2). Most of the ammonia is converted into nitrate-containing fertilizers.

In Azotierung nitrogen is fixed to display cyanamide according to the following reaction equation:

Further meaning

In addition, the determination of nitrogen in the soil the organic matter will be referred to as nitrogen fixation, if an unfavorable carbon to nitrogen ratio (C / N ratio) exists. The reason is here in the nitrogen requirements of the degrading microorganisms. So can be when applying low-nitrogen mulch materials such as sawdust, wood chips or Rindenhäcksel observe a nitrogen deficiency of crops. Therefore, it may be favorable to such materials composted before, or in addition to give a nitrogen fertilizer. The fixed nitrogen is released with the decomposition of organic substances in the long term again.