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Difficulty: MediumNutrient and Biogeochemical Cycles

A soil sample taken from an uncultivated grassland with no leguminous plants shows a steady increase in fixed nitrogen compounds. Laboratory analysis confirms the activity of free-living, aerobic bacteria capable of directly fixing atmospheric nitrogen (N2N_2) into biological compounds without forming root nodule associations. Which microorganism is responsible for this nitrogen fixation process?

  1. AzotobacterAnswer
  2. B
    Nitrobacter
  3. C
    Pseudomonas
  4. D
    Nitrosomonas

Answer

Azotobacter is the free-living aerobic bacterium responsible for fixing atmospheric nitrogen in soil without forming root nodule symbioses.
The option stating 'Azotobacter' is correct because Azotobacter is a genus of non-symbiotic, free-living, aerobic bacteria that convert atmospheric nitrogen (N2N_2) into ammonia and organic nitrogen compounds in soil ecosystems.

Step-by-Step Solution

1
Analyze the ecological condition described in the stem
Nitrogen fixation occurs non-symbiotically (no leguminous host plants) under aerobic soil conditions.
Different nitrogen-fixing organisms operate under distinct ecological constraints (symbiotic vs. free-living, aerobic vs. anaerobic).
2
Evaluate the metabolic roles of candidate bacteria
Azotobacter is a free-living aerobic nitrogen fixer. Rhizobium requires leguminous host roots, Clostridium is anaerobic, Nitrosomonas/Nitrobacter are nitrifiers, and Pseudomonas is a denitrifier.
Identifying the specific metabolic role prevents confusing nitrifying, denitrifying, and nitrogen-fixing bacteria.

Key Concept

Free-living nitrogen fixation in the nitrogen cycle
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Topics

Nutrient and Biogeochemical Cycles

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