Question

The sequence of events occuring upon application of urea in soil are:
(A). Hydrolysis
(B). Nitrification
(C). Denitrification
(D). Leaching
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (A), (B), (D), (C)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

Ammonium (\(\text{NH}_4^+\)) is held on clay exchange complexes and resists leaching. Nitrogen must first undergo nitrification to nitrate (\(\text{NO}_3^-\)) before significant leaching or denitrification can occur.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
When amide-based urea fertilizer is applied to soil, it undergoes a series of biochemical and physical transformations within the nitrogen cycle.

Step 2: Detailed Explanation:
1. Hydrolysis (A): Urea is first hydrolyzed by the extracellular enzyme urease to produce ammonium carbonate: \[ \text{CO(NH}_2)_2 + 2\text{H}_2\text{O} \xrightarrow{\text{urease}} (\text{NH}_4)_2\text{CO}_3 \] 2. Nitrification (B): The resulting ammonium (\(\text{NH}_4^+\)) is oxidized by soil autotrophic bacteria to nitrite and then to nitrate (\(\text{NO}_3^-\)).
3. Leaching (D): Nitrate is a highly mobile anion that is not adsorbed by negatively charged clay colloids. Consequently, it moves downward through the soil profile with water.
4. Denitrification (C): Nitrate that leaches into deeper, oxygen-depleted (anaerobic) zones is reduced by facultative anaerobic bacteria into gaseous dinitrogen oxide (\(\text{N}_2\text{O}\)) or nitrogen gas (\(\text{N}_2\)).
5. Thus, the transformation sequence is: Hydrolysis \(\rightarrow\) Nitrification \(\rightarrow\) Leaching \(\rightarrow\) Denitrification.

Step 3: Final Answer:
The sequential transformation pathway is (A), (B), (D), (C).