What is Nitrogen Cycle?
The Nitrogen Cycle is the process by which atmospheric nitrogen is converted to a form that is absorbable by plants. This occurs through free-living and symbiotic bacteria. The Nitrogen cycle process is necessary because getting atmospheric nitrogen into living organisms is difficult.
Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (where it exists as tightly bonded, triple covalent N2 ) even though the Nitrogen molecules make up about 78 percent of the atmosphere; therefore, the only way Nitrogen enters the living world is through the Nitrogen cycle by the help of free-living and symbiotic bacteria that incorporate nitrogen into their macromolecules through specialized biochemical pathways leading to nitrogen fixation.
The nitrogen cycle involves the complex process by which nitrogen is naturally added and removed from the soil. It is a sequence of reactions indicating the various means by which atmospheric nitrogen is added and removed from the soil.
Cyanobacteria play a key role in nitrogen fixation, these bacteria live in most aquatic ecosystems where sunlight is present and are able to fix atmospheric nitrogen gas (from nitrogen gas) into ammonia (NH3 ) that can be incorporated into the macromolecules of the organism.
Another type of bacteria that helps to fix nitrogen is the Rhizobium bacteria these bacteria live symbiotically in the root nodules of leguminous plants like peas, beans, and peanuts. The Rhizobium bacteria provide them with the organic nitrogen they need. The third type of bacteria that can fix nitrogen is the Free-living bacteria, such as Azotobacter.
Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes, such as primary production and decomposition, are limited by the available supply of nitrogen. The nitrogen gas that enters living systems by nitrogen fixation is eventually converted from organic nitrogen (the form useful to plants) back into nitrogen gas by bacteria a process called Denitrification. This process of converting Nitrogen gas to organic nitrogen and back to nitrogen gas again occurs in three steps in terrestrial systems: ammonification, nitrification, and denitrification.
Simple Nitrogen Cycle Steps
- First, the ammonification process converts nitrogenous waste from living animals or from the remains of dead animals into ammonium (NH4+ ) by certain bacteria and fungi.
- Second, this ammonium is then converted to nitrites (NO2 ) by nitrifying bacteria, such as Nitrosomonas, through nitrification. Subsequently, nitrites are converted to nitrates (NO3 ) by similar organisms.
- Lastly, the process of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium, convert the nitrates into nitrogen gas, thus allowing it to re-enter the atmosphere.
Nitrogen Cycle Process
Nitrogen enters the living world from the atmosphere through nitrogen-fixing bacteria through a process called Nitrogen fixation which involves soil organisms that add a reasonable amount of nitrogen to the soil. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need.
Nitrogen Cycle Diagram
Nitrogen Cycle the Steps and Process
- Symbiotic nitrogen fixation: Some bacteria such as Rhizobium leguminosarium which live in the root nodules of leguminous plants can fix atmospheric nitrogen directly into the plant. The plant supplies carbohydrates for use by the bacteria while the bacteria supply the plant with combined nitrogen this process whereby organisms benefit from each other is called symbiosis.
- Electrical discharge: Nitrogen can also be fixed into the soil during lightning and thunderstorm. Nitrogen in the air combines with oxygen to form nitric oxide or nitrogen (II) oxide which further undergoes oxidation to form nitrogen dioxide or nitrogen (IV) oxide. The nitrogen (iv) oxide formed will dissolve in rainwater to form nitrous (HNO2 ) and nitric acid (HNO3 ) which later dissociates to form nitrate in the soil.
- Nonsymbiotic nitrogen fixation: Some bacteria such as Azotobacter and Clostridium also live freely in the soil and can fix atmospheric nitrogen into the soil either aerobically or anaerobically.
- Ammonification and nitrification: The process involving the formation of ammonium compounds from the dead and decaying of plants and animals and their waste products like urine and feces is called ammonification.
- A further reaction known as nitrification – involves the conversion of ammonium compounds first into nitrate by nitrifying bacteria called nitrosomonas. These nitrites are converted by oxidation to nitrates by another bacteria called nitrobacter. Plants can only absorb nitrates from the soil.
- Denitrification: This is the process that involves the conversion of nitrate to nitrogen gas by certain bacteria. The nitrogen gas so formed can escape into the air. Denitrification is the only major stage in which nitrogen can be lost from the soil while other stages involve the fixing of nitrogen into the soil.
Importance of Nitrogen Cycle
Without the Nitrogen Cycle, it will be impossible to convert the Nitrogen in the atmosphere into useful compounds that can be used by green plants which provide food for man as well as animals that eat the plants that man also eats as meat.