Nitrifying bacteria participate in the nitrogen cycle mainly by:

• A. Reducing nitrate (NO_3^-) to nitrogen gas (N_2)
• B. Fixing atmospheric nitrogen (N_2) into organic compounds
• C. Converting ammonia (NH_3) into nitrite (NO_2^-) and nitrate (NO_3^-)
• D. Decomposing organic matter into ammonia (NH_3)

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the role of nitrifying bacteria in the nitrogen cycle. These bacteria are involved in the process of nitrification, which is the biological oxidation of ammonia to nitrate.
2. Step 2: Recall that nitrification occurs in two main stages: first, ammonia (NH_3) is oxidized to nitrite (NO_2^-), and second, nitrite is further oxidized to nitrate (NO_3^-).
3. Step 3: Identify the types of bacteria responsible for each stage: ammonia-oxidizing bacteria convert NH_3 to NO_2^-, and nitrite-oxidizing bacteria convert NO_2^- to NO_3^-.
4. Step 4: Recognize that nitrifying bacteria do not fix atmospheric nitrogen (N_2) into organic compounds; that process is carried out by nitrogen-fixing bacteria.
5. Step 5: Also note that nitrifying bacteria do not reduce nitrate to nitrogen gas nor decompose organic matter into ammonia; these are functions of other groups of bacteria in the nitrogen cycle.