Higher plants are autotrophic organisms that can synthesize all of their molecular components from inorganic nutrients from the local environment. The mineral nutrients taken up by plants are converted into carbohydrates, lipids and amino acids. It is called nutrient assimilation.
Nitrogen is a key element in many of the compounds present in the plants. It is found in nucleotides and amino acids that form the building blocks of nucleic acids and proteins.
Biological Nitrogen Cycle
The atmosphere contains 79% nitrogen by volume, however this not available to plants directly because plants are unable to break stable triple covalent bond between two nitrogen atoms. The plants are able to assimilate nitrogen present in the form of nitrate or ammonia. This results in further movement of nitrogen in the food chain. The nitrogen is returned to the soil through the death and decomposition of plants and animals by the action of bacteria and certain fungi that convert organic nitrogen to gaseous nitrogen.
The cycle of fixation of gaseous nitrogen, assimilation of fixed nitrogen by plants and then animals, and return of gaseous nitrogen to the atmosphere by denitrifying bacteria is called the biological nitrogen cycle.
The conversion of molecular nitrogen into other forms such as ammonia or nitrate is known as nitrogen fixation. It can occur as a result of both natural and industrial processes.
Under the conditions of elevated temperature (200 °C)’ and high pressure (about 200 atmosphere), molecular nitrogen will combine with hydrogen to form ammonia (Haber process). About 12% nitrogen is removed each year from atmosphere by industrial methods.
About 5% of natural fixation occurs as a result of lightening during thunderstorm. It causes water vapours, oxygen and nitrogen in the atmosphere to combine to form nitric acid which is brought down by rain to the surface of the earth. The remaining 95% of natural nitrogen fixation occurs through certain micro-organisms, mainly bacteria and cyanobacteria (blue-green algae).
Several steps are involved in nitrogen fixation by micro-organisms:
- The ammonia produced during biological nitrogen fixation is assimilation by green plants into amino acids, proteins and other nitrogenous products. Some plants may be consumed by animals. The animals waste and dead plants and animals and micro-organisms undergo decay during which complex organic nitrogenous compounds are decomposed into simpler compounds such as amino acids.
- Ammonification: A group of soil bacteria together with certain fungi converts amino acids into ammonia. This is called ammonification. The ammonia produced reacts with other chemicals present in the soil to form ammonium salts such as ammonium carbonate.
- Nitrification: Two groups of soil bacteria oxidize ammonia of ammonium salts to nitrite (Nitrosomonas) and nitrite to nitrate (Nitrobacter). The process is called nitrification.
- Denitrification: Finally, the nitrates into the soil are reduced by certain soil bacteria to nitrogen and volatile nitrogen oxides, which escape into the atmosphere. This process is called denitrification. Denitrifying bacteria are especially active under anaerobic conditions in wet soils with high organic matter content.
Biological Nitrogen Fixation
The nitrogen fixed through the action of micro-organisms is termed as biological nitrogen fixation.
Biological nitrogen fixation is carried out by free-living bacteria and cyanobacteria, and by bacteria that are in symbiotic association with plants. These organisms contain enzyme systems that can catalyze chemical reactions involving molecular nitrogen.
The nitrogen fixing micro-organisms are known as free fixers.
Nitrogen-fixing bacteria can be aerobic, facultative or anaerobic.
Aerobic Nitrogen-Fixing Bacteria: These bacteria flourish in aerobic conditions. These include Azotobacter that fix nitrogen during night.
Facultative Nitrogen-Fixing Bacteria: This type of bacteria is able to grow under both aerobic and anaerobic conditions. However, they generally fix nitrogen only under anaerobic condition.
Anaerobic Nitrogen-Fixing Bacteria: They do not require oxygen. These may either by photosynthetic. For example; Rhodospirillum; or non-photosynthetic, such as Clostridium.
Although free-living nitrogen fixing bacteria are found in many soils. However, they contribute to the nitrogen content of the soil only under very special soil conditions such as presence of sufficient decayed plant material and high water contents.
The cyanobacteria that fix nitrogen consists of chains of cells in long filaments contain specialized larger colorless cells with thick walls called heterocysts. These cells are able to fix nitrogen. About forty species of cyanobacteria are capable of fixing nitrogen. These are more active in wet tropical soil, e.g., rice fields. Cyanobacteria uses sunlight energy for nitrogen fixation.