Nutrient Cycles

  • The carbon cycle
  • Global warming
  • The nitrogen cycle
  • Natural and artificial fertilisers
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The Carbon Cycle

  • CO2 makes up 0.04% of the atomosphere
  • Photosynthesis removes CO2 from the atmosphere
  • Respiration released CO2 into the atmosphere
  • The concentration of CO2 is greater at night due to photosynthesis not taking place
  • CO2 levels have increased due to the combustion fossil fuels and deforestation
  • Oceans act as a CO2 sink keeping the CO2 levels revatively constant. When levels are too high, the gas dissolves in the water and photosynthetic organisms e.g.phytoplankton use it to form the macromolecules that make up their bodies

Saprobiotic organisms

  • When plants and animals die, they are broken down by saprobiotic organisms
  • They secrete enzymes which break down complex molecules into smaller more manageable ones which can then be absorbed by diffusion
  • CO2 is released by the saprobiotic organism as it respires
  • If decay is prevented organisms may become fossilised into coal, oil or peat. Shells and bones of aquatic organisms form carbon-containing sedimentary rocks such as chalk and limestone.The carbon is eventually returned when the rocks are weathered
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The Carbon Cycle


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Global Warming

The greenhouse effect

  • Of the small amount of radiation from the sun that reaches the earth, some if reflected back as hea and is lost in space. However some of the heat is trapped by greenhouse gases which traps the heat close to the earth.This is the greenhouse effect.

Greenhouse gases (Carbon dioxide)

  • The most important greenhouse gas, due to there being so much of it and it remaining in the atmosphere for so long
  • Human activities e.g. increased combustion of fossil fuels has caused its concentration to increase

Greenhouse gases (Methane)

  • Produced when decomposers break down the dead remains or organisms
  • Produced when microorganisms in the intestines or primary consumers, such as cattle, digest the food that has been eaten
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Global Warming

Consequences of global warming

  • Change in niches- Due to each organism beind adapted to a particular niche, this will cause the distribution of organisms to alter. Organisms will have to migrate to new areas where they will compete for the available niches, this may result in the loss of native species.
  • Melting of polar ice caps- cause sea levels to rise.
  • Rising sea levels- flooding of low level land, sea water would extend further up rivers and as a result make cultivation of crop plants difficult.
  • High temperatures and less rainfall- More drought resistant species would have to be grown. The distribution of wild plants would change, with only xerophytes be able to survive in areas that are extremely dry. This change would also affect the distribution of animal species as their food sources alter.
  • Life cycles and populations of insect pests alter- Insects carry many human and crop pathogens, and the change in climate may result in them moving to areas that they haven't previously inhabited, causing problems in places that there has previously been no issue.
  • Benefits- increased rainfall would fill reservoirs, the warmer temperatures would allow crops to grow where it is presently too cold and would increase the rate of photosynthesis and so may increase the rate of harvesting.
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Nitrogen Cycle


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Nitrogen Cycle


  • The production of ammonia from organic ammonium containing compounds.
  • Saprobiotic microorganisms, mainly fungi and bacteria, feed on the organic matter (e.g. urea, proteins, nucleic acids and vitamins) releasing ammonia, which then forms ammonium ions in the soil. here nitrogen returns to the non living component of the ecosystem.


  • The conversion of ammonium ions to nitrate ions. This is an oxidation reaction and so releases energy.
  • Carried out by free-living soil microorganisms called nitrifying bacteria
  • Conversion in two stages 1. oxidation of ammonium ions to nitrite ions (NO2-) 2. oxidation of nitrite ions to nitrate ions (NO3-)
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Nitrogen Cycle

Nitrogen Fixation

  • Process by which nitrogen gas is converted into nitrogen containing compounds
  • Can be carried out industrially or naturally when lightening passes through the atmosphere
  • The most important form is carried out by microorganisms of which there are two types:
    1. Free-living nitrogen fixing bacteria: Reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen-rich compunds are released from them when they die and decay.
    2. Matualistic nitrogen-fixing bacteria: These bacteria live in nodules on the roots of leguminous plants. They obtain carbohydrates from the plamt and plant acquires amino acids from the bacteria.


  • When the soil becomes waterlogged, there is limited oxygen which results in an increase in anaerobic denitrifying bacteria
  • Anaerobic bacteria convert soil nitrates into gaseous nitrogen
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Natural vs Artificial Fertilisers

Need for fertilisers

  • Food production is intensive and takes place in concentrated specific areas of land that are used repeatedly to achieve a maximum yield.
  • Mineral ions are continuously taken up but are not returned
  • Fertilisers muct be used to replenish the most mineral ions
  • Natural (organic) fertilisers: Dead and decaying remains of plants and animals as well as animal waste, manure and bone meal
  • Artificial (inorganic) fertilisers: mined from rocks and deposits and then converted into different froms and blended together to give the appropriate balance of minerals.
  • Fertilisers contain the three elements: nitrogen, phophorus and potassium
  • Research shows that a combination of both fertilisers gives the greatest long-term productivity

How fertilisers increase productivity

  • Plants require minerals for growth e.g. nitrogen is a component of proteins and DNA. Nitrates cause plants to have a greater leaf area which increases the rate of photosynthesis and therefore crop productivity.
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Environmental consequences of using nitrogen ferti

  • Reduced species diversity- Nitrogen-rich soils favour the growth of rapidly growing species which will out-compete many other species causing them to die out, reducing the species diversity.
  • Leaching- The process by which nutrients are removed from the soil and drain into streams and rivers. This can have a very harmful effect if  the water source is used as drinking water
  • Eutrophication- The process by which nutrients build up in water
    1. Leaching causes nitrate concentrations to increase resulting in an algae bloom
    2. The dense surface of algae absorbs light and prevents it from penetrating to lower depths
    3. Light becomes the limiting factor for the growth of plants at lower depths resulting in death
    4. An increase in dead organisms results in an increase in saprobiotic algae
    5. Saprobiotic bacteria require oxygen for their respiration, creating an increased demand
    6. The concentration of oxygen in the water is reduced and nitrates are released from decaying organisms
    7. Aerobic organisms die due to lack of oxygen
    8. Less competicition for anaerobic organisms whose population increases
    9. Anearobic organisms further decompose dead material, releasing nitrates and toxic wastes e.g. hydrogen sulphide making the water putrid
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