Nitrogen Cycle

Step 1: Understanding the process.
Nitrification is a two-step process where ammonia (NH₃) is first oxidized to nitrites (NO₂⁻) and then to nitrates (NO₃⁻) by nitrifying bacteria. This process is crucial in the nitrogen cycle, as it makes nitrogen available to plants.
Step 2: Analyzing the options.
(1) Nitrate assimilation: This refers to the process by which plants take up nitrate and convert it into organic nitrogen, but it is not the process of converting ammonia to nitrate.
(2) Nitrification: Correct — Nitrification is the correct process in which ammonia is converted into nitrate through bacterial activity.
(3) Ammonia assimilation: This process refers to the incorporation of ammonia into amino acids and other organic compounds, not the conversion into nitrate.
(4) Denitrification: Denitrification is the process where nitrate is reduced to nitrogen gas, opposite of nitrification.
Step 3: Conclusion.
The correct answer is (2) Nitrification, as it directly involves the conversion of ammonia into nitrate.