The correct option is C. ammonification and nitrification. Detailed Explanation To understand why option C is correct, we need to break down the processes involved in making nitrogen available to plants, specifically in the form of nitrates.

1. Nitrogen Cycle Overview:
2. Nitrogen is an essential nutrient for plants, primarily taken up in the form of nitrates (NO₃⁻). The nitrogen cycle describes how nitrogen moves through the environment, including the atmosphere, soil, and living organisms.
3. Ammonification:
4. This is the first step in the process of converting organic nitrogen into a form that plants can use.
5. When plants and animals die, or when they excrete waste, organic nitrogen compounds (like proteins and nucleic acids) are broken down by decomposers (bacteria and fungi) into ammonia (NH₃).
6. This process is called ammonification. The equation can be simplified as: [ \text{Organic Nitrogen} \rightarrow \text{Ammonia (NH₃)} ]
7. Nitrification:
8. The ammonia produced from ammonification is then converted into nitrates through a two-step process known as nitrification.
9. In the first step, ammonia is oxidized to nitrites (NO₂⁻) by bacteria such as Nitrosomonas. The reaction can be represented as: [ \text{NH₃} + \text{O₂} \rightarrow \text{NO₂⁻} + \text{H₂O} + \text{H⁺} ]
10. In the second step, nitrites are further oxidized to nitrates by bacteria such as Nitrobacter: [ \text{NO₂⁻} + \text{O₂} \rightarrow \text{NO₃⁻} ]
11. The end product of this process is nitrates (NO₃⁻), which are readily absorbed by plant roots.

Why Other Options Are Incorrect

• Option A: Deamination and Nitrification:
• Deamination is the process of removing an amino group from an amino acid, resulting in ammonia. While it does produce ammonia, it is not the primary process that converts organic nitrogen into ammonia in the context of the nitrogen cycle. Therefore, this option is incomplete.
• Option B: Ammonification and Denitrification:
• Denitrification is the process where nitrates are converted back into nitrogen gas (N₂) by bacteria, thus removing nitrogen from the soil and returning it to the atmosphere. This process does not contribute to making nitrogen available to plants; instead, it reduces the nitrogen available. Hence, this option is incorrect.
• Option D: Deamination and Denitrification:
• Similar to option A, deamination does not directly lead to the production of nitrates. Moreover, denitrification, as explained, removes nitrates from the soil. Therefore, this option is also incorrect.

Summary of Key Points

• Ammonification converts organic nitrogen into ammonia, which is the first step in making nitrogen available to plants.
• Nitrification then converts ammonia into nitrates, the form that plants can absorb.
• Other processes like denitrification remove nitrogen from the soil, making them irrelevant for the question.
• Understanding the nitrogen cycle is crucial for grasping how nitrogen becomes available to plants.

This thorough understanding of the nitrogen cycle and the specific processes involved will help you in your studies and exams related to agricultural science.