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Difficulty: HardEcology and Environment

In terrestrial and aquatic ecosystems, nitrogen undergoes several critical biological transformations to cycle between atmospheric, organic, and soil-bound forms. Match each nitrogen cycle process with its primary ecological and biochemical transformation:

  • Nitrogen FixationConversion of atmospheric nitrogen gas (N2N_2) into bioavailable ammonia or ammonium (NH4+NH_4^+) by specialized bacteria
  • NitrificationTwo-step aerobic biological oxidation of ammonium (NH4+NH_4^+) into nitrite (NO2−NO_2^-) and subsequently into plant-usable nitrate (NO3−NO_3^-)
  • DenitrificationAnaerobic microbial reduction of soil nitrates (NO3−NO_3^-) into gaseous nitrogen forms (N2ON_2O and N2N_2) returned to the atmosphere
  • AmmonificationDecomposer-driven breakdown of organic nitrogen compounds in waste and dead biomass into inorganic ammonium (NH4+NH_4^+)

Answer

Nitrogen Fixation matches the conversion of atmospheric nitrogen gas into bioavailable ammonia/ammonium; Nitrification matches the aerobic oxidation of ammonium into nitrite and nitrate; Denitrification matches the anaerobic reduction of soil nitrates into gaseous nitrogen released back into the atmosphere; Ammonification matches the decomposer-driven breakdown of organic nitrogen waste into inorganic ammonium.
Each nitrogen transformation corresponds to a distinct microbial function: Nitrogen fixation incorporates atmospheric N2N_2 into bioavailable ammonium; nitrification oxidizes ammonium to nitrite and nitrate; denitrification reduces nitrates back into gaseous dinitrogen under anaerobic conditions; and ammonification breaks down organic cellular waste into inorganic ammonium.

Step-by-Step Solution

1
Identify the pathway that brings elemental gaseous nitrogen from the atmosphere into biological food webs.
Nitrogen Fixation converts inert atmospheric N2N_2 into bioavailable ammonia and ammonium.
Plants cannot directly assimilate triple-bonded atmospheric N2N_2 and rely on specialized bacteria to fix it.
2
Identify the aerobic oxidation sequence that converts soil ammonium into higher oxidized forms of nitrogen.
Nitrification converts NH4+NH_4^+ first into nitrite (NO2−NO_2^-) and then into nitrate (NO3−NO_3^-).
Nitrate is the primary oxidized, highly soluble form of nitrogen preferred for uptake by many terrestrial plants.
3
Identify the pathway that prevents nitrogen accumulation in soil and closes the global nitrogen cycle.
Denitrification reduces soil nitrates (NO3−NO_3^-) back into nitrogen gas (N2N_2).
Anaerobic bacteria use nitrate as an alternate terminal electron acceptor, releasing nitrogen back into the atmosphere.
4
Identify the recycling pathway that handles dead biomass and excretion products.
Ammonification decomposes organic proteins and nucleic acids into inorganic ammonium (NH4+NH_4^+).
Decomposers hydrolyze organic polymers, liberating inorganic nitrogen back into the soil for re-entry into nitrification or plant uptake.

Key Concept

Ecological Biogeochemical Cycles: Nitrogen Transformation Mechanisms
Estimated Time:1m 30s
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Topics

Ecology and Environment

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