The Nitrogen Cycle: KS3 Biology Guide

Nitrogen makes up 78% of the air, yet most living things cannot use it directly. The nitrogen cycle describes how nitrogen moves from the atmosphere into the soil and through living organisms and back again — with specialised bacteria acting as essential gatekeepers at every step.

Why do living things need nitrogen?

Nitrogen is a structural component of some of the most important molecules in life:

Amino acids — the building blocks of proteins, which form enzymes, muscle fibres, hormones, antibodies, and almost every structural material in the body

DNA and RNA — the molecules that store and transmit genetic information

ATP — the universal energy-transfer molecule used in every living cell

Plants absorb nitrogen from the soil as dissolved nitrate ions (NO₃⁻) through their root hair cells. Animals cannot absorb nitrogen from soil or air — they obtain it by eating plants or other animals. The challenge for the whole biosphere is converting atmospheric nitrogen gas (N₂) into a form that plants can actually take up.

What are the main stages of the nitrogen cycle?

Stage | Process | Organisms involved
Nitrogen fixation | N₂ gas converted to ammonium (NH₄⁺) | Nitrogen-fixing bacteria (Rhizobium, free-living Azotobacter); lightning
Nitrification | Ammonium → nitrite → nitrate (NO₃⁻) | Nitrifying bacteria (Nitrosomonas, Nitrobacter)
Assimilation | Plants absorb nitrate and build it into proteins | Plants (via roots)
Decomposition | Dead organisms and waste broken down to ammonium | Decomposing bacteria and fungi
Denitrification | Nitrate converted back to N₂ gas | Denitrifying bacteria (Pseudomonas)

What is nitrogen fixation?

Nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺) — forms that can enter the soil and be used by plants. This is chemically very difficult: the N≡N triple bond is one of the strongest in chemistry.

Natural fixation occurs in two ways:

Nitrogen-fixing bacteria — bacteria such as Rhizobium live inside root nodules of leguminous plants (peas, beans, clover, soya). They receive carbohydrates from the plant and in return supply the plant with ammonium ions — a mutualistic relationship that makes legumes naturally good at enriching soil.

Lightning — the enormous energy of a lightning bolt can split N₂ molecules; nitrogen then reacts with oxygen to form nitrogen oxides, which dissolve in rainwater and reach the soil as dilute nitric acid.

Farmers also add artificial nitrogen fertilisers (produced industrially by the Haber process) to boost soil nitrogen, supplementing the natural cycle.

What is nitrification?

Nitrification is the conversion of ammonium ions in the soil into nitrate ions, carried out in two steps by different groups of bacteria:

Ammonium → nitrite (NO₂⁻): Bacteria such as Nitrosomonas oxidise ammonium to nitrite.

Nitrite → nitrate (NO₃⁻): Bacteria such as Nitrobacter oxidise nitrite to nitrate.

Nitrate (NO₃⁻) is the form that plants absorb through root hair cells by active transport. Inside the plant, nitrogen is incorporated into amino acids and then proteins. When animals eat plants, nitrogen passes up the food chain.

Nitrification is inhibited in waterlogged or very acidic soils, which is why these conditions reduce soil fertility.

What is denitrification?

Denitrification is the conversion of nitrate in the soil back into nitrogen gas (N₂), which escapes into the atmosphere, completing the cycle. It is carried out by denitrifying bacteria such as Pseudomonas denitrificans, which use nitrate instead of oxygen for respiration in the absence of air.

Denitrification occurs mainly in:

Waterlogged soils — where oxygen is absent

Compacted soils — where air cannot circulate

From a farmer's perspective, denitrification is problematic because it removes usable nitrogen from the soil, reducing its fertility. Good drainage and avoiding soil compaction help reduce denitrification losses.

What role do decomposers play in the nitrogen cycle?

