In environmental microbiology and the nitrogen cycle: the direct conversion of atmospheric molecular nitrogen (N2) into ammonia (NH3) is known as which process?

Introduction / Context:
Nitrogen moves through ecosystems via tightly linked microbial processes. A frequent exam task is to identify which named step converts inert atmospheric N2 into a reduced, biologically available form that plants and many microbes can use directly or after further transformation.

Given Data / Assumptions:

• Atmospheric nitrogen gas is represented as N2 and is highly stable.
• The product asked for is ammonia (NH3), a reduced form of nitrogen.
• We must match these clues to the correct nitrogen-cycle term.

Concept / Approach:
Nitrogen fixation is the biological (or industrial/abiotic) reduction of N2 to NH3. Biological fixation is catalyzed by the enzyme nitrogenase, found in free-living bacteria (e.g., Azotobacter, Clostridium), cyanobacteria (e.g., Anabaena, Nostoc), and symbionts (e.g., Rhizobium in legume nodules). This step introduces reactive nitrogen into ecosystems.

Step-by-Step Solution:
Identify the substrate and product: N2 → NH3. Recall the named step that performs this reduction: nitrogen fixation. Distinguish from other steps (nitrification, denitrification, ammonification) that act on already fixed nitrogen, not on N2. Select the option that exactly describes the conversion: nitrogen fixation.

Verification / Alternative check:
Standard nitrogen cycle diagrams show nitrogen fixation as the entry point of atmospheric nitrogen into biospheric pools, before nitrification (NH3 → NO2− → NO3−) and before assimilation by plants or microbes.

Why Other Options Are Wrong:

• Nitrification: Oxidation of NH3/NH4+ to nitrite and nitrate; does not use N2.
• Denitrification: Reduction of nitrate/nitrite to gaseous N2 or N2O; the reverse of fixation.
• Ammonification: Decomposition of organic nitrogen to NH3/NH4+; does not act on N2.
• Assimilation: Uptake of nitrate or ammonium into biomass; not the conversion of N2 to NH3.

Common Pitfalls:
Confusing ammonification (organic N → NH3) with fixation (N2 → NH3); remembering that only fixation uses atmospheric N2 helps avoid this error.

Final Answer:
Nitrogen fixation is the conversion of N2 into NH3.