NUTRIENT CYCLES

• EXTRA-CELLULAR DIGESTION: digestion that takes place outside of the cell. Saprobionts do this. They secrete enzymes onto the food which cause it to break down so that the nutrients can be absorbed.
• SAPROBIONTIC NUTRITION: obtaining nutrients from dead organic matter and animal waste using extra-cellular digestion
• MYCORRHIZAE: are symbiotic relationships between fungi and roots of plants
• SYMBIOSIS: when two species live closely together and one or both species depends on the other for survival. Mutualism is a type of symbiosis where both species benefit.
• NUTRIENT CYCLES

  • nutrients don’t have an extraterrestrial source
  • there is limited availability of nutrient ions in a usable form.
  • it’s important therefore that elements such as carbon, nitrogen and phosphorus are recycled.
  • the flow of nutrients within an ecosystem isn’t linear, but mostly cyclic.
• ALL NUTRIENT CYCLES HAVE ONE SIMPLE SEQUENCE:

  1. the nutrients is taken up by producers (plants) as simple, inorganic molecules.
  2. the producer incorporates the nutrient into complex organic molecules
  3. when the producer is eaten, the nutrient passes into consumers (animals)
  4. it then passes along the food chain when these animals are eaten by other consumers
  5. when the decomposers and consumers die, their complex molecules are broken down by Saprobiontic microorganisms (decomposers) that release the nutrient in its original simple form
• SAPROBIONTS (DECOMPOSERS): these are microorganisms such as bacteria and fungi that feed on the remains of dead plants and animals (and their faeces and urine), breaking them down, digesting them externally.
• HOW DO SAPROBIONTS BREAK DOWN WASTE?

  • feed on remains of dead plants / animals and their waste products eg. Faeces, urea and break down the organic molecules
  • by secreting enzymes for extra cellular digestion
  • saprobionts absorb soluble needed nutrients
• IMPORTANCE OF SAPROBIONTS

  • they are in many ways the driving force that ensure that nutrients are released for reuse
  • without them nutrients would remain locked up as part of complex molecules that cannot be taken up and used again by plants
• ROLE OF MYCORRHIZAE

  • fungi act like extensions of the plants root system made of thin strands called hyphae
  • vastly increase total surface area for the absorption of water and minerals
  • the mycorrhizae act like a sponge and so holds water and minerals in the neighbourhood of the roots
  • this enables the plant to better resist drought and to take up inorganic ions more readily
  • the mycorrhizae plays a part in nutrient cycles by improving the uptake of relatively scarce ions such as phosphate ions

  MYCORRHIZAE ARE SYMBIOTIC (MUTUALISTIC) ASSOCIATIONS BETWEEN FUNGAL HYPHAE AND PLANT ROOTS.
• Symbiotic relationship between fungi and roots of plants = mycorrhizae

  • mutualistic relationship
  • the plant benefits from improved water and inorganic ion uptake while the fungus receives organic compounds such as sugars and amino acids from the plant
• WHY IS THE NITROGEN CYCLE NEEDED?

  needed to create:

  • amino acids / proteins
  • nucleic acids (DNA, RNA)
  • ATP
• NITROGEN CYCLE

  • 78% of the atmosphere is nitrogen
  • plants take up most of the nitrogen they require as nitrate ions (NO$^-3$), from the soil.
  • these ions are absorbed, using active transport by the roots.
  • animals obtain nitrogen-containing compounds by eating and digesting plants
  • this release of nitrate ions by decomposition is most important because, in natural ecosystems, there are very few nitrate ions available from other sources.
• NITROGEN CYCLE 


  Natural ions are very soluble and easily leach (wash) through the soil beyond the reach of plant roots

  • in natural ecosystems, the nitrate concentrations are restored by the recycling of nitrogen containing compounds
  • in agricultural ecosystems, the concentration of soil nitrate can be further increased by the addition of fertilisers
  • Nitrogen also enters the ecosystem by lightning which fixes atmospheric nitrogen.
• THE FOUR NITROGEN CYCLE PROCESSES:

  • nitrogen-fixation
  • nitrification
  • denitrification
  • ammonification
• NITROGEN-FIXATION

