The nitrogen cycle

What you'll learn

The nitrogen cycle is a fundamental process that recycles nitrogen through ecosystems. You need to understand how nitrogen moves between the atmosphere, soil, living organisms, and back again. This topic combines knowledge of nutrient cycles, decomposition, and the role of microorganisms in maintaining soil fertility.

Key terms and definitions

Nitrogen cycle — the processes by which nitrogen is converted between its various chemical forms and circulated through the atmosphere, soil, and living organisms

Decomposers — bacteria and fungi that break down dead organic matter and waste products, releasing nutrients back into the soil

Nitrifying bacteria — microorganisms that convert ammonia into nitrites and then nitrates through the process of nitrification

Denitrifying bacteria — microorganisms that convert nitrates in the soil back into nitrogen gas, which is released into the atmosphere

Nitrogen fixation — the conversion of atmospheric nitrogen gas into nitrogen-containing compounds that plants can absorb and use

Nitrates — nitrogen-containing compounds (NO₃⁻) that plants absorb through their roots to make proteins and nucleic acids

Ammonium compounds — nitrogen-containing compounds (NH₄⁺) formed when decomposers break down proteins and urea

Legumes — plants such as peas, beans, and clover that have root nodules containing nitrogen-fixing bacteria

Core concepts

Why nitrogen is essential for living organisms

Nitrogen is a crucial element found in all proteins and nucleic acids (DNA and RNA). Despite making up 78% of the atmosphere, nitrogen gas (N₂) cannot be used directly by most organisms because the strong triple bond between nitrogen atoms makes it extremely unreactive.

Plants require nitrogen to:

• Synthesise amino acids and proteins for growth
• Produce chlorophyll for photosynthesis
• Make DNA and RNA for cell division and inheritance

Animals obtain nitrogen by eating plants or other animals. They use the nitrogen-containing compounds to build their own proteins and nucleic acids.

The four key processes in the nitrogen cycle

1. Nitrogen fixation

Nitrogen fixation converts unreactive nitrogen gas from the atmosphere into usable nitrogen compounds. This occurs through three main routes:

Biological nitrogen fixation:

• Carried out by nitrogen-fixing bacteria found in soil and root nodules
• Root nodules are small swellings on the roots of leguminous plants (peas, beans, clover)
• The bacteria convert nitrogen gas into ammonia or ammonium compounds
• Plants provide the bacteria with sugars from photosynthesis
• The bacteria provide the plant with nitrogen compounds — a mutualistic relationship
• Free-living nitrogen-fixing bacteria in the soil (such as Azotobacter) also perform this function

Lightning:

• High energy from lightning breaks nitrogen molecules apart
• Nitrogen reacts with oxygen to form nitrogen oxides
• These dissolve in rainwater and enter the soil as nitrates
• Relatively small contribution to the nitrogen cycle

Industrial processes:

• The Haber process manufactures ammonia for fertilisers
• Not part of the natural cycle but adds nitrogen compounds to agricultural systems

2. Decomposition

When organisms produce waste or die, decomposers break down complex organic nitrogen compounds into simpler forms.

The decomposition process:

• Decomposers (bacteria and fungi) secrete enzymes onto dead matter
• These enzymes break down proteins and urea into ammonia
• Ammonia then forms ammonium compounds (NH₄⁺) in the soil
• Decomposition is fastest in warm, moist, aerobic conditions with neutral pH
• The rate slows in cold, dry, waterlogged, or very acidic/alkaline conditions

Sources of organic nitrogen:

• Dead plant and animal material
• Faeces from animals
• Urea in animal urine

3. Nitrification

Nitrifying bacteria convert ammonium compounds into nitrates through a two-stage process called nitrification. This occurs in well-aerated soils.

Stage 1:

• Nitrifying bacteria (such as Nitrosomonas) oxidise ammonium compounds to nitrites (NO₂⁻)
• Nitrites are toxic to plants and usually don't accumulate

Stage 2:

• Different nitrifying bacteria (such as Nitrobacter) oxidise nitrites to nitrates (NO₃⁻)
• Nitrates are the main form of nitrogen absorbed by plant roots

Nitrification requires oxygen, so waterlogged soils with little oxygen have reduced nitrification rates. Both stages release energy that the bacteria use for their metabolism.

4. Denitrification

Denitrifying bacteria convert nitrates back into nitrogen gas, which returns to the atmosphere. This process is called denitrification.

Key features:

• Occurs in anaerobic (oxygen-poor) conditions such as waterlogged soils
• Denitrifying bacteria use nitrates instead of oxygen for respiration
• Reduces soil fertility by removing nitrates that plants could use
• Farmers avoid denitrification by improving drainage

How plants absorb and use nitrogen

Plants absorb nitrates from the soil through their root hair cells by active transport. This requires energy from respiration because nitrates are often absorbed against a concentration gradient.

