Biology · Ecology · Biogeochemical Cycles

Nitrogen Cycle

How nitrogen moves through Earth’s systems

This lesson explains how matter moves through Earth's systems, where it is stored, how organisms obtain it, and how biological, chemical, and geological processes return it to other parts of the cycle.

Atmospheric Nitrogen (N₂)~78% of Earth's atmosphereNitrogen FixationN₂ → NH₃ / NH₄⁺Ammonificationorganic N → NH₄⁺NitrificationNH₄⁺ → NO₂⁻ → NO₃⁻DenitrificationNO₃⁻ → N₂plant / microbial assimilation
A visual overview of the major reservoirs and transfers in the nitrogen cycle.
What Is the Nitrogen Cycle?

The nitrogen cycle describes how nitrogen moves among the atmosphere, soil, water, and living organisms. Nitrogen is essential because it is part of amino acids, proteins, nucleic acids, and other biological molecules.

Although the atmosphere is about 78% nitrogen gas (N₂), most organisms cannot use atmospheric N₂ directly. Nitrogen must first be converted into chemically usable forms. This is why several microbial processes are central to the nitrogen cycle.

The Nitrogen Cycle at a Glance
Atmospheric N₂large reservoir, not directly usable by most organismsFixationN₂ → NH₃ / NH₄⁺Ammonificationorganic N → NH₄⁺NitrificationNH₄⁺ → NO₂⁻ → NO₃⁻DenitrificationNO₃⁻ → N₂Assimilationplants take up usable Nplants → animals → decomposers
Nitrogen moves through several chemical forms. The arrows represent transformations, not simply physical movement.
Key Processes in the Nitrogen Cycle
01

Nitrogen Fixation

Certain bacteria convert atmospheric N₂ into ammonia (NH₃) or ammonium (NH₄⁺). Lightning and industrial processes can also fix nitrogen.

02

Nitrification

Nitrifying bacteria convert ammonium into nitrite (NO₂⁻) and then nitrate (NO₃⁻).

03

Assimilation

Plants absorb usable inorganic nitrogen, especially nitrate and ammonium, and incorporate it into organic molecules. Animals obtain nitrogen by consuming organisms.

04

Ammonification

Decomposers convert organic nitrogen in dead organisms and waste into ammonium.

05

Denitrification

Denitrifying bacteria convert nitrate back into atmospheric N₂, returning nitrogen to the atmosphere.

Why Nitrogen Form Matters

One of the most important ideas in the nitrogen cycle is that nitrogen changes chemical form. A question may describe the same element while asking about different compounds.

N₂

Atmospheric nitrogen gas. Abundant, but not directly usable by most organisms.

NH₃ / NH₄⁺

Ammonia and ammonium. Produced by fixation and ammonification; ammonium can be used by plants.

NO₂⁻ → NO₃⁻

Nitrite and nitrate. Nitrification produces these forms, and plants commonly take up nitrate.

Human Impacts on the Nitrogen Cycle

Humans have greatly increased the movement of nitrogen through ecosystems. Synthetic fertilizers add usable nitrogen to agricultural systems. Fossil fuel combustion releases nitrogen oxides, while wastewater and livestock operations can add nitrogen to soil and water.

Fertilizer Runoff

Excess nitrate can enter waterways and contribute to eutrophication, excessive algal growth, and oxygen depletion.

Combustion

High-temperature combustion can produce nitrogen oxides that contribute to air pollution and atmospheric chemical reactions.

Agriculture

Legumes, fertilizer use, manure, and soil management can all alter the rate at which nitrogen enters and leaves agricultural ecosystems.

Water Quality

Too much biologically available nitrogen can change aquatic community structure and nutrient balance.

Common Misconceptions
Misconception: Plants use atmospheric N₂ directly as their main nitrogen source.
Correct: Most plants cannot use atmospheric N₂ directly; usable nitrogen must be supplied through processes such as fixation and nitrification.
Misconception: Nitrogen fixation and nitrification are the same process.
Correct: Fixation converts N₂ into reduced nitrogen forms such as NH₃/NH₄⁺, while nitrification converts ammonium to nitrite and nitrate.
Misconception: Denitrification adds nitrate to soil.
Correct: Denitrification removes nitrate by converting NO₃⁻ into N₂ gas, returning nitrogen to the atmosphere.
Misconception: Only plants contain nitrogen.
Correct: All organisms need nitrogen-containing molecules, and nitrogen moves through food webs.
If You See This on a Test, Think This
A question says bacteria convert NH₄⁺ to NO₂⁻ and NO₃⁻. What process is this?
Nitrification.
Nitrate decreases while atmospheric N₂ increases. What process is likely occurring?
Denitrification.
A plant takes nitrate from soil and builds amino acids. What process is this?
Assimilation.
Quick Practice
Put these processes in a logical sequence beginning with atmospheric N₂: fixation, nitrification, assimilation.
Fixation makes usable nitrogen forms; nitrification can produce nitrate; assimilation incorporates usable nitrogen into organisms.
What role do decomposers play in the nitrogen cycle?
They carry out ammonification, converting organic nitrogen in dead matter and waste into ammonium.
Why is nitrogen fixation ecologically important?
It converts atmospheric N₂ into forms that can enter biological nutrient pathways.
What happens to nitrate during denitrification?
It is reduced and ultimately converted to atmospheric N₂.
Key Takeaways

Remember These Ideas

  • Atmospheric N₂ is abundant but is not directly usable by most organisms.
  • Nitrogen fixation converts N₂ into biologically useful forms.
  • Nitrification produces nitrite and nitrate from ammonium.
  • Assimilation incorporates usable nitrogen into organisms; ammonification returns organic nitrogen to ammonium.
  • Denitrification returns nitrogen to the atmosphere as N₂.
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