Correct option is B
Introduction
The global nitrogen cycle transforms nitrogen among its many oxidation states through a series of microbially mediated processes.
These conversions move nitrogen between atmospheric, soil, aquatic, and biological pools, and are essential to ecosystem productivity and nutrient availability.
Information Booster
A. Nitrification — the aerobic, two-step microbial oxidation of ammonia to nitrite and then to nitrate (NH₄⁺ → NO₂⁻ → NO₃⁻).
The first step is carried out by ammonia-oxidising bacteria such as Nitrosomonas, and the second by nitrite-oxidising bacteria such as Nitrobacter.
These chemoautotrophs derive energy from the oxidation and require oxygen, so nitrification is favoured in well-aerated soils and waters.
B. Ammonification (mineralization) — the microbial decomposition of organic nitrogen compounds (proteins, amino acids, urea, nucleic acids) from dead plant and animal matter into ammonia/ammonium.
Decomposer bacteria and fungi return nitrogen from the organic pool to an inorganic form that plants and microbes can reuse.
C. Dissimilatory nitrate reduction (DNRA) — the anaerobic reduction of nitrate to ammonium, where nitrate serves as a terminal electron acceptor for energy generation rather than for building biomass.
Unlike denitrification, DNRA conserves nitrogen within the ecosystem by retaining it as bioavailable ammonium, and it tends to dominate in highly reducing, carbon-rich environments.
D. Denitrification — the anaerobic, stepwise reduction of nitrate to gaseous nitrogen (NO₃⁻ → NO₂⁻ → NO → N₂O → N₂), performed by facultative anaerobes such as Pseudomonas.
It returns nitrogen to the atmosphere, closing the cycle, but also releases nitrous oxide (N₂O), a potent greenhouse gas and ozone-depleting agent.
E. Assimilatory nitrate reduction — the uptake and reduction of nitrate to ammonium for incorporation into biomass (amino acids and proteins), driven by the organism's nutritional nitrogen requirement.
This differs fundamentally from the dissimilatory route because the nitrogen is retained in the cell rather than used for respiration.
All five, together with biological nitrogen fixation (the conversion of atmospheric N₂ to ammonia), constitute the complete suite of nitrogen transformations that regulate the movement and availability of nitrogen across the biosphere.
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