Part of N₂ cycle that is opposed to N₂ fixation is that

Question

Accepted Answer

The question asks about the part of the nitrogen (N₂) cycle that is opposed to N₂ fixation. The correct answer is Option 1: Denitrification. Let's explore why this is the correct answer and why the other options are incorrect.

1. **Denitrification (Correct Answer):** Denitrification is the process by which nitrates (NO₃⁻) are reduced and ultimately converted back into nitrogen gas (N₂), which is released into the atmosphere. This process is carried out by certain bacteria under anaerobic conditions. Denitrification is essentially the reverse of nitrogen fixation. While nitrogen fixation involves converting atmospheric nitrogen gas (N₂) into a form usable by plants (such as ammonia, NH₃), denitrification converts these usable forms back into atmospheric nitrogen, thus opposing the fixation process.

2. **Ammonification (Incorrect):** Ammonification is the process by which organic nitrogen from dead plants and animals is converted into ammonia (NH₃) or ammonium ions (NH₄⁺) by decomposer organisms. This process does not oppose nitrogen fixation; rather, it is part of the nitrogen cycle that makes nitrogen available in a form that can be further processed by nitrification or taken up by plants.

3. **Nitrification (Incorrect):** Nitrification is the biological oxidation of ammonia (NH₃) to nitrite (NO₂⁻) followed by the oxidation of nitrite to nitrate (NO₃⁻). This process is carried out by nitrifying bacteria and is a step in the nitrogen cycle that follows ammonification. Nitrification does not oppose nitrogen fixation; instead, it transforms ammonia into nitrates, which plants can absorb and use.

4. **Carbonisation (Incorrect):** Carbonisation is not a part of the nitrogen cycle. It refers to the conversion of organic substances into carbon or carbon-containing residues through pyrolysis or destructive distillation. Since carbonisation is unrelated to the nitrogen cycle, it does not oppose nitrogen fixation.

In summary, denitrification is the process that directly opposes nitrogen fixation by converting fixed nitrogen back into atmospheric nitrogen gas, thereby completing the nitrogen cycle. This makes Option 1 the correct answer.