UPSC Prelims 2013 Q34: Environment & Ecology – Biogeochemical Cycles & Earth Systems
Which of the following adds/add nitrogen to the soil?
1. Excretion of urea by animals
2. Burning of coal by man
3. Death of vegetation
Select the correct answer using the codes given below.
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Explanation
What adds nitrogen to the soil?
| Process | Adds nitrogen to the soil? | How |
|---|---|---|
| 1. Excretion of urea by animals | Yes | Urea breaks down into ammonia in the soil |
| 2. Burning of coal by man | No | It sends nitrogen oxides into the air, not into the soil |
| 3. Death of vegetation | Yes | Decomposers release nitrogen from dead plants (ammonification) |
Steps of the nitrogen cycle:
| Step | What happens | Done by |
|---|---|---|
| Nitrogen fixation | Air nitrogen to ammonia | Rhizobium, Azotobacter, lightning |
| Ammonification | Dead matter and urea to ammonia | Decomposers |
| Nitrification | Ammonia to nitrite to nitrate | Nitrosomonas, Nitrobacter |
| Denitrification | Nitrate back to nitrogen gas | Pseudomonas |
Answer: (c) 1 and 3 only.
Why each option is right or wrong
- (a) Incorrect. The death of vegetation also adds nitrogen to the soil.
- (b) Incorrect. Burning of coal adds nitrogen oxides to the air, not the soil.
- (c) CORRECT. Urea from animals and dead vegetation add nitrogen to the soil.
- (d) Incorrect. Statement 2 is false.
Key point
Nitrogen enters soil through fixation, animal waste (urea) and decay of dead matter; nitrification makes nitrate; denitrification returns it to the air.
Remember
Urea and dead plants add nitrogen to soil.
Common mistakes
- Counting coal burning as a soil nitrogen source.
- Mixing up nitrification and denitrification.
- Forgetting the bacteria: Nitrosomonas, Nitrobacter, Pseudomonas.
Exam tip
Dead things and wastes feed the soil; burning feeds the air.
Sources
- NCERT Class XI — Biology (Mineral Nutrition: nitrogen cycle)
- NCERT Class XII — Biology (Ecosystem: nutrient cycling)