7.7 Decay

Decomposition

  • ‘rotting’

  • Process by which dead organic matter breaks down into simpler organic or inorganic substances (CO2 / water / minerals)

  • Decomposition essential for recycling of elements throughout an ecosystem

    • example: leaves fall from trees & slowly break down in soil (decompose)

      • this recycles carbon, nitrogen, magnesium, etc.. - making them available for plants to absorb & use again


Detritus feeders & decomposers

  • decomposition carried out by 2 groups of organisms: detritus feeders & decomposers

  • Detritus feeders: small animals (worms / woodlice) that feed on dead organic matter

  • Decomposers: microorganisms (bacteria / fungi)


Factors effecting rate of decomposition

  1. Oxygen availability

    • more oxygen → more aerobic respiration → more energy available → faster growth & decomposition

  2. Temperature

    • higher temp → particles have more kinetic energy & enzymes closer to optimum temp → higher rate of reactions → more decomposition

    • However if temperature rises too high, enzymes denature (decreasing rate of reaction) & so decreases rate of decomposition

  3. Water content

    • decomposers need water to survive - so rate of decomposition increases in moist conditions

    • However if soil becomes waterlogged - oxygen levels fall, decreasing rate of decomposition


Compost

  • mixture of decayed organic material used to fertilise & improve soil

  • Prepared by leaving dead plant waste in ideal conditions (plenty oxygen, warm, moist) until mostly decomposed

  • Spread across soil, providing nutrients for plants


Biogas generators

  • some decomposing microorganisms break down organic material without oxygen

    • they carry out anaerobic respiration - so is called ‘anaerobic decay’

  • The mixture of gases produced by anaerobic decay is known as ‘biogas’ - most important of the gases is methane

  • When methane is combusted (burned) it releases lots of energy (used for cooking / heating / generating electricity)

  • Biogas generators are large containers which animal and plant waste is left to decay anaerobically

    • oxygen cannot be in the container - otherwise microorganisms respire aerobically (less methane produced)

  • Once biogas removed from tank, sludgy material is left over (containing high density of useful minerals)

    • Spread on fields like fertiliser