Uses of nitrates in plants:

• Combined with sugars from photosynthesis to make amino acids
• Amino acids joined together to form proteins
• Proteins used for enzymes, structural proteins, and growth
• Production of DNA and RNA bases
• Synthesis of chlorophyll

Without sufficient nitrates, plants show deficiency symptoms:

• Stunted growth due to reduced protein synthesis
• Yellowing of older leaves (chlorosis) as chlorophyll cannot be made
• Poor root development

The complete nitrogen cycle overview

The nitrogen cycle operates continuously in ecosystems:

1. Nitrogen fixation converts atmospheric N₂ into ammonium compounds
2. Nitrification converts ammonium compounds into nitrites then nitrates
3. Plant uptake — plants absorb nitrates and build them into proteins
4. Consumption — animals eat plants and use plant proteins to make animal proteins
5. Decomposition — when organisms die or produce waste, decomposers return nitrogen to the soil as ammonium compounds
6. Denitrification — in anaerobic conditions, some nitrates are converted back to N₂ gas

This creates a continuous cycle that maintains nitrogen availability in ecosystems.

Human impacts on the nitrogen cycle

Human activities significantly affect the nitrogen cycle, particularly through farming practices.

Fertiliser application:

• Farmers add artificial fertilisers containing nitrates to increase crop yields
• Excess fertiliser can be washed into waterways (leaching), causing eutrophication
• Organic fertilisers (manure, compost) release nitrogen more slowly as decomposers break them down

Crop rotation:

• Growing legumes in rotation adds nitrogen compounds to soil
• Following crops benefit from increased soil nitrogen
• Reduces need for artificial fertilisers

Drainage improvement:

• Better drainage maintains aerobic conditions
• Promotes nitrification and reduces denitrification
• Increases nitrate availability for crops

Eutrophication, step by step

Leaching is often mentioned in one line, but exam questions ask for the chain of events, and marks are given for the order:

1. Nitrate is washed from farmland into a river or lake
2. The extra nitrate allows algae and other water plants to grow rapidly, forming a bloom at the surface
3. The bloom blocks light from the plants below, which die
4. The algae themselves are short-lived and also die
5. Decomposer bacteria multiply on all this dead material and respire, using up the dissolved oxygen
6. Fish and other animals that need dissolved oxygen suffocate

The point most often missed: the fish are killed by a shortage of oxygen, not poisoned by the nitrate. That is why the deaths follow the algal growth rather than happening at the same time as it.

Managing soil nitrogen

Several farming practices make sense only once the four processes are understood.

Liming an acid soil. Nitrifying bacteria, like enzymes, have an optimum pH, and very acid soil slows them down. Adding lime reduces the acidity, so ammonia is converted to nitrate faster — the lime supplies no nitrogen itself.

Timing fertiliser application. Nitrate is soluble, which is exactly why roots can absorb it and exactly why rain can carry it away. Fertiliser spread just before heavy rain is largely leached into waterways. Smaller amounts applied in step with the crop's growth waste far less.

Cover crops. A field left as bare soil through a wet winter loses nitrate two ways at once: leaching by rain, and denitrification in waterlogged, oxygen-poor soil. A cover crop sown in the autumn is not grown to be harvested — it holds nitrogen in plant tissue over the winter and returns it to the soil when it is ploughed in.

Fertiliser or legumes? Both raise soil nitrogen, and a question may ask for an advantage of each rather than which is better:

• Manufactured fertiliser acts quickly and the dose can be controlled precisely
• Legumes cost less, need no energy for manufacture, and their roots improve soil structure

Why harvesting breaks the cycle

In an undisturbed ecosystem, plants die where they grew. Decomposers release their nitrogen into the same soil, so the cycle closes and fertility is maintained without anything being added.

Harvesting removes the plants. Every crop carries nitrogen compounds off the field in the grain, fruit or leaves, and none of it returns. This is why a field cropped year after year with nothing added yields a little less each season, and why fertiliser, manure or a legume in the rotation is not an optional improvement but a replacement for a step the farmer has removed.

The same reasoning explains a case that surprises students: soil beneath tropical rainforest is often poor in nitrate even though the vegetation above it is dense. Decomposition there is rapid and the nitrogen released is taken up almost immediately, so very little accumulates in the soil. Nearly all the nitrogen is in the living plants — which is why clearing the forest and farming the ground exhausts it within a few seasons.

Wording that costs marks

Bacteria do not "make nitrogen". Nitrogen-fixing bacteria convert nitrogen gas into compounds organisms can use; nitrogen atoms are rearranged, never created. Saying an organism makes an element implies matter can be created, which is the opposite of what a cycle demonstrates.

Similarly, be precise about direction. Nitrifying and denitrifying bacteria have confusingly similar names, so link each to the way it moves nitrogen: nitrifying produces nitrate (a farmer wants this), denitrifying takes nitrate away as nitrogen gas (a farmer does not).

Worked examples

Example 1: Describing the role of bacteria (4 marks)

Question: Describe the role of bacteria in making nitrogen available to plants.

