Define denitrification.

Explanation

### Step-by-Step Solution:

1. **Understanding Denitrification**: Denitrification is a biological process that involves the conversion of nitrate ions (NO3-) into nitrogen gas (N2) or, to a lesser extent, nitrous oxide (N2O). This process is crucial for the nitrogen cycle.

2. **Relation to Nitrification**: Denitrification is closely related to nitrification, which is the process where ammonia (NH3) is oxidized to nitrite ions (NO2-) and then to nitrate ions (NO3-). Nitrification occurs first, leading to the formation of nitrates in the soil.

3. **Process of Denitrification**: In denitrification, the nitrate ions present in the soil are reduced back to nitrogen gas. This reduction process involves the use of specific bacteria that facilitate the conversion of nitrates to nitrogen gas.

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