  • nitrogen is converted (reduced) into nitrigen containing compounds (ammonia)
  • it can be carried out industrially and also occurs naturally when lightning passes through the atmosphere.
  • by far most important form of nitrogen fixation is carried out by microorganisms (nitrogen-fixing bacteria) which can break the triple bind between the 2 nitrogen atoms using Nitrogenase enzymes and fix the nitrogen ammonium ions. 2 main types:
  • free-living nitrogen-fixing bacteria
  • mutualistic (symbiotic) nitrogen fixing bacteria
• TWO TYPES OF NITROGEN FIXING BACTERIA

  • FREE LIVING NITROGEN FIXING BACTERIA: these bacteria reduce gaseous nitrogen to ammonia, which they can use to manufacture amino acids. Nitrogen-rich compounds are released from them when they die and decay.
  • MUTUALISTIC (SYMBIOTIC) NITROGEN-FIXING BACTERIA: these bacteria live in nodules on the roots of (leguminous) plants such as peas and beans. They obtain carbohydrates from the plant and the plant carries amino acids from the bacteria.
• AMMONIFICTION

  ORGANIC NITROGEN CONTAINING COMPOUNDS CONVERTED TO AMMONIA / AMMONIUM IONS

  1. nitrogen containing compounds include: urea, proteins, nucleic acids and vitamins found in faeces and dead organisms
  2. Saprobionts (fungi and bacteria) carry out the decomposition of these dead matter and waste products
  3. they secrete enzymes for extracellular digestion and then feed on faeces and dead organisms materials
  4. this releases ammonia, which then forms ammonium ions in the soil (saprobiontic nutrition)
  5. this is where nitrogen returns to the non-living components of the ecosystem
• NITROGEN CONTAINING COMPOUNDS:

  • urea
  • proteins
  • nucleic acid
  • chlorophyll
  • vitamins found in faeces and dead organisms
• NITRIFICATION

  CONVERSION OF AMMONIUM IONS TO NITRITE IONS TO NITRATE IONS

  • this is an oxidation reaction so releases energy
  • requires free-living nitrifying bacteria

  CONVERSION OCCURS IN TWO STAGES:

  1. oxidation of ammonium ions to nitrite ions NO2-
  2. oxidation of nitrite ions to nitrate ions NO3-

  • then nitrates (nutrients) can be absorbed by plant root hair cells by active transport

  Nitrifying bacteria
• NITRIFYING BACTERIA (NITRIFICATION)


  Nitrifying bacteria require oxygen to carry out these conversions and so they require a soil that has many air spaces.

  • to raise productivity, it’s important for farmers to keep soil strcytyre light and well aerated by ploughing
  • good drainage also prevents the air spaces from being filled with water and so prevents air being forced out of the soil
• DENITRIFICATION

  NITRATES IN THE SOIL ARE CONVERTED TO NITROGEN GAS

  • when soils become waterlogged = have a low oxygen concentration, type of microorganism present changes
  • fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and theres an increase in anaerobic denitrifying bacteria
  • denitrifying bacteria anaerobically respire
  • these convert soil nitrates into gaseous nitrogen

  WHY IS THIS BAD

  reduces availability of nitrogen-containing compounds for plants

  • for land to be productive, soil needs to be well aerated for crops to prevent build up of denitrifying bacteria
• PHOSPHORUS CYCLE

  • phosphorus is required for the production of nucleic acids (DNA and RNA), ATP and phospholipids
  • found in rocks and dissolved in oceans in form of phosphate ions PO43-

  1. Brought to surface by geological uplifting of rocks
  2. weathering and erosion of these rocks helps phosphate ions to become dissolved and so available for absorption by plants which incorporate them into their biomass

  • phosphate ions dissolved in water in soil can be assimilated (absorbed and used to make more complex molecules by plants / producers)
  • phosphorus cycle lacks a gaseous phase
• PHOSPHORUS CYCLE: 1

  1. phosphate ions in rocks are released into the soil or into sea / rivers by weathering / erosion
  2. plants and aquatic plants take in phosphate ions through roots and incorporate them into their biomass
  3. this is enhanced by mycorrhizae which increase the rate of assimilation
  4. phosphate ions are transferred through the food chain as animals eat plants and are then eaten by other animals
  5. excess phosphate ions are lost from animals in their waste products to the soil
  6. sea bird waste is guano which contains a high concentration of phosphate ions which goes to the soil
• PHOSPHORUS CYCLE: 2

