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Difficulty: HardNitrogen Gas, Nitrogen Cycle, and Oxides of Nitrogen

Arrange the following sequential transformations of nitrogen in the biological nitrogen cycle, beginning with atmospheric molecular nitrogen (N2N_2) and concluding with its re-release into the atmosphere:

  1. 1Nitrogen fixation of atmospheric nitrogen gas (N2N_2) into ammonium ions (NH4+NH_4^+) by diazotrophic bacteria.
  2. 2Nitrosification converting ammonium ions (NH4+NH_4^+) into nitrite ions (NO2−NO_2^-) by *Nitrosomonas* bacteria.
  3. 3Nitration oxidizing nitrite ions (NO2−NO_2^-) into nitrate ions (NO3−NO_3^-) by *Nitrobacter* bacteria.
  4. 4Assimilation of nitrate ions (NO3−NO_3^-) into organic nitrogenous macromolecules such as plant proteins.
  5. 5Denitrification reducing soil nitrate ions (NO3−NO_3^-) back into gaseous molecular nitrogen (N2N_2) by anaerobic bacteria.

Answer

The correct sequence of nitrogen cycle transformations is: Nitrogen fixation (N2→NH4+N_2 \rightarrow NH_4^+), Nitrosification (NH4+→NO2−NH_4^+ \rightarrow NO_2^-), Nitration (NO2−→NO3−NO_2^- \rightarrow NO_3^-), Assimilation (NO3−→organic plant proteinsNO_3^- \rightarrow \text{organic plant proteins}), and Denitrification (NO3−→N2NO_3^- \rightarrow N_2).
The biological nitrogen cycle proceeds in a specific logical sequence. First, atmospheric nitrogen gas (N2N_2) must undergo nitrogen fixation to become ammonium ions (NH4+NH_4^+). Next, nitrification occurs in two steps: nitrosification converts ammonium (NH4+NH_4^+) to nitrite (NO2−NO_2^-) via *Nitrosomonas*, followed by nitration converting nitrite (NO2−NO_2^-) to nitrate (NO3−NO_3^-) via *Nitrobacter*. Plants then assimilate these nitrate ions (NO3−NO_3^-) into organic plant proteins. Finally, denitrifying bacteria reduce residual soil nitrates (NO3−NO_3^-) back into atmospheric nitrogen gas (N2N_2) under anaerobic conditions.

Step-by-Step Solution

1
Identify the initial entry point of atmospheric nitrogen into the biosphere.
Gaseous nitrogen (N2N_2) is fixed into ammonium ions (NH4+NH_4^+) by nitrogen-fixing bacteria (e.g., *Rhizobium*, *Azotobacter*).
Atmospheric N2N_2 has a strong triple covalent bond (N≡NN \equiv N) and cannot be directly utilized by higher plants without preliminary biological fixation.
2
Determine the initial oxidation stage of nitrification (nitrosification).
Ammonium ions (NH4+NH_4^+) are oxidized to nitrite ions (NO2−NO_2^-) by *Nitrosomonas*.
Nitrification occurs in two distinct microbial steps, starting with ammonium conversion to nitrite.
3
Determine the second oxidation stage of nitrification (nitration).
Nitrite ions (NO2−NO_2^-) are oxidized to nitrate ions (NO3−NO_3^-) by *Nitrobacter*.
Nitrate (NO3−NO_3^-) is the most readily absorbed and utilized form of inorganic nitrogen for plants.
4
Trace the biological uptake of bioavailable soil nitrogen.
Plants absorb NO3−NO_3^- ions and assimilate them into plant proteins and nucleic acids.
Inorganic nitrate is reduced inside plant tissues and converted into organic amino acids.
5
Identify the pathway responsible for returning gaseous nitrogen to the atmosphere.
Denitrifying bacteria (e.g., *Pseudomonas denitrificans*) convert unabsorbed soil nitrates (NO3−NO_3^-) into atmospheric N2N_2 gas under anaerobic conditions.
Denitrification closes the global nitrogen loop by replenishing free atmospheric N2N_2.

Key Concept

Nitrogen Cycle Transformations and Microorganisms
Estimated Time:2m 0s
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Topics

Nitrogen Gas, Nitrogen Cycle, and Oxides of Nitrogen

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