19.3 Nutrient cycles

2026 Syllabus Objectives

  1. Describe the carbon cycle, limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion
  2. Describe the nitrogen cycle with reference to: decomposition of plant and animal protein to ammonium ions, nitrification, nitrogen fixation by lightning and bacteria, absorption of nitrate ions by plants, production of amino acids and proteins, feeding and digestion of proteins, deamination, and denitrification
  3. State the roles of microorganisms in the nitrogen cycle, limited to: decomposition, nitrification, nitrogen fixation and denitrification (generic names of individual bacteria are not required)

The Role of Decomposers 🔄

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One of the most important but often overlooked groups of organisms in any ecosystem is the decomposers. These organisms play a vital role in nutrient cycling.

What Are Decomposers?

Decomposers are organisms that feed on:

  • Organic waste material from animals and plants
  • Dead bodies of organisms

Many fungi and bacteria are decomposers.

Why Are Decomposers Important?

Decomposers are extremely important because they help to release substances from dead organisms. These released substances can then be used by other living organisms.

Two of the most important substances recycled by decomposers are:

  • Carbon
  • Nitrogen

The Carbon Cycle 🌍

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Why Is Carbon Important?

Carbon is a very important component of living things because carbon atoms are part of the molecules of:

  • Carbohydrates
  • Fats
  • Proteins

About 0.04% of the air is carbon dioxide.

Key Processes in the Carbon Cycle

1. Photosynthesis 🌱

When plants photosynthesise, carbon atoms from carbon dioxide become part of glucose and starch molecules in the plant.

2. Respiration

  • Some of the glucose is broken down by the plant in respiration
  • The carbon in the glucose becomes part of a carbon dioxide molecule again
  • This carbon dioxide is released back into the air

3. Feeding 🦁

Some of the carbon-containing compounds in the plant will be eaten by animals. The animals respire, releasing some of it back into the air as carbon dioxide.

4. Decomposition

When the plant or animal dies:

  • Decomposers will feed on them
  • Decomposers also feed on waste materials, such as urine and faeces from animals
  • The carbon becomes part of the carbon-containing compounds in the decomposers' bodies
  • When the decomposers respire, they release carbon dioxide into the air again

5. Formation of Fossil Fuels ⚡

Sometimes, dead bodies and waste materials are not broken down by decomposers. This occurs in places where there is little oxygen, such as:

  • The seabed in the deep oceans
  • Waterlogged ground

In these conditions:

  • Decomposers may not be able to respire aerobically
  • Waste substances become buried in sediments
  • Over long periods of time, they are gradually changed into coal, oil or natural gas

These are called fossil fuels – not because they contain fossils, but because they were formed millions of years ago from living organisms.

6. Combustion 🔥

When fossil fuels are burnt, the carbon in them combines with oxygen from the air in a process called combustion. Wood can also undergo combustion.

The carbon and oxygen combine to form carbon dioxide, which is released into the air again.

Summary of Carbon Cycle Processes

Processes that remove CO2\mathrm{CO}_2 from the air: Photosynthesis

Processes that add CO2\mathrm{CO}_2 to the air: Respiration (from plants, animals, and decomposers) and combustion

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