Introduction

Nitrogen is one of the most abundant elements on Earth. In fact, about 78% of the Earth's atmosphere is made up of nitrogen gas (N₂). Despite its abundance, most living organisms cannot use nitrogen gas directly. Instead, nitrogen must first be converted into forms that plants and other organisms can absorb and use.

The nitrogen cycle is the continuous movement and transformation of nitrogen between the atmosphere, soil, living organisms, and decomposers. Tiny microorganisms, especially bacteria, play a crucial role in this cycle by converting nitrogen into different chemical forms. Without the nitrogen cycle, plants could not grow, food chains would collapse, and life on Earth would not be possible.

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Why Is Nitrogen Important?

Nitrogen is essential for all living organisms because it is needed to make:

• Proteins
• Amino acids
• DNA
• RNA
• Chlorophyll (in plants)

Without nitrogen:

• Plants cannot grow properly.
• Animals cannot build or repair tissues.
• Cells cannot reproduce normally.

Nitrogen is therefore one of the most important nutrients in every ecosystem.

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What Is the Nitrogen Cycle?

The nitrogen cycle is the continuous movement and transformation of nitrogen between:

• The atmosphere
• Soil
• Plants
• Animals
• Decomposers

Unlike the carbon cycle, nitrogen changes into several different chemical forms as it moves through ecosystems.

The major stages are:

• Nitrogen fixation
• Nitrification
• Assimilation
• Consumption
• Ammonification (decomposition)
• Denitrification

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Figure 1. The nitrogen cycle transforms nitrogen into different chemical forms as it moves through ecosystems.

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Stage 1: Nitrogen Fixation

Although nitrogen gas (N₂) is abundant in the atmosphere, plants cannot use it directly.

Nitrogen fixation is the process that converts nitrogen gas into usable nitrogen compounds.

This is carried out mainly by nitrogen-fixing bacteria that live:

• In the soil
• Inside the root nodules of legumes (such as peas, beans, and clover)

Some nitrogen is also fixed naturally by lightning, which converts atmospheric nitrogen into nitrogen compounds that are washed into the soil by rain.

The bacteria convert nitrogen gas into ammonia (NH₃) or ammonium ions (NH₄⁺).

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Figure 2. Nitrogen-fixing bacteria living in the root nodules of legumes convert atmospheric nitrogen into forms plants can use.

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Stage 2: Nitrification

Other soil bacteria convert ammonia into forms that plants can absorb.

This process is called nitrification.

It occurs in two main steps:

1. Ammonia is converted into nitrites (NO₂⁻).
2. Nitrites are converted into nitrates (NO₃⁻).

Nitrates are the main form of nitrogen absorbed by plants.

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Stage 3: Assimilation

Assimilation occurs when plants absorb nitrates from the soil through their roots.

Plants use nitrogen to produce:

• Proteins
• DNA
• Chlorophyll

Animals obtain nitrogen by eating plants or by eating other animals.

In this way, nitrogen moves through food chains.

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Figure 3. Plants absorb nitrates through their roots and use them to build important biological molecules.

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Stage 4: Consumption

Nitrogen moves through ecosystems as organisms feed on one another.

For example:

Grass → Rabbit → Fox

• Grass absorbs nitrates from the soil.
• The rabbit obtains nitrogen by eating the grass.
• The fox obtains nitrogen by eating the rabbit.

Nitrogen is transferred from one trophic level to the next through feeding relationships.

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Stage 5: Ammonification (Decomposition)

When plants and animals die or produce waste, decomposers break down the organic matter.

This process is called ammonification.

Decomposers such as bacteria and fungi convert organic nitrogen into:

• Ammonia (NH₃)
• Ammonium ions (NH₄⁺)

These compounds return nitrogen to the soil, where it can be reused.

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Stage 6: Denitrification

Some bacteria convert nitrates back into nitrogen gas.

This process is called denitrification.

The nitrogen gas is released into the atmosphere, completing the cycle.

Without denitrification, nitrogen would gradually build up in soils and water.

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Figure 4. Different groups of bacteria drive the major stages of the nitrogen cycle.

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Human Impacts on the Nitrogen Cycle

Human activities have changed the natural nitrogen cycle.

Fertiliser Use

Farmers add nitrogen-rich fertilisers to increase crop growth.

However, excess fertiliser may be washed into rivers and lakes.

This can lead to:

• Algal blooms
• Reduced oxygen levels
• Fish deaths

This process is known as eutrophication.

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Burning Fossil Fuels

Vehicle engines and power stations release nitrogen oxides (NOₓ) into the atmosphere.

These gases contribute to:

• Air pollution
• Acid rain
• Smog

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Deforestation

Removing forests reduces plant uptake of nitrogen and can increase soil erosion and nutrient loss.

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Wastewater and Agriculture

Animal waste and untreated sewage release additional nitrogen compounds into waterways, affecting aquatic ecosystems.

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Figure 5. Excess nitrogen from fertilisers can cause eutrophication, harming aquatic ecosystems.

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Why the Nitrogen Cycle Is Important

The nitrogen cycle is essential because it:

• Supplies plants with usable nitrogen.
• Supports the production of proteins and DNA.
• Maintains soil fertility.
• Supports food chains.
• Recycles nitrogen between living organisms and the environment.

Without this cycle, life on Earth would quickly run out of usable nitrogen.

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Worked Example

Question

Match each stage of the nitrogen cycle with its description.

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Real-World Connection

Many farmers rotate crops with legumes such as beans, peas, and clover because these plants contain nitrogen-fixing bacteria in their root nodules. The bacteria naturally enrich the soil with nitrogen, reducing the need for artificial fertilisers and helping to improve soil fertility for future crops.

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Did You Know?

Although the air around us is nearly 78% nitrogen, humans and other animals cannot use nitrogen gas directly. We obtain all of the nitrogen needed to build our proteins and DNA by eating plants or by eating animals that have eaten plants. In other words, nearly all usable nitrogen enters food chains through plants.

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Key Terms

Ammonification – The process in which decomposers convert organic nitrogen into ammonia or ammonium.

Assimilation – The uptake of nitrates by plants to build proteins, DNA, and other molecules.

Denitrification – The process in which bacteria convert nitrates back into nitrogen gas.

Eutrophication – The enrichment of water with nutrients, leading to excessive algal growth and oxygen depletion.

Nitrogen cycle – The continuous movement and transformation of nitrogen through the atmosphere, soil, living organisms, and decomposers.

Nitrogen fixation – The conversion of atmospheric nitrogen gas into ammonia or ammonium by bacteria or lightning.

Nitrogen-fixing bacteria – Bacteria that convert atmospheric nitrogen into forms that plants can use.

Nitrates (NO₃⁻) – The main form of nitrogen absorbed by plants.

Nitrification – The bacterial conversion of ammonia into nitrites and then nitrates.

Root nodules – Structures on the roots of legumes that contain nitrogen-fixing bacteria.

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Key Takeaways

• The nitrogen cycle converts atmospheric nitrogen into forms that living organisms can use.
• Nitrogen-fixing bacteria are essential because they convert nitrogen gas into usable nitrogen compounds.
• Plants absorb nitrates from the soil and use them to make proteins, DNA, and chlorophyll.
• Nitrogen moves through ecosystems as organisms feed on one another and is recycled by decomposers and bacteria.
• Human activities, especially fertiliser use and fossil fuel combustion, can disrupt the nitrogen cycle and damage ecosystems.