Mark scheme answer:

• Decomposers/bacteria break down proteins/urea in dead organisms/waste (1 mark)
• This releases ammonia/ammonium compounds into the soil (1 mark)
• Nitrifying bacteria convert ammonium compounds to nitrites then nitrates (1 mark)
• Nitrates are absorbed by plant roots (by active transport) (1 mark)

Examiner tip: Notice this question asks you to "describe" — you need to provide clear steps showing the sequence of events. Each stage earns a separate mark.

Example 2: Explaining nitrogen fixation (3 marks)

Question: Explain how nitrogen-fixing bacteria in root nodules help legume plants grow.

Mark scheme answer:

• Bacteria convert nitrogen gas into ammonia/ammonium compounds/nitrogen compounds (1 mark)
• Plants use these nitrogen compounds to make amino acids/proteins (1 mark)
• This allows the plant to grow even in nitrogen-poor soils (1 mark)

Examiner tip: The command word "explain" requires you to give reasons why something happens. Link the process to the benefit for the plant.

Example 3: Analysing data on soil conditions (5 marks)

Question: A farmer measured nitrate levels in three fields with different soil conditions. Field A had well-drained soil (12 mg/kg nitrate), Field B had waterlogged soil (3 mg/kg nitrate), and Field C had acidic soil (5 mg/kg nitrate). Explain the differences in nitrate levels.

Mark scheme answer:

• Field A has the highest nitrate levels because well-drained soil is aerobic (1 mark)
• Aerobic conditions allow nitrifying bacteria to convert ammonium to nitrates (1 mark)
• Field B has the lowest levels because waterlogged soil is anaerobic (1 mark)
• Anaerobic conditions favour denitrifying bacteria which convert nitrates to nitrogen gas (1 mark)
• Field C has low levels because acidic conditions reduce bacterial activity/nitrification (1 mark)

Examiner tip: When analysing data, always quote the actual values and link them to biological processes. Compare the results systematically.

Common mistakes and how to avoid them

• Confusing nitrogen fixation and nitrification — Nitrogen fixation converts N₂ gas to ammonium compounds; nitrification converts ammonium compounds to nitrates. Remember: fixation fixes atmospheric nitrogen, nitrification makes nitrates.

• Stating plants absorb nitrogen gas from the air — Plants cannot use nitrogen gas directly. They absorb nitrates through their roots from soil, not N₂ from the atmosphere. Only nitrogen-fixing bacteria can convert N₂.

• Forgetting that denitrification removes nitrogen from soil — Students often think all bacteria add nitrogen to soil. Denitrifying bacteria actually reduce soil fertility by converting nitrates to nitrogen gas in waterlogged conditions.

• Not specifying which form of nitrogen moves at each stage — Be precise: use "nitrogen gas," "nitrates," "ammonium compounds," or "proteins" rather than just writing "nitrogen" throughout your answer.

• Describing bacteria as plants or animals — Bacteria are microorganisms, not plants. They don't photosynthesise (unless specifically stated as photosynthetic bacteria, which isn't relevant here).

• Confusing decomposers with detritivores — Decomposers are bacteria and fungi that secrete enzymes externally. Detritivores (woodlice, earthworms) are animals that physically break down matter but aren't part of the nitrogen cycle at GCSE level.

Exam technique for "The nitrogen cycle"

• Command words matter — "State" requires simple facts (1-2 words), "describe" needs clear steps in sequence, "explain" demands reasons linking cause and effect. For nitrogen cycle questions, "explain" commonly appears and requires you to link processes to outcomes.

• Use precise terminology — Always specify the exact form of nitrogen compound (nitrates, ammonium, nitrogen gas) and name the specific bacteria type (nitrifying, denitrifying, nitrogen-fixing, decomposers). Generic answers like "bacteria convert nitrogen" score poorly.

• Draw arrows correctly on diagrams — If asked to complete a nitrogen cycle diagram, ensure arrows show the direction of nitrogen movement and label both the process AND the form of nitrogen moving. For example: "nitrates → proteins (absorption and synthesis)" not just "uptake."

• Extended response questions — These often ask you to explain how the cycle maintains soil fertility or how farmers can increase nitrogen availability. Structure answers logically: start with nitrogen fixation/fertilisers adding nitrogen → plants uptake → consumption → decomposition returns nitrogen → nitrification makes it available again. Show it's a continuous cycle.

Quick revision summary

The nitrogen cycle recycles nitrogen through ecosystems. Nitrogen-fixing bacteria convert atmospheric N₂ into ammonium compounds. Decomposers break down dead matter releasing ammonia. Nitrifying bacteria convert ammonium to nitrites then nitrates in aerobic soils. Plants absorb nitrates and build proteins. Animals consume plants and use the nitrogen. When organisms die, decomposers return nitrogen to soil. Denitrifying bacteria convert nitrates back to N₂ in waterlogged anaerobic conditions, completing the cycle.