Correct Option: B. Soil Nitrates Detailed Explanation: Denitrifying bacteria play a crucial role in the nitrogen cycle, which is essential for maintaining the balance of nitrogen in ecosystems. These bacteria are primarily responsible for converting nitrates (NO₃⁻) and nitrites (NO₂⁻) in the soil back into nitrogen gas (N₂), which is then released into the atmosphere. This process is known as denitrification.

1. Understanding Denitrification:
2. Denitrification is a microbial process that reduces nitrates and nitrites to nitrogen gas. This process is vital because it helps to remove excess nitrogen from the soil, preventing problems such as nutrient pollution in water bodies.
3. The general reaction for denitrification can be summarized as follows: [ 2NO_3^- + 10e^- + 12H^+ \rightarrow N_2 + 6H_2O ]
4. In this reaction, nitrates (NO₃⁻) are reduced to nitrogen gas (N₂) through a series of intermediate steps involving nitrites (NO₂⁻).
5. Role of Soil Nitrates:
6. Soil nitrates are the primary substrates for denitrifying bacteria. When these bacteria encounter nitrates in the soil, they utilize them as an electron acceptor in their metabolic processes, leading to the production of nitrogen gas.
7. This process typically occurs in anaerobic (low-oxygen) conditions, which are often found in waterlogged soils or sediments.

Why Other Options Are Incorrect:

• A. Ammonium Salts:
• Ammonium salts (NH₄⁺) are not directly involved in the denitrification process. Instead, they are typically converted to nitrates through a process called nitrification, which is carried out by nitrifying bacteria. Denitrifying bacteria do not use ammonium salts as a substrate for producing nitrogen gas.
• C. Thunderstorms:
• Thunderstorms can contribute to the nitrogen cycle by facilitating the formation of nitrates through lightning, which converts atmospheric nitrogen (N₂) into nitrates (NO₃⁻) that can be deposited into the soil. However, thunderstorms do not directly liberate nitrogen gas from denitrifying bacteria. This option is unrelated to the denitrification process.
• D. Soil Nitrites:
• While soil nitrites (NO₂⁻) are indeed intermediates in the denitrification process, the primary substrate that denitrifying bacteria utilize to produce nitrogen gas is nitrates (NO₃⁻). Therefore, while nitrites are involved, they are not the direct source of gaseous nitrogen in this context.

Revision Summary:

• Denitrifying bacteria convert soil nitrates (NO₃⁻) into nitrogen gas (N₂) through the process of denitrification.
• This process is essential for maintaining nitrogen balance in ecosystems and preventing nutrient pollution.
• Ammonium salts are not used in denitrification; they are converted to nitrates first.
• Thunderstorms contribute to the nitrogen cycle but do not directly involve denitrifying bacteria.
• Soil nitrites are intermediates but not the primary source of nitrogen gas in denitrification.