Nutrient Cycles

• Nutrient Cycles:

  • elements that are important to organisms are constantly being recycled between biotic and abiotic phases
  • unlike energy, nutrient cycle between the biotic and abiotic components of ecosystems within which they exist as organic and inorganic compound. In most nutrient cycles;
  • the nutrients in a simple, inorganic form are taken up by producers (plants/algae)
  • the producers use the nutrients to synthesise more complex organic molecules
  • they are then passed along the food chains as producers and animals are eaten by other consumers
  • when producers and consumers die, they are decomposed by saprobionts and the nutrients are eventually returned to their original inorganic form to complete the cycle
• The Nitrogen Cycle:

  • living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogen-containing compounds
  • although 78% of the atmosphere is nitrogen, most organisms cannot use nitrogen gas directly
  • plant roots absorb most of the nitrogen they require in the form of nitrates from the soil, using active transport
  • this is where nitrogen enters the living component of the ecosystem. Animals then obtain nitrogen-containing compounds by eating and digesting plants
• The nitrogen cycle:

  there are 4 main stages to the nitrogen cycle:

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

  • nitrogen gas, $N_2$ is converted (by reduction) into ammonium compounds
  • this is carried out by nitrogen fixing bacteria
  • they contain an enzyme called nitrogenase
  • these bacteria can live freely in the soil or can live in root nodules of plants called legumes
  • the bacterium receives water and sugars from the plant and in return supplies the plant with ammonia (a symbiotic relationship)
  • ammonium ions can then be absorbed and assimilated by plants, used to make proteins, nucleotides and other nitrogen-containing compounds
• Ammonification:

  • when organisms die (or excrete urea or produce faeces), the nitrogen-containing compounds (proteins and nucleic acids) within them are converted into ammonia by saprobionts
  • ammonia forms ammonium ions within the soil
  • saprobionts are decomposers (microorganisms) that break down the remains of dead organisms and waste products using extracellular digestion
• Nitrification:

  • ammonium ions in the soil are oxidised into nitrites, and then these are further oxidised into nitrates
  • this is carried out by nitrifying bacteria
  • plants can then absorb nitrates from the soil through their roots by active transport
  • nitrifying bacteria require oxygen, so they require a soil that has many air spaces
  • to raise productivity, it is important for farmers to keep soil well aerated by ploughing/raking. Good drainage also prevents the air spaces from being filled with water and so prevents air being forced out of the soil
• Denitrification:

  • when soils become waterlogged and have low oxygen concentration, fewer aerobic nitrifying and nitrogen fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria
  • these convert soil nitrates into gaseous nitrogen through reduction reactions, This reduces the availability of nitrates for plants
• The phosphorus cycle:

  • phosphorus is an important biological element as it is a component of ATP, phospholipids and nucleic acids
  • unlike the nitrogen cycle, in the phosphorus cycle the main reservoir is in mineral form rather than in the atmosphere - the phosphorus cycle lacks a gaseous phase altogether
• The phosphorus cycle:

  • phosphorus exists mostly as phosphate ions in the form of sedimentary rock deposits
  • the weathering and erosion of these rocks releases phosphate ions into the soil. These can be absorbed by plants from their roots , helped by mycorrhizae
  • weathering of rocks also releases phosphate ions into the sea and so available for absorption by aquatic producers, such as algae, which incorporate them into their biomass (assimilation)
• The phosphorus cycle:

  • the phosphate ions pass into animals which eat the algae/plants and up through the food chain
  • large amounts of phosphate ions are returned to soils by excretion of guano by sea birds
  • on the death of plants and animals, saprobionts decompose them, releasing phosphate ions into the water or soil. Some phosphate ions remain in parts of animals, such as bones or shells, that are very slow to breakdown
  • phosphate ions released by decomposition and dissolved out of rocks are transported by streams and rivers into lakes and oceans where they form sedimentary rocks, thus completing the cycle
• Mycorrhizae:

  • mycorrhizae are associations between certain types of fungi and the roots of many plant. The fungal hyphae (long thin strands protruding from their cells) act like extensions of the plant's root system and vastly increases the total surface area for the absorption of water and minerals
  • the mycorrhizae act like a sponge and so hold water and minerals close to the roots. This enables the plant to better resist drought and to take up inorganic ions more readily. The mycorrhizae improve the uptake of relatively scarce ions such as phosphate ions
• Mycorrhizae:

  • the mycorrhizal relationship between plants and fungi is symbiotic - 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
• Fertilisers:

  • intensive food production makes large demands on the soil because mineral ions are continually taken up by the crops being grown on it
  • in natural ecosystems, the minerals that are removed from the soil by plants are returned when the plant is decomposed by microorganisms on its death
  • in agricultural systems, the crop is harvested and then transported from its point of origin for consumption. The urine, faeces and dead remains of the consumers are rarely returned to the same area of land. Under these conditions the concentrations of mineral ions in agricultural land will fall
• Fertilisers:

  • it is therefore necessary to replenish these mineral ions otherwise their reduced concentrations will become the main limiting factor to plant growth. Productivity will consequently be reduced
  • to reduce this loss of mineral ions, fertilisers need to be added to the soil

  there are 2 types:

  • natural (organic) fertilisers, which consist of the dead and decaying remains of plants and animals as well as animal wastes such as manure, slurry and bone meal
  • artificial (inorganic) fertilisers, which are mined from rocks and deposits and then converted into different forms and blended to give the appropriate balance of minerals for a particular crop. Compounds containing the three elements; nitrogen, phosphorus and potassium are almost always present
• Fertilisers:

  • nitrogen is an essential component of biological molecules such as proteins and is needed for growth and, therefore, an increase in the surface area of leave. This increases the rate of photosynthesis and increases crop productivity

  the use of nitrogen-containing fertilisers also 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
  • leaching, which may lead to pollution of watercourses
  • eutrophication, caused by leaching of fertiliser into watercourses
• Leaching:

  • this is the process by which nutrients are removed from soil
  • rainwater will dissolve any soluble nutrients, such as nitrate ions, and carry them deep into the soil, eventually beyond the reach of plant roots
  • the leached nitrate ions move into watercourses, such as streams and rivers, that in turn may drain into freshwater lakes
  • the leached nitrate ions are also harmful to the environment as they can cause eutrophication
• Eutrophication:

  • this is the process by which nutrient concentrations increase in bodies in water
  • in most lakes and rivers, there is naturally very little nitrate, meaning nitrate is a limiting factor for plant and algal growth
  • once nitrate leaches into the watercourse, it ceases to be a limiting factors and plants and algae grow exponentially
• Eutrophication process:

  • fertilisers containing nitrate/phosphate leach into lakes
  • an 'algal bloom' occurs (rapid algal growth) - a dense layer of algae forms on the surface of the water, absorbing light and preventing it form reaching lower depths
  • aquatic plants below the surface cannot absorb light and therefore cannot photosynthesise and die
  • when the plant die, they are decomposed by saprobionts, whose populations suddenly increase
  • the increase in aerobic respiration by saprobionts uses up oxygen
  • the dissolved oxygen in the water becomes depleted, causing many fish to die because they do not have enough oxygen to respire aerobically
• Eutrophication:

  • without the aerobic organisms, there is less competition for the anaerobic organisms, whose populations now rise
  • the anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes