Question

What is the main process by which nitrogen is returned to the atmosphere from the soil?

• A. Nitrogen fixation
• B. Ammonification
• C. Denitrification
• D. Nitrification

Hint:

{Denitrification = Nitrates → Nitrogen gas}. It is the key step that returns nitrogen back to the atmosphere in the nitrogen cycle.

Answer 1

The Correct Option is C

Concept: The nitrogen cycle describes the movement of nitrogen through the atmosphere, soil, water, and living organisms. Nitrogen undergoes several biological and chemical transformations in this cycle. Important processes in the nitrogen cycle include:

• Nitrogen Fixation – Conversion of atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)) by nitrogen-fixing bacteria.
• Nitrification – Conversion of ammonia into nitrites (\(NO_2^-\)) and then nitrates (\(NO_3^-\)).
• Ammonification – Conversion of organic nitrogen from dead organisms and wastes into ammonia.
• Denitrification – Conversion of nitrates back into nitrogen gas (\(N_2\)) which returns to the atmosphere.

Step 1: Understanding how nitrogen returns to the atmosphere.
The process that releases nitrogen gas back into the atmosphere is denitrification. In this process, certain bacteria convert nitrates (\(NO_3^-\)) present in the soil into nitrogen gas (\(N_2\)) or nitrous oxide (\(N_2O\)).
Step 2: Role of denitrifying bacteria.
Denitrification is carried out by anaerobic bacteria such as Pseudomonas and Clostridium, especially in waterlogged or oxygen-poor soils.
Step 3: Eliminating other options.

• Nitrogen fixation converts atmospheric nitrogen into usable forms.
• Nitrification converts ammonia into nitrates.
• Ammonification converts organic nitrogen into ammonia.

Thus, the process responsible for returning nitrogen to the atmosphere is denitrification.