Correct Option: D. Changing of ammonia to nitrites, then nitrates Detailed Explanation: Nitrification is a crucial process in the nitrogen cycle, which is essential for soil fertility and plant growth. It involves the conversion of ammonia (NH₃) into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). This process is carried out by specific types of bacteria in the soil, and it occurs in two main steps:

1. Ammonia Oxidation: The first step of nitrification is the oxidation of ammonia to nitrites. This is primarily performed by bacteria known as ammonia-oxidizing bacteria (AOB), such as Nitrosomonas. During this step, ammonia is converted into nitrite through the following reaction: [ \text{NH}_3 + \text{O}_2 \rightarrow \text{NO}_2^- + 3\text{H}^+ ]
2. Nitrite Oxidation: The second step involves the oxidation of nitrites to nitrates, which is carried out by nitrite-oxidizing bacteria (NOB), such as Nitrobacter. The reaction for this step is: [ \text{NO}_2^- + \text{O}_2 \rightarrow \text{NO}_3^- ]

Thus, the overall process of nitrification can be summarized as: [ \text{NH}_3 \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^- ] This process is vital because nitrates are a form of nitrogen that plants can readily absorb and utilize for growth. Why Other Options Are Incorrect:

• Option A: Conversion of nitrates to nitrogen This option describes denitrification, not nitrification. Denitrification is the process where nitrates are reduced to nitrogen gas (N₂) or nitrous oxide (N₂O), which is the opposite of what nitrification entails. Therefore, this option is incorrect.
• Option B: Fixing nitrogen into plants This option refers to nitrogen fixation, which is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) by certain bacteria (like Rhizobium in legumes). This process is different from nitrification, which deals with the conversion of ammonia to nitrites and nitrates. Thus, this option is also incorrect.
• Option C: Conversion of nitrates to nitrites This option describes a part of the denitrification process, where nitrates can be converted back to nitrites. However, nitrification specifically refers to the conversion of ammonia to nitrites and then to nitrates, making this option incorrect as well.

Summary of Key Points:

• Nitrification is the biological process that converts ammonia into nitrites and then into nitrates.
• It occurs in two steps: ammonia oxidation (NH₃ to NO₂⁻) and nitrite oxidation (NO₂⁻ to NO₃⁻).
• Nitrification is essential for making nitrogen available to plants in a usable form.
• Other options (A, B, C) describe processes that are either the reverse of nitrification or entirely different processes related to nitrogen cycling.

This thorough understanding of nitrification will help you grasp its importance in ecosystems and agricultural practices.