  7. When plants and animals die / release waste products (urine, faeces) saprobionts break down the organic compounds by extracellular digestion

  8. The phosphate ions are released into the soil for assimilation by plants

  9. some phosphate ions remain in parts of animals, such as bones or shells, that are very slow to break down

  10. Phosphate ions are released By decomposition and out of rocks are transported by streams / rivers where they form sedimentary rocks
• NEED FOR FERTILISERS

  NITRATES AND PHOSPHATES ARE LOST FROM THE SYSTEM WHEN:

  1. CROPS ARE HARVESTED

  • crops take in minerals from the soil as they grow and use them to build their own tissues
  • when crops are harvested, they’re removed form the field rather than dying and decomposing there
  • this means the mineral ions (phosphates / nitrates) aren’t returned to the soil by Saprobionts

  2. ANIMALS ARE MOVED

  • they eat grass and other plants which takes in their nutrients so removing them elsewhere or for slaughter, the nutrients aren’t replaced through remains and waste products
• NEED FOR FERTILISERS

  WHAT FERTILISERS DO

  • necessary to replenish these mineral ions because otherwise their reduced concentrations will become the main limiting factor to plant growth

  1. Resulting in reduced productivity
  2. to offset this loss of mineral ions, fertilisers need to be added to the soil

  • fertilisers improve the efficiency of energy transfer (more energy can be used for growth)
  • increase productivity
• WHAT IS NATURAL / ORGANIC FERTILISER?

  Consist of the dead and decaying remains of plants and animals as well as animal wastes
• WHAT TYPE OF FERTILISER IS NATURAL FERTILISER?

  organic
• EXAMPLES OF NATURAL FERTILISER

  • manure
  • compost
  • slurry
  • bone meal
• ADVANTAGES OF NATURAL FERTILISER

  • less soluble than artificial fertilisers, so the minerals are released more slowly as they’re decomposed which prevents leaching and means they last longer
  • cheap as organic wastes need to be disposed of
  • improves soil structure by binding soil particles together and provides food for soil organisms such as earthworms which improves drainage and aeration
• DISADVANTAGES OF NATURAL FERTILISER


  • exact nutrients can’t be controlled
  • less concentrated in minerals than artificial fertilisers, so more needs to be spread on a field to have a similar effect
  • may contain unwanted substances eg. Weed seeds, fungal spores, heavy metals
• WHAT TYPE OF FERTILISER IS ARTIFICIAL FERTILISER?

  inorganic
• WHAT IS ARTIFICIAL FERTILISER?

  mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop
• EXAMPLES OF ARTIFICIAL FERTILISER

  • NPK
  • pure chemicals eg. Ammonium nitrate as powders / pellets
• ADVANTAGES OF ARTIFICIAL FERTILISER

  • can control concentration of each nutrient
  • increase crop yield
• DISADVANTAGES OF ARTIFICIAL FERTILISERS

  • inorganic substances more water soluble so larger quantities leach which leads to eutrophication
  • expensive
• HOW FERTILISERS INCREASE PRODUCTIVITY

  PLANTS REQUIRE MINERALS FOR THEIR GROWTH

  • nitrogen is an essential component of amino acids, ATP and nucleotides in DNA - which are needed for plant growth
  • where nitrate ions are readily available, plants are likely to develop earlier, grow taller and have a greater leaf area - this increases rate of photosynthesis and improves crop productivity
• HOW FERTILISERS INCREASE PRODUCTIVITY

  • research suggests that combination of natural and artificial fertilisers gives the greatest long term increase in productivity
  • however, it’s important that minerals are added in appropriate quantities as there’s a point at which further increases in the quantity of fertiliser no longer results in increased productivity
• EFFECTS OF FERTILISER

  THE USE IF NITROGEN-CONTAINING FERTILISERS HAS SOME DETRIMENTAL EFFECTS:

  • REDUCED SPECIES DIVERSITY: because nitrogen-rich soils favour the growth of grasses, nettles and other rapidly growing species. These outcompete many other species, which die as a result.
  • using fertilisers too much changes the balance of the nutrients in the soil, so too much of a particular nutrient can cause crops and other plants to die
  • LEACHING: may lead to pollution of watercourses
  • EUTROPHICATION: caused by leaching of fertiliser into watercourses