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Difficulty: HardNutrient and Biogeochemical Cycles

The nitrogen cycle involves a sequential series of metabolic transformations mediated by specialized soil microorganisms. What is the correct chronological sequence of these biological processes, starting from the decay of organic waste to the release of free nitrogen gas into the atmosphere?

  1. 1Decomposition of nitrogenous organic matter into ammonium ions (NH4+NH_4^+) by saprophytic decomposers and ammonifying bacteria.
  2. 2Oxidation of ammonium ions (NH4+NH_4^+) into nitrite ions (NO2−NO_2^-) by *Nitrosomonas* bacteria.
  3. 3Oxidation of nitrite ions (NO2−NO_2^-) into nitrate ions (NO3−NO_3^-) by *Nitrobacter* bacteria.
  4. 4Anaerobic reduction of nitrate ions (NO3−NO_3^-) into elemental nitrogen gas (N2N_2) by *Pseudomonas* bacteria.

Answer

The correct sequence of transformations is: (1) Decomposition of nitrogenous organic matter into ammonium ions (NH4+NH_4^+) by saprophytes and ammonifying bacteria, (2) Oxidation of ammonium ions (NH4+NH_4^+) into nitrite ions (NO2−NO_2^-) by Nitrosomonas, (3) Oxidation of nitrite ions (NO2−NO_2^-) into nitrate ions (NO3−NO_3^-) by Nitrobacter, and (4) Anaerobic reduction of nitrate ions (NO3−NO_3^-) into elemental nitrogen gas (N2N_2) by Pseudomonas.
The nitrogen cycle pathway begins with ammonification (conversion of organic wastes into ammonium ions), followed by two sequential nitrifying steps: nitritation (ammonium to nitrite by Nitrosomonas) and nitratation (nitrite to nitrate by Nitrobacter). Finally, denitrification converts nitrate ions back into atmospheric nitrogen gas via Pseudomonas under anaerobic conditions.

Step-by-Step Solution

1
Identify the initial organic reactant stage.
Ammonification converts organic protein/urea waste into inorganic ammonium ions (NH4+NH_4^+).
Complex nitrogen compounds bound in dead organic material must be broken down by saprophytic microbes before chemoautotrophic bacterial oxidation can occur.
2
Identify the first stage of nitrification (nitritation).
Nitrosomonas oxidizes ammonium ions (NH4+NH_4^+) to nitrite ions (NO2−NO_2^-).
Ammonium serves as the specific electron donor and substrate for Nitrosomonas.
3
Identify the second stage of nitrification (nitratation).
Nitrobacter oxidizes nitrite ions (NO2−NO_2^-) to nitrate ions (NO3−NO_3^-).
Nitrobacter utilizes the nitrite produced by Nitrosomonas and converts it into nitrate.
4
Identify the terminal atmospheric release stage (denitrification).
Pseudomonas reduces nitrate ions (NO3−NO_3^-) back to atmospheric nitrogen gas (N2N_2).
In low-oxygen environment conditions, denitrifying bacteria utilize nitrate as a terminal electron acceptor, closing the biogeochemical loop.

Key Concept

Sequential biochemical conversions in the nitrogen cycle
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Topics

Nutrient and Biogeochemical Cycles

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