Which of the following statements about the nitrogen cycle are false ?
1. Rhizobium and Frankia cause nitrogen fixation in their independent free - living stage .
2. Azotobacter and Anabaena are anaerobic, symbiotic , free - living , nitrogen fixers.
3. Rhodospirillum is an aerobic , nitrogen -fixing microbe found in the root nodules of legumes .
4. Nitrobacter oxidizes nitrites to nitrates .

Options

A. 1 and 2
B. 2 and 4
C. 1 , 2 and 3
D. 3 and 4

Answer

To determine which statements about the nitrogen cycle are false, we will evaluate each statement one by one.

### Step-by-Step Solution:

1. **Evaluate Statement 1**:
- **Statement**: "Rhizobium and Frankia cause nitrogen fixation in their independent free-living stage."
- **Analysis**: This statement is false. Rhizobium and Frankia are nitrogen-fixing bacteria that form symbiotic relationships with the root nodules of legumes and actinorhizal plants, respectively. They do not fix nitrogen in a free-living stage.

**Hint**: Remember the role of Rhizobium and Frankia in symbiosis with plants.

2. **Evaluate Statement 2**:
- **Statement**: "Azotobacter and Anabaena are anaerobic, symbiotic, free-living nitrogen fixers."
- **Analysis**: This statement is also false. Azotobacter is a free-living nitrogen fixer that operates in aerobic conditions, while Anabaena is a cyanobacterium that can fix nitrogen but is not strictly anaerobic and is often found in symbiotic relationships with certain plants.

**Hint**: Consider the environmental conditions under which Azotobacter and Anabaena thrive.

3. **Evaluate Statement 3**:
- **Statement**: "Rhodospirillum is an aerobic nitrogen-fixing microbe found in the root nodules of legumes."
- **Analysis**: This statement is false. Rhodospirillum is an aerobic nitrogen-fixing bacterium, but it is not found in the root nodules of legumes. Instead, Rhizobium is the bacterium associated with legumes.

**Hint**: Identify the specific bacteria associated with legume root nodules.

4. **Evaluate Statement 4**:
- **Statement**: "Nitrobacter oxidizes nitrites to nitrates."
- **Analysis**: This statement is true. Nitrobacter is a genus of bacteria that plays a crucial role in the nitrogen cycle by oxidizing nitrites (NO2-) to nitrates (NO3-).

**Hint**: Recall the role of Nitrobacter in the nitrification process.

### Conclusion:
Based on the evaluations:
- Statements 1, 2, and 3 are false.
- Statement 4 is true.

Thus, the correct answer is that statements 1, 2, and 3 are false regarding the nitrogen cycle.

### Final Answer:
The false statements about the nitrogen cycle are 1, 2, and 3.