What is Nitrogen Fixation
The amount of nitrogen in the atmosphere is 78%, but despite having such a high amount, atmospheric nitrogen is unusable for plants. Plants completely lack the ability to take nitrogen in gaseous form. Plants take up nitrogen in the form of nitrates (NO3) and nitrites (NO2). The nitrogen present in the form of compounds is called fixed nitrogen. Therefore, the process of converting the free nitrogen gas of the atmosphere into nitrogen compounds is called fixation of nitrogen.
Nitrogen fixation is a process by which nitrogen in the Earth’s atmosphere is converted into ammonia (NH3) or other molecules available to living organisms. Atmospheric nitrogen or molecular dinitrogen (N2) is relatively inert: it does not readily react with other chemicals to form new compounds. The fixation process frees the nitrogen atoms from their all three bonded diatomic form, NNN, to be used in other ways.
Nitrogen fixation is essential for some forms of life because inorganic nitrogen compounds are needed for the biosynthesis of the basic building blocks of plants, animals and other life forms, for example, the nucleotides for DNA and RNA, the coenzyme nicotinamide adenine. For its role in nucleotide metabolism (transferring electrons between molecules), and amino acids for proteins. Therefore, as part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer.
It is, indirectly, relevant to the manufacture of all nitrogen-containing chemical compounds, including explosives, most pharmaceuticals, and dyes. Nitrogen Fixation Nitrogen fixing is done naturally in soil by a wide range of bacteria and archaea, including Azotobacter. Some Nitrogen Fixation bacteria have sympatric relationships with certain plant groups, especially legumes.
The weak association between nitrogen-fixing bacteria and plants is often referred to as ally or non-symbiotic, as seen in nitrogen fixation on rice roots. It also occurs naturally in the air through NOx production by electricity.
All biological nitrogen fixation is done through a metalloenzyme called nitrogenase.
These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium. Microorganisms that can fix nitrogen are prokaryotes (both bacteria and archaea, distributed in their respective domains) called diazotrophs. Some higher plants, and some animals (termites), have formed associations (symbiosis) with diazotrophs.
Nitrogen is fixed by two methods
1. Natural Method :– Nodules are present in the roots of leguminous plants. Some symbiotic bacteria are present in the soil above or near these knots. These bacteria take nitrogen directly from the atmosphere and convert it into nitrate which goes into the soil. Plants synthesize proteins by taking up nitrates from the soil. Thus, after these plants are cut, the amount of nitrogenous manure in the field increases as compared to earlier. For this reason, plants of the leguminous family are cultivated in it to make the land more fertile.
2. By Electric Immersion :- During the rainy season or on other days, due to the action of electric discharge, atmospheric nitrogen and oxygen combine to form nitric oxide (NO). Nitric oxide combines with oxygen to form nitrogen dioxide (NO2). This nitrogen dioxide dissolves in rain water and gets converted into nitric acid (HNO3) and nitrous acid (HNO2). These acids combine with lime or other alkaline substances present in the soil to form nitrates and nitrites, which act as fertilizers for plants.
How do plants absorb Nitrogen
Plants take up nitrogen in the form of ammonium or nitrate ions. The process known as nitrogen organization occurs in plants. That is, nitrogen is bound to organic molecules. Thus nitrogen moves in a small cycle produced by plants, animals and bacteria.
Although overall, as seen, the nitrogen cycle begins with the atmospheric one: part of the nitric nitrogen is actually converted by specific denitrifying bacteria which then release the nitrogen into the atmosphere as molecular nitrogen.
In particular, plants are able to absorb nitrogen because there are specific transporters at the level of plant cells of the root system that operate a symporter between nitrates and protons (H+ that dissolve in soil).
Nitrate in the plant cell is processed by an enzyme called “nitrate reductase” which converts nitrate into nitrite. Thereafter, the nitrite is further reduced to the ammonium ion and it is at this point that nitrogen organization can occur.
Only when nitrogen (entered as nitrate) is converted to ammonium (via nitrite) can nitrogen be incorporated into important molecules such as proteins and nucleic acids. Then the food chain does the rest!