Step 1: Concept Overview:
The problem requires identifying the original substance transformed into ammonia during ammonification within the nitrogen cycle.
Step 2: In-Depth Analysis:
Ammonification is a decomposition process. Decomposing plants and animals, along with animal waste, contain organic matter rich in complex nitrogen compounds like proteins, nucleic acids, and urea.
Microbial decomposers (bacteria and fungi) degrade these complex organic molecules. Proteins are broken down into their fundamental components: amino acids.
Subsequently, these decomposers process the amino acids, extracting the amino group (-NH\textsubscript{2}) and transforming it into ammonia (\(NH_3\)) or ammonium ions (\(NH_4^+\)), which are released back into the soil.
The remaining options are incorrect because:
Nitrates and Nitrites are reduced to ammonia through a separate mechanism known as dissimilatory nitrate reduction, not ammonification.
Nitrogen gas (\(N_2\)) becomes ammonia via nitrogen fixation.
Step 3: Conclusion:
Ammonification is the production of ammonia from the decomposition of organic nitrogen, primarily amino acids sourced from proteins.