nutrient cycles

• • Living organisms require nutrients from their environments for growth and other processes (e.g. reproduction)
  • These nutrients are then returned to the environment when organisms produce waste or die and decompose
  • This is due to the waste products and dead organisms being digested (decomposed) by microorganisms
  • The products of this decomposition are available to plants as nutrients in the soil
  • These plants can then sustain organisms in higher trophic levels (consumers)
• • In stable communities, the processes that remove nutrients (e.g. plant growth) are balanced by the processes that return these nutrients (e.g. decomposition of dead plants and animals)
  • This means these nutrients are constantly being cycled in ecosystems
• • Two examples of these nutrient cycles are:
  • The nitrogen cycle
  • The phosphorous cycle
• nitrogen cycle has four main steps

  • nitrogen fixation
  • ammonification
  • nitrification
  • denitrification
• Nitrogen fixation

  1. Atmospheric nitrogen gas is converted into nitrogen-containing compounds

  2. Nitrogen-fixing bacteria such as Rhizobium convert nitrogen into ammonia

  3. Ammonia forms ammonium ions that can be used by plants

  4. Nitrogen-fixing bacteria are found in root nodules of leguminous plants

  5. Bacteria have a symbiotic relationship with plants - bacteria provide nitrogen, plants provide carbohydrates
• Ammonification

  1. Nitrogen compounds in waste and dead organisms are converted into ammonia by saprobionts

  2. Ammonia forms ammonium ions in the soil
• Nitrification

  1. Ammonium ions in soil are converted by nitrifying bacteria into nitrates

  2. Nitrosomonas convert ammonium into nitrites

  3. Nitrobacter convert nitrites into nitrates
• Denitrification

  1. Denitrifying bacteria use nitrates in soil during respiration

  2. This produces nitrogen gas which returns to atmosphere

  3. Occurs in anaerobic conditions with little or no oxygen
• The phosphorus cycle

  1. Phosphorus in rocks is slowly released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) by the process of weathering

  2. Phosphate ions are taken up from the soil by plants through their roots or absorbed from water by algae

  3. Phosphate ions are transferred to consumers during feeding

  4. Phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts

  5. The phosphate ions can now be taken up and used once again by producers or may be trapped in sediments that, over very long geological time periods may turn into phosphorus-containing rock once again
• The phosphorus cycle

  Shows how phosphorus is recycled in ecosystems
• Phosphorus

  • Plants and animals require it to produce certain biological molecules such as phospholipids (for cell membranes), nucleic acids (DNA and RNA) and ATP
• plants need nitrogen which is required to make the amino acids that form proteins, which are needed for cell growth
• plants need phosphorus which is required to make phospholipids for cell membranes, nucleic acids and ATP
• plants need magnesium which is required to make chlorophyll
• plants need Calcium which is required to make calcium pectate for the middle lamella (the layer that provides support to plants by holding the cell walls of two plant cells together) and is important for membrane permeability
• • Plants obtain these elements in the form of mineral ions that they actively absorbed from the soil through root hair cells
• saprobionts

  decompose waste and dead matter via extracellular digestion, making inorganic ions available to other organisms, carry out ammonification (convert nitrogen compounds into waste and dead matter into ammonia and forms ammonium ions in soil)
• nitrogen-fixing bacteria


  convert atmospheric nitrogen gas into nitrogen-containing compounds such as ammonia which then forms ammonium ions in soil that can be used by plants
• nitrifying bacteria


  convert ammonium ions in soil into nitrogen containing compounds that can be used by plants, called nitrates

  some nitrifying bacteria convert ammonium ions into nitrates, different bacteria then convert nitrites into nitrates
• denitrifying bacteria

  use nitrates during respiration releasing nitrogen gas in the process
• mycorrhizal fungi

  increase SA of root systems, helping plants absorb water and scarce mineral ions from soil
• Agricultural ecosystems

  Ecosystems where crops and livestock are grown
• Nutrient uptake in agricultural ecosystems

  1. Crops and livestock take in nutrients (e.g. inorganic 'mineral' ions) from the soil (or from the grass that grows in the soil) as they grow

  2. Crops and livestock use these nutrients to generate biomass
• Agricultural ecosystems

  • They are not like natural ecosystems because the crops or livestock are eventually removed from the fields instead of dying and decomposing there naturally
• Crops and livestock are removed from agricultural ecosystems

  The mineral ions (e.g. nitrates and phosphates) now contained in the biomass are not returned to the soil by microorganisms
• The interruption of nutrient recycling processes (e.g. the nitrogen and phosphorus cycles) in agricultural ecosystems can lead to a decrease in the concentration of nutrients in the soil</b>
• Decrease in soil nutrient concentration in agricultural ecosystems

  Eventually leads to a decrease in crop yields or meat and milk yields from livestock
• Fertilisers


  A way of replacing the minerals lost from agricultural ecosystems
• Adding fertilisers to fields

  Ensures crops and livestock can continue to grow and increase in biomass as normal, ensuring yields remain high
• Fertilisers can be used to add important mineral ions, such as ions of nitrogen, phosphorus and potassium, back into the soil
• two types of fertilisers:

  • Natural fertilisers
  • Artificial fertilisers
• Natural fertilisers

  • Natural fertilisers are made up of organic matter in the form of the dead and decomposing remains of organisms and their waste products
  • This includes manure, composted vegetables, crops residues (crop parts left over after harvesting) and sewage
  • They can also improve soil structure, which helps in reducing soil erosion and increase the water-holding ability of the soil
  • Nutrients from natural fertilisers are released over long time periods
  • The nutrients present are not very concentrated so relatively large amounts are needed
• Artificial fertilisers

  • Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain pure chemical compounds (e.g. ammonium nitrate)
  • As the exact chemical composition is known, it is easier to how much to apply and the effects they will have on crop yields
  • The nutrients present are concentrated so smaller amounts are needed
  • This means transport costs are lower
  • These fertilisers are easy to apply evenly and are clean, making them easy to handle
• Leaching

  Mineral ions (nitrate and phosphate) from excess fertiliser are transported by rainwater or irrigation water into nearby bodies of water (ponds, lakes, streams, rivers)
• Fertilisers are very effective in ensuring high crop yields, so they are often applied to fields by farmers in greater quantities than are actually needed by the crop plants
• Crop plants are unable to use all the fertiliser provided, so the soluble nitrate and phosphate ions in the excess fertiliser remain in the soil water
• Fertilisers are applied just before heavy rainfall

  Leaching is more likely to occur
• Leaching can lead to a potentially damaging process known as eutrophication
• Eutrophication


  When the mineral ions from excess fertiliser leach from farmland into waterways, they cause rapid growth of algae at the surface of the water
• Algal bloom

  Rapid growth of algae at the surface of the water