Nitrogen is a limiting nutrient for both (A)____ and (B) ecosystems.

Answer

**Step-by-Step Solution:**

1. **Understanding Ecosystems**: Identify the two main types of ecosystems: natural ecosystems and artificial (or agricultural) ecosystems. Natural ecosystems include terrestrial (land) and aquatic (ocean) environments.

2. **Role of Nitrogen**: Recognize that nitrogen is a crucial nutrient for all living organisms. It is a major component of amino acids, proteins, nucleic acids (DNA and RNA), and other vital biomolecules.

3. **Nitrogen Availability**: Note that nitrogen gas (N₂) makes up about 80% of the Earth's atmosphere, but it is not in a form that organisms can use directly.

4. **Nitrogen Fixation**: Understand the process of nitrogen fixation, where nitrogen gas is converted into usable forms such as ammonium (NH₄⁺) by nitrogen-fixing bacteria. This process is essential for making nitrogen available to plants.

5. **Nitrification**: Learn about nitrification, where ammonium is further converted into nitrites (NO₂⁻) and then nitrates (NO₃⁻), which are the forms of nitrogen that plants can absorb and utilize for growth.

6. **Denitrification**: Acknowledge the process of denitrification, where nitrates are converted back into nitrogen gas, completing the nitrogen cycle.

7. **Limiting Nutrient Concept**: Conclude that nitrogen is a limiting nutrient because its availability directly affects the productivity and sustainability of both natural and agricultural ecosystems. Without sufficient nitrogen, these ecosystems cannot support life effectively.

**Final Answer**: Nitrogen is a limiting nutrient for both (A) natural ecosystems and (B) agricultural ecosystems.

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