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Difficulty: MediumNutrient and Biogeochemical Cycles

An agricultural soil receives ammonium-based fertilizer. Arrange the subsequent biological transformations and processes in their natural sequential order, starting from the conversion of ammonium ions and ending with the synthesis of plant proteins. What is the correct sequence of these steps?

  1. 1Conversion of ammonium ions (NH4+\text{NH}_4^+) into nitrite ions (NO2−\text{NO}_2^-) by *Nitrosomonas* bacteria.
  2. 2Oxidation of nitrite ions (NO2−\text{NO}_2^-) into nitrate ions (NO3−\text{NO}_3^-) by *Nitrobacter* bacteria.
  3. 3Absorption of soluble nitrate ions (NO3−\text{NO}_3^-) from soil water by plant root hair cells.
  4. 4Biochemical reduction and assimilation of absorbed nitrates into amino acids and structural proteins.

Answer

The correct order of steps is: Conversion of ammonium ions into nitrite ions by Nitrosomonas -> Oxidation of nitrite ions into nitrate ions by Nitrobacter -> Absorption of soluble nitrate ions from soil water by plant root hair cells -> Biochemical reduction and assimilation of absorbed nitrates into amino acids and structural proteins.
Nitrification occurs in two sequential steps: *Nitrosomonas* oxidizes ammonium ions into nitrite, and *Nitrobacter* oxidizes nitrite into nitrate. The dissolved nitrate is then absorbed by root hairs and assimilated into plant amino acids and proteins.

Step-by-Step Solution

1
Identify the initial step of nitrification.
Ammonium ions (NH4+\text{NH}_4^+) are first oxidized into nitrite ions (NO2−\text{NO}_2^-) by specialized nitrifying bacteria (*Nitrosomonas*).
Ammonium cannot be taken up efficiently by most plants until converted by nitrifying bacteria.
2
Determine the second phase of nitrification.
Nitrite ions are further oxidized to nitrate ions (NO3−\text{NO}_3^-) by *Nitrobacter*.
Nitrite is toxic to plants and must be converted to nitrate before plant uptake.
3
Identify the mechanism of plant uptake.
Plant root hairs absorb dissolved nitrate ions (NO3−\text{NO}_3^-) from the soil solution.
Nitrate is the primary soluble form of nitrogen utilized by higher plants.
4
Determine the final assimilation step.
Absorbed nitrates are incorporated into organic molecules, producing amino acids and proteins within plant tissues.
Inorganic nitrate must be biochemically converted into organic nitrogenous compounds for plant biomass growth.

Key Concept

Sequential Nitrification and Plant Nitrogen Assimilation
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Topics

Nutrient and Biogeochemical Cycles

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