Which of these holds true for the assimilation process in the nitrogen cycle?

Options

A. Plants and animals consume nitrogen
B. Ammonia changes into nitrates and nitrites.
C. Nitrates in the soil change into nitrogen gas.
D. Nitrogen in living things returns to soil as ammonia.

Answer

To determine which statement holds true for the assimilation process in the nitrogen cycle, let's analyze each option step by step:

1. **Understanding the Assimilation Process**:
- The assimilation process in the nitrogen cycle refers to how plants and animals incorporate nitrogen into their biological systems. This primarily involves the uptake of nitrogen in forms that can be used to synthesize proteins and nucleic acids.

2. **Evaluating the Options**:
- **Option 1**: "Plants and animals consume nitrogen."
- This statement is true. Plants absorb nitrogen from the soil in the form of nitrates and ammonium, while animals obtain nitrogen by consuming plants or other animals.

- **Option 2**: "Ammonia changes into nitrate and nitrite."
- This statement describes the nitrification process, not assimilation. Nitrification is the conversion of ammonia into nitrite and then nitrate by bacteria in the soil.

- **Option 3**: "Nitrate in soil changes into nitrogen gas."
- This statement refers to denitrification, which is the process where nitrates are converted back into nitrogen gas, thus returning nitrogen to the atmosphere. This is not part of the assimilation process.

- **Option 4**: "Nitrogen in living things returns to soil as ammonia."
- This statement pertains to the decomposition process, where dead organisms are broken down, returning nitrogen to the soil as ammonia. This is not directly related to assimilation.

3. **Conclusion**:
- The correct answer is **Option 1: "Plants and animals consume nitrogen."** This accurately describes the assimilation process where nitrogen is incorporated into living organisms.