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Difficulty: HardNutrient and Biogeochemical Cycles

A waterlogged agricultural field experiences prolonged anaerobic conditions following excessive irrigation. Laboratory analysis of the soil reveals a marked decline in soil nitrate (NO3−NO_3^-) concentration accompanied by a corresponding release of dinitrogen gas (N2N_2) into the atmosphere. Which microbial process and bacterial genus are primarily responsible for this nitrogen transformation?

  1. Denitrification by Pseudomonas speciesAnswer
  2. B
    Nitrification by Nitrosomonas species
  3. C
    Nitrogen fixation by Azotobacter species
  4. D
    Ammonification by Clostridium species

Answer

Denitrification by Pseudomonas species
Denitrification is the anaerobic microbial reduction of soil nitrates (NO3−NO_3^-) to atmospheric dinitrogen gas (N2N_2). Species of the bacterial genus Pseudomonas thrive in oxygen-depleted environments such as waterlogged soils, utilizing nitrate during anaerobic respiration and consequently reducing soil nitrogen levels.

Step-by-Step Solution

1
Analyze environmental conditions and chemical change
Waterlogging creates anoxic (anaerobic) soil conditions, leading to the reduction of soil nitrate (NO3−NO_3^-) into gaseous dinitrogen (N2N_2).
When oxygen is depleted, facultative anaerobic microbes utilize nitrate as an alternative terminal electron acceptor in cellular respiration.
2
Identify the specific metabolic pathway
The conversion of nitrate (NO3−NO_3^-) to gaseous nitrogen (N2N_2) is denitrification.
Denitrification returns fixed soil nitrogen back to the atmospheric reservoir as dinitrogen gas.
3
Match the pathway with the correct microbial agent
Pseudomonas species (and Thiobacillus denitrificans) are classic denitrifying bacteria.
Nitrosomonas performs nitrification (ammonia oxidation), Azotobacter fixes N2N_2 gas, and ammonification decomposes organic residues into ammonium.

Key Concept

Denitrification in Anaerobic Soil Microenvironments
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Topics

Nutrient and Biogeochemical Cycles

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