APES 1.5 Notes - Nitrogen Cycle

Main Ideas / Lessons: Nitrogen Cycle (APES 1.5 Notes)

Goal of the lesson

• Understand the steps and interactions between reservoirs in the nitrogen cycle.
• Be able to explain how a chemical transformation affects an ecosystem (FRQ practice).

What the nitrogen cycle is

The nitrogen cycle is the movement of nitrogen molecules between:

• Sources: release nitrogen into the atmosphere
• Sinks: take in increasing amounts of nitrogen
• Reservoirs: major storage locations of nitrogen

Key difference from the carbon cycle

• Nitrogen reservoirs generally hold nitrogen for shorter periods than carbon reservoirs.
• Example: nitrogen in plants persists only while plants are alive. When plants die, nitrogen returns quickly to the soil and is taken back up again.

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Nitrogen Reservoirs and Why Fixation Is Required

Atmospheric reservoir

• About 78% of Earth’s atmosphere is nitrogen gas (N₂).

Core problem

• Most atmospheric nitrogen is N₂, which is not usable by plants or animals.

Biological importance of nitrogen

Nitrogen is a key limiting nutrient because organisms need it to make:

• DNA (in every cell)
• Amino acids (building proteins)

Result: why nitrogen fixation is needed

• The cycle requires nitrogen fixation to convert atmospheric N₂ into usable forms.

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Step-by-Step Methodology / Process (Chemical Transformations)

1) Nitrogen Fixation (first and most critical step)

Definition: Conversion of atmospheric N₂ into biologically usable nitrogen:

• NH₃ (ammonia) and/or NO₃⁻ (nitrate)

Two ways it happens:

Natural nitrogen fixation

• Done by nitrogen-fixing bacteria
  • Freely in soil: convert atmospheric N₂ into ammonium (NH₄⁺)
  • In symbiosis with legumes (peas, beans)
    • Bacteria live in root nodules
    • Mutualism:
      • bacteria get amino acids/food from plants
      • plants get fixed nitrogen
• Also can occur via lightning storms (less emphasized than bacteria)

Synthetic (human-caused) nitrogen fixation

• Humans convert N₂ into nitrates (NO₃⁻) through industrial processes
• Requires a lot of energy
• Produces fertilizer that increases crop yields

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2) Assimilation

Definition: Plants and animals take nitrogen into their bodies and incorporate it into:

• DNA
• proteins

How organisms get nitrogen:

• Plants (primary assimilators): absorb nitrogen from soil as nitrates (NO₃⁻) or ammonia (NH₃/NH₄⁺) through roots
• Animals: get nitrogen by eating plants (or eating animals that ate plants)

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3) Ammonification (decomposition → ammonia)

Definition: Soil microbes/decomposers convert waste and dead biomass back into ammonia.

• Organic nitrogen returns to NH₃ (and associated forms)

Role in the cycle:

• Restores nitrogen to the soil so it can be transformed again.

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4) Nitrification (oxidation sequence in soil)

Definition (sequential conversion):

• NH₄⁺ (ammonium) → NO₂⁻ (nitrite) → NO₃⁻ (nitrate)

Performed by: soil bacteria Key point:

• This step makes nitrogen available in a form plants commonly use, especially nitrate.

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5) Denitrification (return to atmosphere)

Definition: Conversion of soil nitrogen (especially NO₃⁻) back into a gas.

• Gas produced: nitrous oxide (N₂O)
• Further outcome: nitrous oxide can eventually break down to form nitrogen gas (N₂), completing the cycle
• Performed by: bacteria in soil

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Human Impacts on the Nitrogen Cycle (Ecosystem/Climate Consequences)

A) Increased nitrous oxide → climate warming

• N₂O is a greenhouse gas
• Produced when agricultural soils undergo denitrification due to waterlogged or over-irrigated conditions
• Human links:
  • land clearing and tilling
  • fertilizer use increases N₂O emissions

B) Ammonia volatilization (excess fertilizer losses)

• Excess fertilizer can release ammonia gas (NH₃) into the atmosphere
• Consequences:
  • Acid precipitation: ammonia + atmospheric water → acidic rain
  • Health impacts: ammonia irritation to human respiratory tracts
  • Loss from fields: nitrogen is lost from fields, reducing farmer returns

C) Leaching → eutrophication (water pollution and ecosystem disruption)

• Leaching: over-applied nitrates exceed soil capacity and are carried away by rain/irrigation runoff
• Eutrophication effects:
  • excess nitrogen fuels algae blooms
  • algae block sunlight → submerged aquatic plants die
  • disrupts aquatic ecosystem balance and productivity

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Speakers / Sources Featured

• Speaker: Mr. Smeeds (the instructor presenting the notes)