The Nitrogen Cycle (College Board AP® Environmental Science): Study Guide

What is the nitrogen cycle?

• The nitrogen cycle is the set of processes by which nitrogen atoms and nitrogen-containing molecules move through different parts of the environment, including:

  • The atmosphere

  • Soil

  • Water

  • Living organisms

• Nitrogen is essential for life because it is a key component of proteins and nucleic acids (DNA and RNA)

• The nitrogen cycle ensures:

  • Nitrogen is converted into forms that plants and animals can use

  • Nitrogen is then recycled back into the atmosphere or soil

• Nitrogen moves through sources (which release nitrogen) and sinks (which absorb nitrogen)

Nitrogen sources

• Nitrogen sources release nitrogen compounds into the environment

• Examples include:

  • Lightning strikes: convert atmospheric nitrogen into nitrates (NO₃⁻)

  • Decomposition of organic material: breakdown of dead plants and animals releases nitrogen as ammonium (NH₄⁺)

  • Fertilizer application: adds nitrogen compounds like nitrates and ammonium to the soil

  • Animal waste: releases ammonia and nitrates into the environment

Nitrogen sinks

• Nitrogen sinks absorb or remove nitrogen from another part of the nitrogen cycle

• Examples include:

  • Atmosphere

  • Soil

  • Plants and animals

  • Oceans

Nitrogen reservoirs

• Nitrogen reservoirs are places where nitrogen compounds are stored within the cycle

• Most reservoirs, like soil and living organisms, hold nitrogen compounds for relatively short periods (days to decades)

  • These are known as short-term reservoirs

• Some reservoirs, like sediments in oceans and sedimentary rocks, can store nitrogen for longer periods (thousands to millions of years)

  • These are known as long-term reservoirs

Key nitrogen reservoirs

• Atmosphere:

  • Contains nitrogen gas (N₂), which makes up about 78% of Earth's atmosphere

• Soil:

  • Holds nitrogen in forms like nitrates (NO₃⁻) and ammonium (NH₄⁺), which plants can absorb

• Living organisms:

  • Store nitrogen in organic forms such as proteins and nucleic acids

Fixation of atmospheric nitrogen

• Fixation is the process that converts nitrogen gas (N₂) from the atmosphere into forms that plants can use

  • E.g. ammonia (NH₃)

• Plants cannot directly use nitrogen gas, so fixation is a crucial step in the nitrogen cycle

Types of nitrogen fixation

1. Biotic fixation:

   • Performed by nitrogen-fixing bacteria like rhizobium (found in legume root nodules)

   • These bacteria use enzymes to convert nitrogen gas into ammonia

2. Abiotic fixation:

   • Occurs through natural processes such as lightning

   • High energy from lightning breaks nitrogen molecules

   • This allows the nitrogen to react with oxygen to form nitrates

The role of bacteria in the nitrogen cycle

• Bacteria are essential for converting nitrogen into usable forms

  • Nitrogen fixation: nitrogen-fixing 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⁺)

Atmospheric nitrogen reservoir

• The atmosphere is the largest nitrogen reservoir

• It contains nitrogen gas in its inert form (N₂)

• Although nitrogen gas is abundant, most organisms cannot use it directly

• Processes like nitrogen fixation make this nitrogen available to plants and animals in usable forms like ammonia and nitrates

• Denitrification returns nitrogen to the atmosphere as gas, completing the cycle

Importance of the atmospheric nitrogen reservoir

• Acts as a constant source of nitrogen for ecosystems

• Supports processes like fixation

  • This enables nitrogen to enter the biosphere

• For example, after a thunderstorm:

  • Nitrogen from the atmosphere is converted into nitrates

  • Nitrates are deposited into the soil by rain

  • This enriches the soil for plant growth