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Difficulty: HardNutrient and Biogeochemical Cycles

A farmer notices a rapid loss of soil nitrogen in a waterlogged maize field that had been treated with ammonium-based fertilizer. Soil analysis confirms that ammonium ions (NH4+\text{NH}_4^+) were first oxidized to nitrites (NO2−\text{NO}_2^-), then further oxidized to nitrates (NO3−\text{NO}_3^-), which were subsequently reduced to gaseous nitrogen (N2\text{N}_2) under anaerobic conditions. Which sequence of bacteria is sequentially responsible for these three specific biochemical transformations?

  1. Nitrosomonas, Nitrobacter, and PseudomonasAnswer
  2. B
    Nitrobacter, Nitrosomonas, and Rhizobium
  3. C
    Azotobacter, Nitrobacter, and Clostridium
  4. D
    Nitrosomonas, Rhizobium, and Nitrobacter

Answer

The sequence Nitrosomonas, Nitrobacter, and Pseudomonas correctly identifies the organisms performing ammonium oxidation, nitrite oxidation, and denitrification respectively.
The first step of nitrification (oxidation of ammonium NH4+\text{NH}_4^+ to nitrite NO2−\text{NO}_2^-) is performed by Nitrosomonas. The second step (oxidation of nitrite NO2−\text{NO}_2^- to nitrate NO3−\text{NO}_3^-) is performed by Nitrobacter. Under waterlogged, oxygen-depleted soil conditions, anaerobic denitrifying bacteria such as Pseudomonas convert soil nitrates back into gaseous elemental nitrogen (N2\text{N}_2), causing a loss of soil fertility.

Step-by-Step Solution

1
Identify the bacterium responsible for converting ammonium ions (NH4+\text{NH}_4^+) to nitrite ions (NO2−\text{NO}_2^-).
Nitrosomonas is the nitrifying bacterium that carries out the first step of nitrification.
Ammonium oxidation is an aerobic process mediated by chemoautotrophic bacteria like Nitrosomonas.
2
Identify the bacterium responsible for converting nitrite ions (NO2−\text{NO}_2^-) to nitrate ions (NO3−\text{NO}_3^-).
Nitrobacter completes nitrification by oxidizing nitrite to nitrate.
Nitrate is the primary form of nitrogen absorbed by plants, generated through nitrite oxidation by Nitrobacter.
3
Identify the bacterium responsible for converting nitrates (NO3−\text{NO}_3^-) into gaseous nitrogen (N2\text{N}_2) in waterlogged, anaerobic soil.
Pseudomonas (or Thiobacillus denitrificans) conducts denitrification, returning nitrogen gas to the atmosphere.
Waterlogged soils lack molecular oxygen, forcing facultative anaerobes like Pseudomonas to use nitrate as a terminal electron acceptor.

Key Concept

Nitrification and Denitrification Pathways in the Nitrogen Cycle
Estimated Time:2m 0s
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Topics

Nutrient and Biogeochemical Cycles

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