The Nitrogen Cycle

What Is the Nitrogen Cycle?

Nitrogen is an essential building block of life. It is a key part of proteins, DNA, and chlorophyll, and every living organism needs it to grow. Nitrogen gas (N2) makes up about 78% of Earth's atmosphere, making it by far the most abundant element in the air — yet most organisms cannot use it in that form at all.

The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, water, and living things, constantly being converted between different chemical forms by bacteria, lightning, and human activity. Like the carbon and water cycles, nitrogen is never created or destroyed — it is recycled endlessly through the biosphere.

Why Nitrogen Gas Is Hard to Use

The two nitrogen atoms in N2 are held together by an extremely strong triple covalent bond, making the molecule almost chemically inert. Plants cannot absorb nitrogen gas through their roots, and animals cannot use it from the air they breathe. Before nitrogen can enter living systems, it must be fixed — converted into a more reactive compound such as ammonia (NH3) or nitrate (NO3-).

The Main Stages of the Nitrogen Cycle

1. Nitrogen Fixation

Nitrogen fixation converts unreactive nitrogen gas into ammonia. This happens mainly through:

• Nitrogen-fixing bacteria living freely in the soil, or in a mutualistic relationship inside the root nodules of legumes (such as peas, beans, and clover)
• Lightning, which provides enough energy to force nitrogen and oxygen in the air to combine into nitrogen oxides that dissolve into the soil with rain
• Industrial fixation, where humans use the Haber process to manufacture ammonia for fertiliser on a massive scale

2. Nitrification

Once ammonia is in the soil, nitrifying bacteria convert it in two steps: first into nitrites (NO2-), then into nitrates (NO3-). Nitrate is the form of nitrogen that plant roots most readily absorb.

3. Assimilation

Plants absorb nitrates through their roots and use the nitrogen to build amino acids, proteins, and nucleic acids. Animals then obtain their nitrogen by eating plants or other animals, passing nitrogen-containing compounds along the food chain.

4. Ammonification (Decomposition)

When plants and animals die, or when animals excrete waste, decomposers (bacteria and fungi) break down the nitrogen-containing organic matter and release ammonia back into the soil, ready to be nitrified again.

5. Denitrification

In waterlogged or oxygen-poor soils, denitrifying bacteria convert nitrates back into nitrogen gas, releasing it into the atmosphere and completing the cycle. This is the only stage that returns nitrogen from the soil back into the air.

Human Impact on the Nitrogen Cycle

Human activity has dramatically increased the amount of nitrogen being fixed each year:

• Synthetic fertilisers, produced by the Haber process, now fix more nitrogen than all natural land-based processes combined
• Growing legume crops deliberately increases nitrogen fixation in farmland soil
• Burning fossil fuels releases nitrogen oxides into the atmosphere, contributing to acid rain and air pollution

While fertilisers boost crop yields, excess nitrates that wash off farmland into rivers and lakes can cause eutrophication — explosive algal growth that depletes oxygen in the water and harms aquatic life.

Key Terms

Term	Definition
Nitrogen fixation	Converting unreactive nitrogen gas into a usable compound like ammonia
Nitrification	Converting ammonia into nitrites, then nitrates
Assimilation	Plants and animals incorporating nitrogen compounds into their tissues
Ammonification	Decomposers releasing ammonia from dead organic matter and waste
Denitrification	Converting nitrates back into nitrogen gas, released into the atmosphere
Haber process	The industrial method for manufacturing ammonia from nitrogen and hydrogen gas
Eutrophication	Excess nutrients causing algal overgrowth and oxygen depletion in water

Common Mistakes

• Thinking plants can absorb nitrogen gas directly from the air — they can only absorb it as nitrates through their roots.
• Confusing nitrification (ammonia to nitrate) with nitrogen fixation (nitrogen gas to ammonia) — they are different stages carried out by different bacteria.
• Forgetting that denitrification is the only process that returns nitrogen to the atmosphere, completing the cycle.
• Assuming fertilisers only help plants grow — excess nitrates can also cause serious water pollution through eutrophication.

Tips and Tricks

• Remember the order: fixation to nitrification to assimilation to ammonification to denitrification, then back to nitrogen gas.
• "Fixing" nitrogen means locking it into a compound; "denitrifying" means freeing it back into gas — the two are opposites.
• Legume root nodules are a classic exam example of natural nitrogen fixation through a mutualistic relationship with bacteria.
• Compare with the carbon cycle: both rely on bacteria and decomposers to keep the element moving, but nitrogen's main gas-to-solid step (fixation) is much harder chemically than carbon's (photosynthesis).