Nitrogen Cycle (HL) (DP IB Environmental Systems & Societies (ESS))

Nitrogen Cycle

• The nitrogen cycle is the process through which nitrogen moves between organic and inorganic forms in ecosystems

  • Organic nitrogen is found in proteins, DNA, and other compounds in living organisms and in dead organic matter

    • After death, organic matter decomposes, returning nitrogen to the soil

  • Inorganic nitrogen exists in the atmosphere (as nitrogen gas, N₂), and as nitrogen compounds like ammonia (NH₃), nitrites (NO₂⁻), and nitrates (NO₃⁻) in soils and water

    • The atmosphere holds the largest store of nitrogen, mostly as N₂ gas, which makes up 78% of the air

The role of bacteria in the nitrogen cycle

• Bacteria are essential for converting nitrogen into usable forms:

  • Nitrogen fixation: bacteria (such as rhizobium) convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can use

    • This can happen in the soil or through symbiotic relationships with plants like legumes

  • Nitrification: nitrifying bacteria convert ammonia (NH₃) into nitrites (NO₂⁻), then into nitrates (NO₃⁻)

    • Plants absorb these nitrates through their roots

  • Denitrification: denitrifying bacteria convert nitrates (NO₃⁻) back into nitrogen gas (N₂)

    • This nitrogen gas then returns to the atmosphere

    • This process happens in anaerobic (low oxygen) conditions, like waterlogged soils

  • Decomposition (ammonification): when plants and animals die, decomposing bacteria break down their nitrogenous compounds into ammonium (NH₄⁺)

Denitrification in anaerobic conditions

• Anaerobic conditions (low oxygen) occur in waterlogged soils

• This is where denitrification takes place

  • Denitrification reduces nitrogen availability for plants by converting nitrates into nitrogen gas

  • Plants in these areas grow poorly because they can’t absorb enough nitrogen, which is essential for growth

  • Some insectivorous plants, like pitcher plants and sundews, have adapted to these environments by obtaining nitrogen from insects instead

Mutualistic relationships and nitrogen fixation

• Plants cannot use atmospheric nitrogen directly

• However, some plants, such as legumes (like peas, beans, and clover), form mutualistic relationships with nitrogen-fixing bacteria

  • These bacteria live in root nodules of the plants and convert nitrogen gas into ammonia

  • This gives these plants a competitive advantage in nitrogen-poor soils

    • This is because they can now access nitrogen from the atmosphere, unlike other plants

Nitrogen flows: transfers and transformations

• Transfer processes (movement of nitrogen without changing its form):

  • Mineral uptake: plants absorb nitrates (NO₃⁻) from the soil

  • Consumption: animals eat plants or other animals, taking in nitrogen in the form of proteins and moving nitrogen through the food chain

  • Excretion: animals release nitrogen back into the environment through waste products (urea, ammonia, faeces)

  • Death and decomposition: dead plants and animals add nitrogen back into the soil when they decompose

• Transformation processes (nitrogen changes form):

  • Nitrogen fixation: Nitrogen gas (N₂) → Ammonia (NH₃)

  • Nitrification: Ammonia (NH₃) → Nitrites (NO₂⁻) → Nitrates (NO₃⁻)

  • Ammonification: Organic nitrogen (proteins) → Ammonium (NH₄⁺)

  • Denitrification: Nitrates (NO₃⁻) → Nitrogen gas (N₂)

• Nitrogen fixation can also occur through lightning

  • The energy from lightning breaks nitrogen molecules (N₂) in the atmosphere

  • This allows nitrogen atoms to combine with oxygen to form nitrogen oxides (NO and NO₂)

  • These nitrogen oxides dissolve in rain and fall to the ground as nitrates (NO₃⁻), a usable form of nitrogen for plants