Nitrogen is a necessary element in the production of proteins. Most nitrogen is found as gas in the atmosphere and is relatively unavailable for protein synthesis. Which of the following releases nitrogen back into the atmosphere?
I Nitrification
II Denitrification
III Nitrogen fixation
(a) I only
(b) II only
(c) III only
(d) I and III only
(e) II and III only

Hint: This cycle helps to reduce nitrate or nitrite into gaseous nitrogen or dinitrogen. This cycle works on the availability of N oxides like nitrite and nitrate which are formed from the autotrophic nitrification pathway.

Complete answer:
Nitrogen is one of the most abundant elements found in the atmosphere. It is found in soils and plants and is the key building block of DNA. It is also one of the important macronutrients needed by the plants. The nitrogen cycle helps understand how nitrogen moves from the atmosphere to the land and back.
In the atmosphere, nitrogen is in the form of gas ${ N }_{ 2 }$ but in soil, it exists as ${ NO }_{ 2 }$ and NO which makes protein synthesis relatively unavailable in plants.
There are five different stages in the nitrogen cycle namely: fixation, mineralization, nitrification, immobilization, and denitrification. Only in the last stage, the nitrogen oxides are converted into atmospheric nitrogen by bacteria. This results in the loss of nitrogen from the soils as the gaseous form of the nitrogen moves into the atmosphere and helps in protein synthesis in plants.
The sequence in which nitrate is converted to dinitrogen is given below:
-Nitrate ${ NO }_{ 3 }$ is reduced to nitrite ${ NO }_{ 2 }$.
-Nitrite is reduced to nitric oxide NO
-Nitric oxide NO is reduced to nitrous oxide ${ N }_{ 2 }$O
-Nitrous oxide is reduced to dinitrogen.
Some strains of bacteria including Thiobacillus denitrificans and Micrococcus denitrificans, with certain other species namely Serratia, Pseudomonas, and Achromobacter are labeled as denitrifiers or bacterias which help in the process of denitrification. Pseudomonas aeruginosa can also work as a denitrifier under anaerobic conditions.

So, the correct answer is ‘option (b) II only’.

Notes:
According to scientists humans are disrupting the nitrogen cycle by altering the amount of nitrogen in the environment. This is mainly due to the combustion of fossil fuels which release nitric acid that combines with other elements in the air to form smog and acid rain.