Examkin

Question

Difficulty: MediumEcology and Environment

In terrestrial ecosystems, nitrogen moves continuously between the atmosphere, soil, and living organisms through biochemical transformations. Arrange the following stages of the nitrogen cycle in sequential order, starting from the capture of atmospheric nitrogen gas and ending with its return to the atmosphere.

  1. 1Nitrogen-fixing soil bacteria convert atmospheric nitrogen gas (N2N_2) into ammonia.
  2. 2Nitrifying bacteria oxidize ammonia into nitrite and subsequently into nitrate ions.
  3. 3Plant roots absorb soil nitrates and assimilate them into organic amino acids and proteins.
  4. 4Denitrifying anaerobic bacteria reduce soil nitrates back into nitrogen gas that is released into the air.

Answer

The correct chronological sequence is: (1) Nitrogen fixation by bacteria, (2) Nitrification of ammonia into nitrates, (3) Assimilation of nitrates by plants, and (4) Denitrification converting nitrates back to nitrogen gas.
The nitrogen cycle begins with nitrogen fixation, where atmospheric nitrogen gas (N2N_2) is converted to ammonia by soil microorganisms. Next, nitrification occurs as specialized bacteria oxidize ammonia to nitrates (NO3−NO_3^-). Plants then assimilate these nitrates into organic compounds such as proteins. Finally, denitrifying bacteria convert excess nitrates back into gaseous nitrogen, completing the cycle and releasing it back into the atmosphere.

Step-by-Step Solution

1
Identify the primary entry mechanism of atmospheric nitrogen into terrestrial soils.
Free atmospheric nitrogen (N2N_2) cannot be directly absorbed by plants; it must first undergo nitrogen fixation by specialized bacteria (such as *Rhizobium* or free-living soil diazotrophs) to produce ammonia.
Atmospheric gas has strong triple covalent bonds that require bacterial enzymes to break down into fixed nitrogen.
2
Determine the biochemical transformation that follows ammonia production.
Ammonia is converted by nitrifying bacteria into nitrites (NO2−NO_2^-) and then into nitrates (NO3−NO_3^-).
Nitrification provides the chemical form of nitrogen that terrestrial plants utilize most efficiently.
3
Identify how nitrates enter the food web.
Plants absorb nitrates through their root systems and assimilate the nitrogen into vital macromolecules such as proteins and nucleic acids.
Assimilation moves inorganic soil nutrients into biological tissue.
4
Identify the closing stage that returns nitrogen to the atmosphere.
Anaerobic denitrifying bacteria reduce nitrates back into molecular nitrogen gas (N2N_2).
Denitrification closes the cycle by replenishing the atmospheric reservoir of nitrogen.

Key Concept

The Nitrogen Cycle and Microbial Nutrient Transformations
Estimated Time:1m 0s
Rate this question

Topics

Ecology and Environment

More questions from this topic