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Difficulty: MediumNutrient and Biogeochemical Cycles

In an agricultural ecosystem, farmers frequently rotate cereal crops with leguminous plants such as cowpeas to maintain soil fertility. Which of the following biological processes explains how legumes contribute to restoring nitrogen levels in the soil?

  1. Conversion of atmospheric gaseous nitrogen into organic nitrogenous compounds by symbiotic bacteria residing in root nodulesAnswer
  2. B
    Oxidation of soil nitrites into nitrates by free-living Nitrobacter present around root zones
  3. C
    Reduction of soil nitrates into free atmospheric nitrogen gas by anaerobic Pseudomonas species
  4. D
    Breakdown of dead organic tissues into ammonium ions by saprophytic fungi and decomposers

Answer

Conversion of atmospheric gaseous nitrogen into organic nitrogenous compounds by symbiotic bacteria residing in root nodules
Leguminous plants form a mutualistic association with nitrogen-fixing bacteria (Rhizobium) contained within their root nodules. These microorganisms fix unreactive atmospheric nitrogen gas (N2N_2) into biologically available compounds like ammonium and amino acids, enriching the soil for subsequent crop rotations.

Step-by-Step Solution

1
Identify how leguminous plants introduce additional nitrogen into soil ecosystems.
Legumes host specialized root nodules containing symbiotic nitrogen-fixing bacteria, primarily species of the genus Rhizobium.
Plants cannot directly assimilate inert atmospheric nitrogen (N2N_2) gas through stomata or roots without biological fixation.
2
Analyze the biochemical transformation occurring within root nodules.
Rhizobium reduces elemental gaseous nitrogen (N2N_2) into ammonia and amino acids.
This process introduces new fixed nitrogen into the plant tissue, which subsequently enriches the soil upon crop decay or harvest residue integration.

Key Concept

Biological Nitrogen Fixation in Soil Ecosystems
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Topics

Nutrient and Biogeochemical Cycles

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