Nitrogen fixation
Molecular nitrogen fixation in to form of ammonia. It is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds that plants can use.
Atmospheric nitrogen (N₂) makes up 78% of air, but plants cannot use it directly because of the strong triple bond (N≡N).
Nitrogen fixation breaks this bond, making nitrogen biologically available.
Types of Nitrogen Fixation
1. Biological Nitrogen Fixation (BNF)
Done by microorganisms using the enzyme nitrogenase.
– Rhizobium (symbiotic with legumes, forms root nodules)
– Frankia (in actinorhizal plants)
– Azotobacter, Clostridium (free-living in soil)
– Anabaena, Nostoc (cyanobacteria in water/lichens)
2. Abiotic (Non-biological) Nitrogen Fixation
Occurs naturally through lightning or industrial processes (Haber–Bosch process).
Lightning provides high energy that converts N₂ → NOx (nitrogen oxides), which dissolve in rain to form nitrates.
Haber–Bosch process:
Nitrification
Nitrification is a key step in the nitrogen cycle, where soil microorganisms convert ammonia (NH₃/NH₄⁺) into nitrate (NO₃⁻) through an aerobic (oxygen-requiring) process
It happens In two main steps:
Ammonia oxidation
Carried out by ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas, Nitrosospira, nitrosococcus) or ammonia-oxidizing archaea (AOA).
Nitrite oxidation
Carried out by nitrite-oxidizing bacteria (NOB) (e.g., Nitrobacter, Nitrospira, and nitrococcus).
Ammonification
Nitrogen present in soil in form of organic nitrogen compounds in presence of bacteria ( Bacillus ramosus, and B. mycoides) and fungi the fixation of organic nitrogen in form of ammonia is called ammonification.
Denitrification
It’s a process in which nitrate or nitrite convert in to nitrogen in presence of heterotrophic microorganisms (pseudomonas and thiobacillus).
Nitrogen assimilation
In plants nitrogen is actively absorbed in form of nitrate and ammonia. With help of a proton gradient across plasma membrane by H+ ATPase drives uptake of nitrogen against concentration gradient. Reduction of nitrate to ammonia occurs in two steps-
In first step, nitrate is reduced in to nitrite by enzyme nitrate reductase , the reaction takes place in cytosol.
In second step, nitrite reduced to ammonium ion by presence of nitrite reductase. This reaction takes place in plastids.
Plant cells to avoid ammonium toxicity by converting ammonia into amino acids. It’s also a enzyme dependent sequential process
The enzymes glutamine synthetase and glutamate synthase ( glutamine -2-oxoglutarate aminotransferase, GOGAT) are involved in this process