Nitrogen is an important element that forms a major portion of proteins(Present in the amino acids) and nucleic acids such as DNA and RNA (present in nitrogenous bases).The largest reservoir of nitrogen is the atmosphere which contain about 78% of Nitrogen gas (N2). Nitrogen is fixed through a process called nitrogen fixation on Nitrification.
Nitrogen fixation combines nitrogen with oxygen to create nitrates and NO3 nitrates can then be used by plants or animals (which eat plants or eat animals that have eaten plants)
Nitrogen gets fixed with the help of free nitrogen fixing bacteria present in the soil or roots of legumes through addition of fertilizers.Both symbiotic and non symbiotic nitrogen fixing bacteria as play an important role in cycling of Nitrogen.The free living aerobic and anaerobic bacteria that help in nitrogen fixation include species such as azotobacter , clostridium and rhizobium.
Rhizobium, a symbiotic nitrogen fixing bacteria living in the nodules present in the roots of legumes fixes a good quantity of atmospheric nitrogen.Nitrogen fixation is also assisted by blue-green algae( cyanobacteria) present in the free form of in Symbiosis with fungi as in certain in lichens and mosses and Ferns.
Nitrogen fixing bacteria use certain enzymes that are capable of fixing Nitrogen gas into nitrates and include free bacteria in soil symbiotic bacteria in legumes and also cyanobacteria or blue green algae in water.During nitrogen fixation, nitrogen is converted to ammonia. Ammonia gas converted to nitrite into nitrates .nitrates are used by plants to form amino acids and nucleic acids.
Many heterotrophic bacteria actinomycetes and fungi metabolize the organic nitrogen and release it in an or inorganic form such as ammonia.The process is known as Ammonification or mineralization. Ammonia gets converted to nitrate. the process is termed as nitrification.
Nitrates are used by autotrophic and heterotrophic organisms.Ammonia further oxidised and form nitrites. the process is facilitated by bacteria Nitrosomonas.
Nitrite is further oxidised and gets converted to nitrate by nitrobacter.Nitrates get leached in soil readily. They loss through denitrification process.In this process molecular or gaseous oxygen nitrous oxide and nitric oxide is Formed From nitrates. the bacteria use nitrates as a source of oxygen.
The total nitrogen fixed annually is estimated to be around 92× 10 6metric tons While the amount of Nitrogen returned back via denitrification is estimated to be around 83×106 metric tons.Nitrogen fixation and nitrification mediated by bacteria are influenced by environmental factors such as temperature moisture content.
Nitrogen gets added to the soil as nitrate after decomposition and decay of animal and plant waste.Decomposer and detritivores ingest the detritus from plants and animals and nitrogen is changed into Ammonia or nitrogen.
Ammonia is converted to nitrite then to nitrate.Some nitrate also get converted to nitrite and then to nitrous oxide by denitrifying bacteria. Nitrous oxide also get converted into nitrogen gas and is released back into atmospheres.
The nitrite that is not used by plants and animals get converted to nitrate by denitrifying bacteria that can be used by plant and animals.
Human Impact on Nitrogen Cycle
Humans alter the cycling of Nitrogen by different ways.
- Application of excess of Nitrogen containing fertilizers affect the distribution of Nitrogen. around 30× 10 6 metric tons of nitrogen is fixed annually in the production of fertilizers.
- The disposal of Nitrogen rich waste in water (particularly irrigation drinking and see which water )leads to its entry into the aquatic ecosystem through run off leaching. Presence of nitrogen is compound in high amount promotes growth of algae and phytoplankton leading to nutrient depletion. The process is termed eutrophication.
- Burning of fossil fuels leads to release of Nitrogen oxide .nitrous oxide gets converted to Nitrogen dioxide after oxidation. it combines with Ozone in the atmosphere.All these activities increases the concentration of Nitrogen in the environment leading to an ecological imbalance.
Nitrogen oxide released in the environment particularly atmosphere get converted to nitric Acid That comes down on earth along with rain and precipitation.The changes the pH of the soil and aquatic ecosystems.