Nutrient cycles

nutrient cycles

• In a functioning ecosystem the elements that living organisms need are constantly recycled

  • These elements are incorporated into biological molecules within the tissues of living organisms, and then released back into the environment when decomposers break down dead or waste matter

• Examples of nutrient cycles are:

  • the nitrogen cycle

  • the phosphorous cycle

the nitrogen cycle

Nitrogen is present as N₂ gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms

• nitrogen fixation

• ammonification

• nitrification

• denitrification

the nitrogen cycle - nitrogen fixation

• N₂ gas is converted into ammonium compounds by nitrogen-fixing bacteria; these bacteria can be free-living in the soil, or may live within root nodules of legume plants

• Ammonium compounds are converted into nitrates, which are then absorbed by plants and used to build plant proteins

the nitrogen cycle - ammonification

• Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of saprobionts such as bacteria and fungi

• This ammonia forms ammonium ions in the soil

the nitrogen cycle - nitrification

• The ammonium ions in the soil are converted into nitrates by nitrifying bacteria

  • Nitrosomonas bacteria convert ammonium ions into nitrites

  • Nitrobacter bacteria then convert nitrites into nitrates

the nitrogen cycle - denitrification

• Denitrifying bacteria use nitrates in the soil for respiration

  • This occurs in anaerobic conditions, such as in waterlogged soil

• This process produces nitrogen gas, which returns to the atmosphere

the phosphorus cycle

• Plants and animals require phosphorus for production of, e.g.:

  • phospholipids

  • nucleic acids (DNA and RNA)

  • ATP

• The phosphorus cycle includes the following processes:

  • phosphorus in rocks is released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) due to weathering

  • Phosphate ions are taken up from the soil by plants, or absorbed from water by algae

  • Phosphate ions are transferred to consumers during feeding

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

  • The phosphate ions can be taken up and used again by producers, or may be trapped in sediments that may turn into phosphorus-containing rock once again

what are saprobionts

organisms that decompose dead and waste organic matter

what do saprobionts do

• Examples include fungi and bacteria

• Decomposition by saprobionts releases chemical elements, such as phosphorus and nitrogen, from within dead and waste material

• The process of decomposition by saprobionts is as follows:

  1. enzymes are secreted onto a food source

  2. extracellular digestion occurs

  3. nutrients are absorbed

• Importantly, not all of the products of extracellular digestion are absorbed by saprobionts; some mineral ions remain in the soil where they can be absorbed by plants

what are mycorrhizae

symbiotic relationships between fungi and the roots of plants

what do mycorrhizae do

• Fungi form long, thin filaments known as hyphae, which connect with plant roots

• The hyphae effectively increase the surface area of the root systems of the plants, increasing absorption of water and inorganic ions

• In return the fungi receive carbon compounds, e.g. glucose, from the plant

role in mineral cycling - saprobionts

Decompose dead and waste matter via extracellular digestion, making inorganic ions available to other organisms
Carry out ammonification by converting nitrogen compounds in waste and dead matter into ammonia

role in mineral cycling - nitrogen fixing bacteria

Convert atmospheric nitrogen gas into nitrogen-containing compounds, such as ammonia

role in mineral cycling - nitrifying bacteria

Convert ammonium ions in soil into nitrates then nitrites

role in mineral cycling - denitrifying bacteria

Convert nitrates into nitrogen gas

role in mineral cycling - mycorrhizal fungi

Increase surface area of root systems, helping plants to absorb water and mineral ions from soil

fertilisers

• In natural ecosystems, decomposition by saprobionts recycles nutrients from waste and dead organisms back into the soil

• In agricultural ecosystems, crops and livestock are harvested and removed, so the nutrients contained in their biomass are not returned to the soil

  • This disrupts nutrient cycles and can lead to reduced soil fertility

• To maintain productivity, fertilisers are used to replace lost mineral ions

natural fertilisers

• Natural fertilisers are made from organic matter, such as:

  • manure

  • compost

  • crop residues

  • sewage

advantages of natural fertilisers

• Release nutrients slowly over time, so nutrients are less likely to be washed away into rivers and lakes after rain

• Contain organic matter, so can improve soil structure and water retention

disadvantages of natural fertilisers

• Contain organic matter, so can improve soil structure and water retention

• Nutrient content is variable and harder to control

artificial fertilisers

• Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain chemical compounds, e.g. ammonium nitrate

advantages of artificial fertilisers

• Nutrients are concentrated and easy to apply

• Precise nutrient content allows controlled dosing

disadvantages of artificial fertilisers

• Are highly soluble in water so can be leached out of soil into rivers and lakes when it rains

• Do not improve soil structure

environmental issues caused by fertilisers

• Fertilisers are often applied in larger quantities than crops require; as a result, excess mineral ions may remain dissolved in the soil water

• These mineral ions can be carried by rainwater into nearby rivers, lakes or streams; this is known as leaching

  • Leaching is more likely to occur:

    • after heavy rainfall

    • when using artificial fertilisers, as the inorganic ions are highly soluble

  • Leaching is less likely to occur with natural fertilisers as organic matter must be decomposed by microorganisms before minerals become water-soluble

environmental issues caused by fertilisers - eutrophication

• Leaching can lead to a process called eutrophication, which occurs as follows:

  1. mineral ions enter water bodies, causing rapid growth of algae at the surface

     • This is known as an algal bloom

  2. algae block sunlight, which prevents aquatic plants below the surface from photosynthesising

  3. these plants, and eventually the algae, begin to die and dead organic matter accumulates

  4. bacteria decompose the dead matter, respiring aerobically and using up the oxygen dissolved in the water

  5. oxygen levels fall and aquatic animals such as fish and insects can no longer survive