Nitrogen Cycle

Nitrogen Cycle is a cycle through which nitrogen gets converted into its various chemical forms which are vital for life on Earth. Nitrogen is the largest constituent of our planet’s atmosphere i.e. 78%. But, nitrogen in this form cannot be used by most organisms. Nitrogen is important for many ecosystems. All life form on earth use nitrogen as it is an important component in Amino acids and Proteins. It is also a part of bases that make u nucleic acids such as RNA and DNA. Nitrogen cycle is important because the nitrogen present in out atmosphere is unreactive and cannot be used by plants in this form. It is therefore vital that gaseous nitrogen is converted in ammonia and nitrates. Due to this particular reason Nitrogen Cycle holds huge importance in every ecosystem.

The four processes which participate in cycling of nitrogen are –

1. Nitrogen Fixation
2. Decay
3. Nitrification
4. Denitrification

We will briefly discuss these processes.

1. Nitrogen Fixation – N2 or the nitrogen molecule is fairly unreactive and in order to make it react with other elements a substantial amount of energy is required to break its atoms. This energy is provided mainly by three processes.
   1. Atmospheric Fixation which is majorly due to lighting. The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
   2. Biological Fixation – Only certain bacteria and archae have the ability to fix nitrogen. They either form a symbiotic relationship with plants of legumes family or plant other than legumes. Some also live in symbiotic relationship with animals such as termites.

• Industrial Fixation – At temperature of 600⁰ C, along with a catalyst, N2 and hydrogen are combined to form ammonia (NH3). Ammonia is used as a fertilizer and can be further processed to form urea.

1. Agriculture may be responsible for half of the nitrogen fixation on earth through the use of fertilizers and by growing legumes like soyabeans and alfalfa

2. Decay – The protein made plants produce organic nitrogen compounds at each trophic level of metabolism. These nitrogen compounds return to the environment chiefly through excretions. These compounds benefit the microorganism of decay. These microorganisms break down the molecules in excretion and dead organisms in to ammonia.
3. Genus Nitrospira converts ammonia to nitrite to nitrates. This ability is called Comammox.Nitrification – Most of the ammonia produced by decay is converted into nitrates. Bacteria which are responsible for Nitrification are –

1. Bacteria of the genus Nitrosomonas
2. Bacteria of the genus Nitrbacter
3. Archeal Microbes and many legumes also carry out the process of nitrification.

4. Denitrification – The above processes remove nitrogen from the atmosphere and convert it into nitrates and nitrites which are useful for the ecosystem. Denitrification converts nitrates and nitrites back into nitrogen gas and thus replenishes the atmosphere. In the process of denitrification some intermediate compounds are also formed – Nitric Oxide (NO), Nitrous Oxide(N2O) which is a greenhouse gas and has 300 times more greenhouse capacity than CO2, Nitrous Acid(HONO).
5. Anammox (Anaerobic ammonia oxidation) – Under anaerobic conditions in marine and freshwater sediments, some other species of bacteria are able to oxidize ammonia forming nitrogen gas.

NH₄⁺ + NO₂⁻ → N₂ + 2H₂O

This annamox reaction accounts for nearly 50% of denitrification which occurs in the oceans.