19.3) Nutrient cycles

19.3) Nutrient cycles


The carbon cycle:

  • Carbon is an element that occurs in all the compounds which make up living organisms.
  • Plants get their carbon from carbon dioxide in the atmosphere and animals get their carbon from plants.
  • The carbon cycle, therefore, is mainly concerned with what happens to carbon dioxide.

Removing CO2 from the atmosphere:

  • Plants use carbon dioxide from the atmosphere for photosynthesis.
  • The carbon becomes part of complex molecules in the plants, such as proteins, fats and carbohydrates.


Passing carbon from one organism to the next:

  • When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal.
  • Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants.
  • The carbon then becomes part of these organisms.


Returning CO2 to the atmosphere:

  • Carbon dioxide is released to the atmosphere through respiration by animals, plants and microorganisms.
  • It is also released by the combustion of wood and fossil fuels (such as coal, oil and natural gas).
  • The use of fossil fuels is gradually increasing the carbon dioxide levels in the atmosphere.
  • Decomposition or decay also releases carbon dioxide.
  • This process happens faster in warm, moist conditions with plenty of oxygen because it involves microorganisms.
  • Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.


The effects of the combustion of fossil fuels:

  • Since the Industrial Revolution, we have been burning the fossil fuels such as coal and petroleum and releasing extra CO2 into the atmosphere. As a result, the concentration of CO2 has increased from 0.029% to 0.035% since 1860.
  • Although it is not possible to prove beyond all reasonable doubt that production of CO2 and other ‘greenhouse gases’ is causing a rise in the Earth’s temperature, ie. global warming, the majority of scientists and climatologists agree that it is happening now and will get worse unless we take drastic action to reduce the output of these gases.
  • Another factor contributing to the increase in atmospheric gaseous CO2 is deforestation.
  • Trees are responsible for removing gaseous CO2 and trapping the carbon in organic molecules (carbohydrates, proteins and fats).
  • When they are cut down the amount of photosynthesis globally is reduced.
  • Often deforestation is achieved by a process called ‘slash and burn’, where the felled trees are burned to provide land for agriculture and this releases even more atmospheric CO2.


The water cycle:

  • The water cycle is also known as the hydrological cycle. It describes how water moves on, above, or just below the surface of our planet.
  • Water molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes. Water can change state.


  • Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and lakes. The warm air rises, carrying water vapour with it.


  • Transpiration from plants releases water vapour into the air.


  • The moist air cools down as it rises. Water vapour condenses back into liquid water, and this condensation process produces clouds.


  • As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet. This is called precipitation (it is not the same as precipitation in Chemistry, refer to A Lin’s notes).


The nitrogen cycle:

  • Nitrogen compounds found in cells include proteins.
  • Nitrogen from the air is converted into soluble ions that plant roots can absorb. It forms part of nitrogen compounds in the plants, and is then passed from one organism to the next.
  • It is returned to the atmosphere as nitrogen gas. This is the nitrogen cycle.
  • When a plant or animal dies, it tissues decompose, partly as a result of the action of saprotrophic bacteria
  • One of the important products of the decay of animal and plant protein is ammonia (NH3, a compound of nitrogen), which is washed into the soil. It dissolves readily in water to form ammonium ions (NH4-).
  • The excretory products of animals contain nitrogenous waste products such as ammonia, urea and uric acid.
  • Urea is formed in the liver of humans as a result of deamination.
  • The organic matter in animal droppings is also decomposed by soil bacteria.


Processes that add nitrates to soil:

Nitrifying bacteria:

  • These are bacteria living in the soil, which use the ammonia from excretory products and decaying organisms as a source of energy.
  • In the process of getting energy from ammonia called nitrification, the bacteria produce nitrates.
  • The ‘nitrite’ bacteria oxidise ammonium compounds to nitrites (NH4- ⇒ NO2-).
  • ‘Nitrate’ bacteria oxidise nitrites to nitrates (NO2- ⇒ NO3-).
  • Although plant roots can take up ammonia in the form of its compounds, they take up nitrates more readily, so the nitrifying bacteria increase the fertility of the soil by making nitrates available to the plants.


Nitrogen-fixing bacteria:

  • This is a special group of nitrifying bacteria that can absorb nitrogen as a gas from the air spaces in the soil, and build it into compounds of ammonia.
  • Nitrogen gas cannot itself be used by plants. When it has been made into a compound of ammonia, however it can easily be changed to nitrates by other nitrifying bacteria.
  • The process of building the gas, nitrogen, into compounds of ammonia is called nitrogen fixation.
  • Some of the nitrogen-fixing bacteria live freely in the soil. Others live in the roots of leguminous plants (peas, beans, clover), where they causes swellings called root nodules.
  • These leguminous plants are able to thrive in soils where nitrates are scarce, because the nitrogen-fixing bacteria in their nodules make compounds of nitrogen available for them.
  • Leguminous plants are also included in crop rotations to increase the nitrate content of the soil.



  • The high temperature of lightning discharge causes some of the nitrogen and oxygen in the air to combine and form oxides of nitrogen.
  • These dissolve in the rain and are washed into the soil as weak acids, where they form nitrates.
  • Although several million tonnes of nitrate may reach the Earth’s surface in this way each year, this forms only a small fraction of the total nitrogen being recycled.


Processes that removes nitrates from the soil:

Uptake by plants:

  • Plant roots absorb nitrates from the soil and combines them with carbohydrates to make amino acids, which are built up into proteins.
  • These proteins are then available to animals, which feed on the plants and digest the proteins in them.



  • Nitrates are very soluble and as rainwater passes through the soil it dissolves the nitrates and carries them away in the run-off or to deeper layers of the soil.


Denitrifying bacteria:

These are bacteria that obtain their energy by breaking down nitrates to nitrogen gas, which then escapes from the soil into the atmosphere.

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