Step 1: Role of nitrogen in living systems.
Nitrogen is a vital nutrient for plants and animals, but the nitrogen present in the atmosphere as \(N_2\) is chemically inert. Because of this, most organisms cannot use atmospheric nitrogen directly. It must first be transformed into usable chemical forms.
Step 2: Conversion of atmospheric nitrogen.
Certain microorganisms have the unique ability to convert atmospheric nitrogen into ammonia (\(NH_3\)). This transformation is carried out by nitrogen-fixing bacteria such as Rhizobium, Azotobacter, and some cyanobacteria. Ammonia produced in this way can then be absorbed by plants and enter the food chain.
Step 3: Examining the given processes.
(A) Nitrification: Conversion of ammonia into nitrites and nitrates — not the correct process here.
(B) Denitrification: Conversion of nitrates back into atmospheric nitrogen — opposite of what is asked.
(C) Nitrogen fixation: Conversion of atmospheric nitrogen into ammonia by bacteria — correct process.
(D) Ammonification: Breakdown of organic nitrogen into ammonia during decomposition — different step of the cycle.
Step 4: Final conclusion.
The bacterial process that converts atmospheric nitrogen into ammonia is known as nitrogen fixation.
\[ \boxed{\text{Nitrogen fixation}} \]