The correct option is B. converting ammonium salt to nitrate. Detailed Explanation: Nitrifying bacteria play a crucial role in the nitrogen cycle, which is essential for maintaining soil fertility. Let's break down the process and understand why option B is correct and why the other options are not.

1. Understanding Nitrification:
2. Nitrification is a two-step process carried out by specific types of bacteria in the soil. It involves the conversion of ammonia (NH₃) or ammonium ions (NH₄⁺) into nitrates (NO₃⁻), which plants can readily absorb and use for growth.
3. The two main groups of nitrifying bacteria involved in this process are:
   • Ammonia-oxidizing bacteria (AOB): These bacteria convert ammonium ions into nitrites (NO₂⁻).
   • Nitrite-oxidizing bacteria (NOB): These bacteria then convert nitrites into nitrates.
4. Why Option B is Correct:
5. Option B states that nitrifying bacteria convert ammonium salt to nitrate. This is precisely what happens during the nitrification process. Ammonium salts, which can come from organic matter decomposition or fertilizers, are converted into nitrates, making nitrogen available to plants.
6. This conversion is vital because nitrates are a primary source of nitrogen for plants, which is necessary for synthesizing proteins and nucleic acids.
7. Why the Other Options are Incorrect:
8. Option A: Converting nitrate to nitrogen:
   • This option describes a process known as denitrification, not nitrification. Denitrifying bacteria convert nitrates back into nitrogen gas (N₂), which is released into the atmosphere. This process reduces the amount of nitrogen available in the soil for plants, thus not contributing to soil fertility.
9. Option C: Converting atmospheric nitrogen to plant nitrogen:
   • This option refers to nitrogen fixation, which is a different process. Nitrogen-fixing bacteria (such as Rhizobium) convert atmospheric nitrogen (N₂) into ammonia, which can then be used by plants. While this is important for soil fertility, it is not the role of nitrifying bacteria.
10. Option D: Converting nitrate to nitrous oxide:
    • Similar to option A, this describes a denitrification process where nitrates are converted into nitrous oxide (N₂O) or nitrogen gas. This process also does not contribute to soil fertility in the same way that nitrification does, as it reduces the available nitrogen in the soil.

Summary of Key Points:

• Nitrifying bacteria convert ammonium salts into nitrates, which are essential for plant growth.
• The nitrification process involves two steps: ammonium to nitrite and then nitrite to nitrate.
• Denitrification and nitrogen fixation are different processes involving other types of bacteria.
• Understanding the roles of different bacteria in the nitrogen cycle is crucial for grasping soil fertility dynamics.

Revision Summary:

• Nitrifying bacteria convert ammonium salts to nitrates, enhancing soil fertility.
• The nitrification process consists of two steps: NH₄⁺ → NO₂⁻ → NO₃⁻.
• Denitrification (nitrate to nitrogen) and nitrogen fixation (atmospheric nitrogen to ammonia) are separate processes.
• Recognizing the specific roles of bacteria in the nitrogen cycle is essential for understanding ecosystem health.