The microbial transformation of nitrates (NO3-) to gaseous nitrogen (N2 and N2O) through sequential reductions is known as:

Introduction:
Nitrogen cycling includes oxidative and reductive steps that move nitrogen between inorganic and gaseous forms. This question asks you to identify the process that removes fixed nitrogen from ecosystems by converting nitrate to gaseous products returned to the atmosphere.

Given Data / Assumptions:

• Denitrification occurs primarily under low oxygen conditions.
• Key gaseous products are nitrous oxide (N2O) and dinitrogen (N2).
• Different processes in the nitrogen cycle must not be confused.

Concept / Approach:
Denitrification is a dissimilatory respiratory pathway where microbes use nitrate and nitrite as alternative electron acceptors, producing NO, N2O, and N2. In contrast, nitrification oxidizes ammonia to nitrite and nitrate, nitrogen fixation reduces N2 to NH3, and ammonification releases NH3 from organic nitrogen via decomposition.

Step-by-Step Solution:
Read the transformation direction: nitrate to gaseous nitrogen. Match to process definitions: respiratory reduction sequence defines denitrification. Exclude oxidative nitrification and reductive fixation to NH3. Select denitrification as the correct term.

Verification / Alternative check:
Gas flux measurements and isotopic tracing in wetlands, sediments, and agricultural soils verify nitrate loss via denitrification to N2/N2O.

Why Other Options Are Wrong:

• Nitrification: Oxidation of NH3 to NO2-/NO3-.
• Nitrogen fixation: Reduction of N2 to NH3.
• Ammonification: Mineralization of organic nitrogen to NH3/NH4+.

Common Pitfalls:
Confusing denitrification with dissimilatory nitrate reduction to ammonium (DNRA); only denitrification yields gaseous nitrogen end products.

Final Answer:
denitrification is the process converting nitrate to gaseous nitrogen.