Nutrient Cycles


Nutrient Cycles

All nutrient cycles have one simple sequence:

  • nutrient is taken up by producers as simple, inorganic molecules
  • producer incorporates the nutrient into complex organic molecules
  • when the producer is eaten, the nutrient passes into consumers
  • it then passes along the food chain when these animals are eaten by other consumers
  • when the producers and consumers die, their complex molecules are broken down by saprobiotic microorganisms that release the nutrient in it original simple form
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Carbon Cycle


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Nitrogen Cycle


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Nitrogen Cycle


  • The production of ammonia from organic ammonium containing compounds
  • Saprobiotic microorganisms feed on these compounds, releasing ammonia, which then forms ammonium ions in the soil


  • Oxidation reaction so releases energy
  • Carried out by nitrifying bactera
  • Ammonium ions are first converted to nitrite ions. These nitrite ions are then converted to nitrate ions

Nitrogen Fixation

  • Nitrogen gas is converted into nitrogen-containing compounds
  • Carried out by microorganisms (free-living or mutualistic nitrogen-fixing bacteria)


  • Anaerobic dentrifying bacteria convert soil nitrates into gaseous nitrogen, reducing the availability of nitrogen-containing compounds for plants
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Greenhouse Effect

The Greenhouse Effect is where heat and light from the sun (solar radiation) is trapped in the earth's atmosphere


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Greenhouse Gasses

Carbon Dioxide

  • 50-70% of global warming is due to CO2
  • Human activities are increasing the concentration of CO2 in the atmosphere


  • Produced when microorganisms break down organic molecules when:
    • decomposers break down the dead remains of organisms
    • microorganisms in intestines of primary consumers digest the food that has been eaten

Global Warming

  • Melting of polar ice caps = extinction fo some wild plants and animals and rises sea levels
  • Rise in sea levels due to thermal expansion = flood low lying land and fertile land, and salt water = cultivation of crop plants difficult
  • Higher temp and less rainfall (drought) = crop failure, only xerophytes or similar plants could survive, consumers affected (only those that can feed on xerophytes survive)
  • Greater rainfall and intense storms = similar but opposite affect
  • Life cycles and populations of insect pests change = spread of diseases
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Types of fertilisers:

  • Natural (organic) fertilisers: dead and decaying remains or organimsms and their wastes
  • Artificial (inorganic) fertilisers: mined from rocks and deposits and blended together

Effect of Nitrogen Fertilisers:

  • reduced species diversity - nitrogen-rich soils favours the growth of grasses, nettles and other rapidly growing species. These out-compete many other species, which die.
  • leaching - leading to pollution of watercourses
  • eutrophication - caused by leaching

Leaching is the process where nutrients are removed from the soil. Rain and water dissolve soluble nutrients, e.g nitrates, and carry them deep into the soil. The leached nitrates get into watercourses, where they are harmful to organisms that drink from the water. They are also harmful to the environment as they cause eutrophication

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Eutrophication is the process where nutrients build up in bodies of water.

  • Most lakes have very little nitrate so nitrate is a limiting factor for plant and algal growth
  • As the nitrate concentration increases, due to leaching, it ceases to be a limiting factor so plants and algae grow rapidly
  • The upper layers of water become densley populated with algae: 'algal bloom'
  • This surface layer of algae absorbs light and prevents it from penetrating to lower plants
  • Light becomes a limiting factor for the growth of plants at lower depths so they die
  • The lack of dead organisms is no longer a limiting factor for saprobiotic microorganisms, so they grow rapidly
  • The saprobiotic bacteria use alot of oxygen up for their respiration, so there is an increase in the demand of O2
  • The concentration of O2 in the water is reduced
  • Oxygen becomes a limiting factor for the population of anaerobic organisms, e.g fish, who die
  • Without the aerobic organisms, there is less competition for the anaerobic organisms, who's population rises rapidly
  • The anaeobic organisms further decompose dead material, releasing more nitrates and some toxic wastes, making the waters putrid
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