decay

• Decomposition, often called 'rotting', is the process by which dead organic matter is broken down into simpler organic or inorganic substances, such as carbon dioxide, water, simple sugars, and minerals.

  Decomposition is essential for the recycling of elements throughout an ecosystem.
• when leaves fall from a tree and are slowly broken down in the soil, we say that they are decomposing. This recycles the carbon, nitrogen, magnesium etc. and makes them available for the plants to absorb and use again.
• Detritus feeders and decomposers

  
  

  Decomposition is carried out by two groups of organisms: detritus feeders (detritivores), and decomposers.

  
  

  Detritus feeders - small animals like worms and woodlice that feed on dead organic matter.

  Decomposers - microorganisms, such as bacteria and fungi
• The rate of decomposition is affected by 3 environmental conditions:

  • Oxygen availability
  • temperature
  • water content
• Oxygen availability

  Higher oxygen ➔ more aerobic respiration ➔ more energy available ➔ faster growth and decomposition 
• Temperature 

  Higher temperature ➔ particles have more kinetic energy & enzymes are closer to optimum temperature ➔ higher rate of reactions ➔ more decomposition 

  However, if temperature rises too high, it will denature enzymes, decreasing the rate of reaction and therefore decreasing the rate of decomposition.
• Water content 

  Decomposers need water to survive, so the rate of decomposition increases in moist conditions. 

  However, if the soil becomes waterlogged then the oxygen levels will fall, which will decrease the rate of decomposition. 
• Compost 

  Compost is a mixture of decayed organic material that is used to fertilise and improve the soil. It is commonly prepared by leaving dead plant waste in ideal conditions (plenty of oxygen, warm, moist) until it is mostly decomposed. It can then be spread across soil, where it provides nutrients for the plants. 
• The image shows how the mixture changes over time as it decomposes. At first there are whole pieces of food, but over time it breaks into smaller pieces, and in the end starts to look like soil.

  
• Biogas generators

  • Some decomposing microorganisms can break down organic material without oxygen. In these cases, the microorganisms carry out anaerobic respiration and so it is called 'anaerobic decay'.
  • The mixture of gases this produces is known as 'biogas', and the most important of the gases is methane.
  • When methane is combusted (burned), it releases a lot of energy, which can be used for cooking, heating, or generating electricity. 

  
• • Biogas generators are large containers in which animal or plant waste is left to decay anaerobically. Importantly, there cannot be any oxygen in the container. If there is oxygen, the microorganisms will respire aerobically and won't produce as much methane.
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  • Once the biogas is removed from the tank, a sludgy material is left over, which contains a high density of useful minerals. This can be spread on fields like fertiliser.