Which is correct

Options

A. Both Azotobacter and Rhizobium fix atmospheric nitrogen in root nodules of plants
B. Cyanobacteria, Anabaena and Nostoc are mobilizers of phosphates and plant nutrition in soil
C. At present it is not possible to grow maize without chemical fertilizers
D. Excessive use of chemical fertilizers may lead to eutrophication of nearby water bodies.

Answer

To determine which statement is correct regarding nitrogen fixation and the use of chemical fertilizers, we can analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Analyze the first option:**
- The statement claims that both Azotobacter and Rhizobium fix atmospheric nitrogen in the root nodules of plants.
- **Explanation:** Azotobacter is a free-living bacterium that fixes nitrogen in the soil, while Rhizobium forms symbiotic relationships with leguminous plants and is responsible for the formation of root nodules where nitrogen fixation occurs. Therefore, this statement is incorrect.

2. **Analyze the second option:**
- The statement suggests that cyanobacteria such as Anabaena and Nostoc are mobilizers of phosphate and plant mutation in the soil.
- **Explanation:** Anabaena and Nostoc are known for their ability to fix nitrogen, not phosphate. They contain specialized cells called heterocysts that facilitate nitrogen fixation. Thus, this statement is also incorrect.

3. **Analyze the third option:**
- The statement claims that it is not possible to grow maize without chemical fertilization.
- **Explanation:** It is indeed possible to grow maize using organic methods, such as organic manure and compost, which do not involve chemical fertilizers. Therefore, this statement is incorrect.

4. **Analyze the fourth option:**
- The statement asserts that excessive use of chemical fertilizers may lead to the eutrophication of nearby water bodies.
- **Explanation:** This is correct. The runoff from fields treated with chemical fertilizers can lead to nutrient loading in water bodies, causing algal blooms and subsequent eutrophication, which depletes oxygen in the water and harms aquatic life. Thus, this statement is correct.

### Conclusion:
The correct answer is option number 4: "Excessive use of chemical fertilizer may lead to the eutrophication of nearby water bodies."

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