19.2 Nutrient Cycles

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the carbon cycle, including: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels, and combustion.
  2. Outline the nitrogen cycle and how it makes nitrogen available for plant and animal protein, including:
    • (a) Decomposition of plant and animal protein to ammonium ions
    • (b) Nitrification
    • (c) Nitrogen fixation by lightning and bacteria
    • (d) Absorption of nitrate ions by plants
    • (e) Production of amino acids and protein
    • (f) Feeding and digestion of proteins
    • (g) Denitrification
  3. Outline the role of fungi and bacteria in decomposition.

Main Content

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Why Do Nutrients Need to Be Recycled?

Living organisms are made of chemical elements such as carbon and nitrogen. When organisms die, these elements must be released and returned to the environment so that they can be used again by new organisms. This is called a nutrient cycle — a continuous pathway through which chemical elements move between living things and the non-living environment (like the air, soil, and water).

Without nutrient cycles, the supply of essential elements would run out, and life on Earth could not continue.

Section 1: The Carbon Cycle

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What Is the Carbon Cycle?

The carbon cycle describes how carbon moves continuously between living organisms and the atmosphere (the air around us). Carbon exists in the atmosphere as a gas called carbon dioxide (CO₂). It is also found locked inside the bodies of living organisms, in the soil, and underground in fossil fuels (coal, oil, and natural gas).

Step-by-Step: How Carbon Moves Through the Cycle

1. Photosynthesis — Carbon Enters Living Things

  • Photosynthesis is the process by which green plants (and some other organisms) use energy from sunlight to turn carbon dioxide from the air and water from the soil into glucose (a type of sugar) and oxygen.
  • This is how carbon is removed from the atmosphere and built into the bodies of plants.
  • Think of it as plants "breathing in" CO₂ and locking that carbon into their tissues.

2. Feeding — Carbon Passes Between Organisms

  • When animals eat plants, they take in the carbon that was stored in the plant's body.
  • This carbon then becomes part of the animal's body — in molecules like proteins, fats, and carbohydrates.
  • Carbon passes further up the food chain when one animal eats another.

3. Respiration — Carbon Is Released Back to the Atmosphere

  • All living organisms — plants, animals, and microorganisms — carry out respiration. This is the process of breaking down glucose to release energy for life processes.
  • As a result of respiration, carbon dioxide is released back into the atmosphere.
  • So both plants and animals are constantly returning CO₂ to the air through respiration.

4. Decomposition — Carbon Is Released From Dead Organisms

  • When organisms die, their bodies contain large amounts of carbon locked in complex molecules.
  • Decomposers (mainly bacteria and fungi) break down the dead material. During this process, they respire and release carbon dioxide back into the atmosphere.
  • Decomposition is essential — without it, carbon would stay locked inside dead bodies forever.

5. Formation of Fossil Fuels — Carbon Gets Trapped Underground

  • Sometimes, organisms die in places where decomposers cannot reach them — for example, at the bottom of swamps, seas, or in areas without oxygen.
  • Over millions of years, the remains of these organisms are buried under layers of rock and sediment, and the pressure and heat underground slowly convert them into fossil fuels — coal, oil, and natural gas.
  • The carbon is now locked deep underground and is removed from the carbon cycle for a very long time.

6. Combustion — Burning Releases Carbon Back to the Atmosphere

  • When humans burn fossil fuels (in cars, power stations, factories), this is called combustion.
  • Combustion releases the carbon that was stored underground back into the atmosphere as carbon dioxide.
  • This is a one-way, very fast release of carbon that took millions of years to form — which is why it is causing a build-up of CO₂ in the atmosphere today.

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