Nitrogen, an essential element required by plant for making amino acids and proteins, cannot be fixed by plants independently. Nitrogen fixation essentially means converting the nitrogen present in the atmosphere to be converted into forms usable by living organisms. Nitrogen fixation in plants is typically done with the help of symbitoic bacteria. In a symbiotic relationship with bacteria, legumes form nodules on the roots to fix nitrogen into a usable form which can then be used by either plants or animals. Beneficial bacteria that fix nitrogen are called diazotrophs. The abundance of nitrogen in the legumes is beneficial not only for the legumes but also for the plants around.
Nitrogen fixation can be done either biologically or abiotically. It’s a process where nitrogen (N2) present in the air is converted to ammonia (NH3). Nitrogen fixation is a very important life process as it biosynthesis the building blocks of life, e.g., nucleotides for DNA and RNA and amino acids for proteins. However, nitrogen fixation can occur due to non-biological processes like lightning, combustion and industrially through Haber-Bosch Processes.
Biological nitrogen fixation happens when nitrogen present in the air is transformed to ammonia by nitrogenase (an enzyme). The ammonium generated with the help of nitorgenase is incorporated into glutamate through glutamine synthetase pathway. But, special care should be taken because oxygen can destroy the enzymes responsible for nitrogenase action. Most microorganisms either respire to reduce the oxygen levels or bind the oxygen with protein like Leghemoglobin.
Plants that help nitrogen fixation include the legume family — clover, alfalfa, soybeans, peanuts, lupines and rooibos. These plants contain symbiotic bacteria known as Rhizobia in their root systems that produce nitrogen compounds to encourage faster growth and race with other plants. Most of the time, when a plant dies, it releases nitrogen which is then available to other plants and this helps in fertilization. Beneficial microorganisms that efficiently fix nitrogen are Azotobacteraceae, Cyanobacteria, Rhizobia and Frankia.
There are also artificial ways of fixing nitrogen like the Haber process, dinitrogen complexes and ambient nitrogen reduction. Haber process works best in environment where there is high pressure and high temperature. Here natural gas is used as a hydrogen source and natural air as a nitrogen source. The dinitrogen complexes method is yet to be discovered full to gain complete benefits and ambient nitrogen reduction method is not reliable as it fixes few nitrogen molecules.