Question

Which nitrogen source is most commonly used by plants for inorganic nitrogen fixation?

• A. Nitrite (\(NO_2^-\))
• B. Nitrate (\(NO_3^-\))
• C. Ammonia (\(NH_3\))
• D. Molecular nitrogen (\(N_2\))

Hint:

Plants mainly absorb nitrogen as {nitrate ((NO_3^-))} from soil. Remember: {Nitrate = Major plant nitrogen source}.

Answer 1

The Correct Option is B

Concept: Plants require nitrogen for the synthesis of essential biomolecules such as amino acids, nucleic acids, and chlorophyll. However, plants cannot directly utilize atmospheric nitrogen ((N_2)). Instead, they absorb nitrogen from the soil mainly in inorganic forms.
Step 1:Forms of nitrogen available in soil.
The major inorganic nitrogen forms present in soil are: [ NO_3^- ; (\text{nitrate}) \quad \text{and} \quad NH_4^+ ; (\text{ammonium}) ]
Step 2:Most commonly absorbed nitrogen form.
Among these forms, nitrate ((NO_3^-)) is the most abundant and commonly absorbed form of nitrogen by plants in well-aerated soils.
Step 3:Utilization in plant metabolism.
Once absorbed, nitrate is reduced first to nitrite and then to ammonium inside plant cells. This ammonium is incorporated into amino acids through biochemical pathways such as the GS-GOGAT cycle. Therefore, the most commonly used inorganic nitrogen source by plants is nitrate ((NO_3^-)).