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Difficulty: HardNutrient and Biogeochemical Cycles

Arrange the following microbial and biochemical transformations of nitrogen in sequential order, beginning with the fixation of atmospheric dinitrogen (N2N_2) gas by symbiotic root nodule bacteria and ending with the release of gaseous dinitrogen back into the atmosphere.

  1. 1Reduction of atmospheric dinitrogen gas (N2N_2) into organic nitrogenous compounds within root nodules by *Rhizobium*.
  2. 2Decomposition of organic nitrogen wastes into ammonium ions (NH4+NH_4^+) by saprophytic ammonifying microorganisms.
  3. 3Oxidation of ammonium ions (NH4+NH_4^+) into nitrite ions (NO2−NO_2^-) by *Nitrosomonas* bacteria.
  4. 4Oxidation of nitrite ions (NO2−NO_2^-) into nitrate ions (NO3−NO_3^-) by *Nitrobacter* bacteria.
  5. 5Reduction of soil nitrate ions (NO3−NO_3^-) back into dinitrogen gas (N2N_2) by *Pseudomonas* under anaerobic conditions.

Answer

The correct sequence of transformations is: (1) Reduction of atmospheric dinitrogen gas (N2N_2) by symbiotic *Rhizobium* inside root nodules, followed by (2) Decomposition of organic nitrogen wastes into ammonium ions (NH4+NH_4^+) by ammonifying saprophytes, then (3) Oxidation of ammonium ions (NH4+NH_4^+) to nitrite (NO2−NO_2^-) by *Nitrosomonas*, followed by (4) Oxidation of nitrite (NO2−NO_2^-) to nitrate (NO3−NO_3^-) by *Nitrobacter*, and finally (5) Reduction of soil nitrates (NO3−NO_3^-) to dinitrogen gas (N2N_2) by *Pseudomonas* under anaerobic conditions.
The biological nitrogen cycle begins with nitrogen fixation by *Rhizobium*, which converts inert atmospheric dinitrogen (N2N_2) into organic amino acids and proteins in legumes. Upon plant death or excretion, ammonifying decomposers convert organic nitrogen into ammonium ions (NH4+NH_4^+). Two-step nitrification follows: first, *Nitrosomonas* oxidizes ammonium to nitrite (NO2−NO_2^-), and second, *Nitrobacter* oxidizes nitrite to nitrate (NO3−NO_3^-). Finally, anaerobic *Pseudomonas* carries out denitrification, reducing nitrates back to atmospheric dinitrogen gas (N2N_2), completing the cycle.

Step-by-Step Solution

1
Identify the initial process fixing elemental nitrogen gas (N2N_2) into biological systems.
Symbiotic fixation by *Rhizobium* in root nodules converts gaseous N2N_2 into organic nitrogen compounds.
Atmospheric nitrogen cannot be directly utilized by plants without biological fixation by specialized prokaryotes.
2
Trace the movement of organic nitrogen through biological consumption and excretion to ammonification.
Saprophytic bacteria and fungi break down organic nitrogen compounds into inorganic ammonium ions (NH4+NH_4^+).
Ammonification is necessary to release bound organic nitrogen from dead tissues and excretions back into soil ionic forms.
3
Determine the first step of nitrification.
Chemoautotrophic *Nitrosomonas* bacteria oxidize ammonium ions (NH4+NH_4^+) to nitrite ions (NO2−NO_2^-).
Nitrification proceeds in two distinct obligate stages, starting with ammonium oxidation.
4
Determine the second step of nitrification.
*Nitrobacter* bacteria oxidize toxic nitrite ions (NO2−NO_2^-) into bioavailable nitrate ions (NO3−NO_3^-).
Nitrate is the chief chemical form of nitrogen absorbed and assimilated by terrestrial plants.
5
Identify the closing pathway of the cycle returning nitrogen to the gaseous state.
Anaerobic denitrifying bacteria such as *Pseudomonas* reduce nitrates (NO3−NO_3^-) back into atmospheric dinitrogen gas (N2N_2).
Denitrification prevents complete accumulation of soil nitrates and restores atmospheric dinitrogen balance.

Key Concept

Biogeochemical Nitrogen Cycle Transformation Pathway
Estimated Time:1m 30s
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Topics

Nutrient and Biogeochemical Cycles

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