1.5 The Nitrogen Cycle

What It Is

• The nitrogen cycle describes the movement of nitrogen between:

  • Atmosphere

  • Soil

  • Water

  • Living organisms

• Nitrogen is essential for life:

  • Key component of proteins, amino acids, nucleic acids (DNA & RNA), and ATP

• Nitrogen is mostly inert as N₂ in the atmosphere (78%) and cannot be used directly by plants or animals

• Specialized bacteria and natural processes convert N₂ into biologically usable forms

Major Nitrogen Reservoirs

• Atmosphere:

  • Largest nitrogen reservoir (~78% N₂ gas)

  • Mostly inert, unavailable to most organisms

• Soil:

  • Holds ammonium (NH₄⁺), nitrate (NO₃⁻), and organic nitrogen

  • Plants absorb these forms for growth

• Living Organisms:

  • Nitrogen stored in proteins and nucleic acids

• Water & Sediments:

  • Nitrogen in dissolved forms (NO₃⁻, NH₄⁺)

  • Sediments act as longer-term storage

• Short-term reservoirs: Soil, water, plants, animals (days–decades)

• Long-term reservoirs: Ocean sediments, sedimentary rocks (thousands–millions of years)

Key Processes in the Nitrogen Cycle

• Nitrogen Fixation

  • Converts N₂ gas → ammonia (NH₃) or ammonium (NH₄⁺)

  • Methods:

    • Biotic fixation: Nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules, Azotobacter, cyanobacteria)

      • Bacteria use nitrogenase enzyme to break N≡N bond

    • Abiotic fixation: Lightning breaks N₂ bonds → forms nitrates (NO₃⁻)

    • Industrial fixation (Haber-Bosch process): Humans produce ammonia for fertilizers

  • Importance: Makes atmospheric nitrogen biologically available

• Nitrification (two-step aerobic process)

  • Step 1: Ammonium (NH₄⁺) → Nitrite (NO₂⁻) by Nitrosomonas bacteria

  • Step 2: Nitrite (NO₂⁻) → Nitrate (NO₃⁻) by Nitrobacter bacteria

  • Importance: Converts nitrogen into nitrate (NO₃⁻), easily absorbed by plants

• Assimilation

  • Plants absorb NH₄⁺ and NO₃⁻ → incorporate nitrogen into amino acids and proteins

  • Animals obtain nitrogen by eating plants or other animals

  • Transfers nitrogen from inorganic → organic form

• Ammonification (Decomposition / Mineralization)

  • Decomposers (bacteria & fungi) break down dead organisms and waste

  • Converts organic nitrogen → ammonium (NH₄⁺)

  • Returns nitrogen to soil for reuse

• Denitrification

  • Anaerobic bacteria convert NO₃⁻ → N₂ or N₂O (nitrous oxide)

  • Occurs in low-oxygen environments: wetlands, waterlogged soils

  • Completes the cycle by returning nitrogen to the atmosphere

  • N₂O acts as a potent greenhouse gas

Additional Nitrogen Notes

• Forms of Nitrogen:

  • Gas: N₂ (inert)

  • Inorganic: NH₄⁺ (ammonium), NH₃ (ammonia), NO₂⁻ (nitrite), NO₃⁻ (nitrate)

  • Organic: Proteins, nucleic acids, dissolved organic N (DON), particulate organic N (PON)

• Nitrogen in Lakes & Aquatic Systems:

  • Sources:

    • Direct deposition (dust, acid rain, NOx from combustion)

    • Runoff from soil, fertilizers, sewage, farm animals

    • Autochthonous N fixation by cyanobacteria and certain bacteria

  • Losses:

    • Outflow from basin

    • Denitrification (NO₃⁻ → N₂/N₂O)

    • Sedimentation

• Cyanobacteria & Bacteria in Fixation:

  • Heterocysts in cyanobacteria (e.g., Anabaena) fix nitrogen when limiting

  • Not all cyanobacteria can fix nitrogen (Microcystis cannot)

  • Wetland plants (e.g., alder) have symbiotic bacteria to fix nitrogen

Human Impacts on the Nitrogen Cycle

• Fertilizers: Excess NH₄⁺ and NO₃⁻ → runoff → eutrophication → algal blooms → hypoxia / dead zones

• Fossil fuel combustion → NOx gases → acid rain and air pollution

• Agriculture increases N₂O emissions → greenhouse gas ~300x CO₂ potency

• Altered N cycling can reduce biodiversity (nitrogen-demanding species outcompete others)

Quick Exam Tips

• Key order: Fixation → Nitrification → Assimilation → Ammonification → Denitrification

• Fixation & denitrification involve the atmosphere (N₂)

• Middle steps mostly occur in soil

• Legumes + Rhizobium = classic nitrogen fixation example