3.5.4 NUTRIENT CYCLES

• Finite supply of nutrients on Earth, which are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle.

  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 - digested by microorganisms
  • Products of this decomposition are available to plants as nutrients in the soil. These plants can then sustain organisms in higher trophic levels (consumers).
• • Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA).
  • Cannot use nitrogen in gaseous form - rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds, which can be taken up by plants.

  The nitrogen cycle shows how nitrogen is converted into a usable form and then passed on between different living organisms and the non-living environment.
• Nitrogen fixation:

  • Atmospheric nitrogen gas converted into ammonium ions by nitrogen-fixing bacteria. 
  • Convert nitrogen into ammonia, which forms ammonium ions (in solution) that can then be used by plants.
  • (Some) Nitrogen-fixing bacteria found in root nodules of leguminous plants - have a symbiotic (mutually beneficial) relationship with the plants - bacteria provide plants with nitrogen compounds and the plants provide the bacteria with carbohydrates. Other nitrogen-fixing bacteria are found in the soil.
• Nitrification:

  Nitrifying bacteria first convert ammonium ions in the soil into nitrites, and then other nitrifying bacteria convert those into nitrates, which can be used by plants.
• Denitrification:

  • Nitrates in the soil are converted into nitrogen gas by denitrifying bacteria — they use nitrates to carry out respiration and produce nitrogen gas. 
  • This happens under anaerobic conditions e.g. in waterlogged soils.
• Ammonification:

  • Nitrogen compounds from waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts (type of decomposer including some fungi and bacteria)
  • This ammonia forms ammonium ions in the soil
• Plants and animals require phosphorus to produce certain biological molecules such as phospholipids (for cell membranes), nucleic acids (DNA and RNA) and ATP.

  Phosphorus is found in rocks and dissolved in the oceans in the form of phosphate ions (PO43-). Phosphate ions dissolved in water in the soil can be assimilated (absorbed and then used to make more complex molecules) by plants and other producers.
• Phosphorus Cycle

  • Phosphorus in rocks slowly released into soil + water sources in form of phos ions by weathering.
  • Ph ions taken up from soil by plants through roots/absorbed from water by algae. Mycorrhizae greatly increase rate at which phosphorus assimilated. Transferred through food chain.
  • Ph ions in waste products and dead organisms released into soil/water during decomposition by saprobionts - which also release ph ions from urine and faeces.
  • Can now be assimilated again/trapped in sediments that over long geological time periods may turn into phosphorus-containing rock once again.
• Guano - seabird waste. Contains a high proportion of phosphate ions. Returns a significant amount of phosphate ions to soils (particularly in coastal areas). Often used as a natural fertiliser.
• Role of saprobionts in decomposition -

  • Break down dead organisms to release phosphate, ammonia/ammonium compounds.
  • Feed on the remains of dead plants and animals and on their waste products, breaking them down (decomposers).
  • They secrete enzymes and digest their food externally, then absorb the required nutrients (extracellular digestion). During this process, organic molecules are broken down into inorganic ions.
• Role of mycorrhizae in facilitating uptake of water, inorganic ions by plants -

  • Types of fungi form symbiotic relationship with roots of plants.
  • The fungi are made up of long, thin strands called hyphae, which connect to the plant’s roots and increase the surface area for increased absorption of water and mineral ions (incl. phosphate ions).
  • The fungi obtain organic compounds like glucose, from the plant.
• Nitrates and phosphates in soil are lost by harvesting plants and removing livestock:

  • Crops take in minerals from the soil as they grow and are used in tissues. Crops harvested - removed from the field where they’re grown rather than being allowed to die and decompose there. So mineral ions they contain are not returned to the soil by decomposers in the nitrogen or phosphorus cycles.
  • Animals take in nutrients by eating plants. Removed for slaughter/transfer fields - nutrients aren’t replaced through their remains or waste products.
• Adding fertiliser replaces the lost minerals, so more energy from the ecosystem can be used for growth, increasing the efficiency of energy transfer.

  • Artificial fertilisers are inorganic - contain pure chemicals (e.g. ammonium nitrate). Composition tailored for specific crops.
  • Natural fertilisers are organic matter - dead, decaying remains of plants, animals and their waste.
• Leaching - Water-soluble compounds in the soil are washed away, e.g. by rain or irrigation systems, often into nearby waterways. Can lead to eutrophication.

  Inorganic ions in chemical fertilisers are relatively soluble, so excess minerals that are not used immediately are more likely to leach into waterways. 
• Leaching...

  • More likely to occur if fertiliser applied just before heavy rainfall.
  • Less likely with natural fertilisers as the nitrogen and phosphorus are still contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants - release into the soil for uptake by plants more controlled. 
  • The leaching of phosphates is less likely than the leaching of nitrates because phosphates are less soluble in water.
• Eutrophication - Caused by excess nutrients. 

  • Mineral ions leached from fertilised fields stimulate the rapid growth of algae in ponds and rivers.
  • Large amounts of algae block light from reaching the plants below.
  • Eventually the plants die because they’re unable to photosynthesise enough.
  • Bacteria feed on the dead plant matter. The increased numbers of bacteria reduce the oxygen concentration in the water by carrying out aerobic respiration.
  • Fish and other aquatic organisms die because there isn’t enough dissolved oxygen.
• Using fertilisers may also change the balance of nutrients in the soil — too much of a particular nutrient can cause crops and other plants to die.