Decomposers — bacteria and fungi that break down dead organic matter — are essential for returning nitrogen to the soil. When plants and animals die, or when animals excrete waste, nitrogen-containing compounds (proteins, urea, uric acid) enter the soil. Decomposers break these complex molecules down into simpler ammonium ions (NH₄⁺) in a process called decomposition (or ammonification).

Without decomposers:

Nitrogen locked in dead organisms would never re-enter the soil

Soil nitrate levels would fall continuously as plants removed them

The entire cycle would grind to a halt within years

The rate of decomposition is affected by temperature (faster in warm conditions), moisture (requires water), and oxygen availability (aerobic decomposers work faster than anaerobic ones).

How does human activity affect the nitrogen cycle?

Human activity disrupts the nitrogen cycle in several ways:

Artificial fertilisers: Vast quantities of nitrogen fertiliser are applied to agricultural land globally. When it rains, excess nitrate runs off into rivers and lakes, causing eutrophication — algal blooms, oxygen depletion, and the death of aquatic organisms.

Burning fossil fuels: Combustion at high temperatures converts atmospheric N₂ into nitrogen oxides (NOₓ), which dissolve in rainwater to form acid rain, damaging soils and freshwater ecosystems.

Sewage and waste water: Nitrogen-rich sewage discharged into waterways accelerates eutrophication.

Deforestation: Removing forests exposes soil to heavy rain, increasing the leaching (washing out) of nitrates into waterways.

The global nitrogen cycle is now significantly altered by human activity — more reactive nitrogen is released annually by human action than by all natural fixation processes combined.

Frequently asked questions

What is the role of bacteria in the nitrogen cycle?

Bacteria are indispensable at every stage. Nitrogen-fixing bacteria (Rhizobium, Azotobacter) convert atmospheric N₂ into ammonium that plants can use. Nitrifying bacteria (Nitrosomonas, Nitrobacter) convert ammonium to nitrate in the soil. Decomposing bacteria break down proteins in dead matter and waste, releasing ammonium back into the soil. Denitrifying bacteria return nitrate to N₂ gas, completing the cycle. Without bacteria, the nitrogen cycle would stop and life on Earth as we know it could not be sustained.

Why can't plants absorb nitrogen gas directly from the air?

Atmospheric nitrogen (N₂) contains a triple bond (N≡N) that is one of the strongest chemical bonds in nature. Plants lack the enzyme (nitrogenase) needed to break this bond and convert N₂ into usable compounds. Plants can only absorb nitrogen in the ionic form NO₃⁻ (nitrate) dissolved in soil water. Converting N₂ into usable nitrogen — nitrogen fixation — requires either specialised bacteria with nitrogenase, the energy of lightning, or industrial high-temperature and high-pressure processes.

What is denitrification and why does it matter?

Denitrification is the conversion of soil nitrate (NO₃⁻) back into nitrogen gas (N₂) by denitrifying bacteria operating in oxygen-poor conditions. It matters because it removes fixed nitrogen from the soil, reducing fertility, and because it returns nitrogen to the atmosphere, balancing the inputs from fixation. Without denitrification, nitrogen would gradually accumulate in the soil and oceans. However, excess denitrification — caused by waterlogged or compacted soils — can significantly reduce agricultural productivity.

What is eutrophication and how does it link to the nitrogen cycle?

Eutrophication is the excessive enrichment of a body of water with nutrients, particularly nitrates and phosphates, causing explosive algal growth. When nitrogen-rich fertiliser washes off farmland into rivers and lakes, algae bloom rapidly. The algae block sunlight, killing aquatic plants below. When the algae die, aerobic decomposing bacteria consume vast amounts of dissolved oxygen, causing oxygen levels to crash. Fish and other aquatic animals suffocate in the resulting "dead zone." Eutrophication is a direct consequence of human interference with the natural nitrogen cycle.

Explore nutrient cycling and biological systems at every scale — from soil bacteria to global climate — with Professor Darwin at aitutors.me.