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7.2.3 Decomposition

Decomposition

Definition

Decomposition

The breakdown of dead organisms and waste by microorganisms, which releases nutrients back into the soil and air.

  1. Decomposition is the breakdown of dead organisms and waste by microorganisms.
  2. It is carried out mainly by bacteria and fungi, which are called decomposers.
  3. They release enzymes onto the dead material and then absorb the digested products.
  4. This returns carbon, nitrogen and mineral ions to the soil and air for producers to use again.

Factors that affect the rate of decay

  1. Temperature: warmth speeds up decay because it raises the activity of the decomposers' enzymes, but too high a temperature denatures the enzymes and decay slows or stops.
  2. Water: moist conditions speed up decay because decomposers need water to live and to digest, while dry material decays very slowly.
  3. Oxygen: most decomposers respire aerobically, so a good supply of oxygen speeds up decay.
Key Idea

Decay is fastest when it is warm, moist and has plenty of oxygen, because these conditions suit the decomposers best.

Calculating the rate of decay

  1. The rate of decay tells you how fast the material is breaking down.
  2. Work it out by dividing the change in the measurement by the time taken: rate=change in measurementtime taken\text{rate} = \dfrac{\text{change in measurement}}{\text{time taken}}rate=time takenchange in measurement​.
  3. For example, if a compost heap loses 60 g of mass in 30 days, the rate of decay is 6030=2\dfrac{60}{30} = 23060​=2 grams per day.
Practical

Required practical: temperature and the rate of decay of milk

  • Aim: to investigate how temperature affects the rate at which fresh milk decays.
  • Add the same volume of fresh milk and a fixed volume of the alkali sodium carbonate solution to a tube, with a few drops of phenolphthalein so the mixture starts pink.
  • Add lipase and start a stopwatch, because the lipase breaks the fats in the milk into fatty acids, which lower the pH.
  • Time how long it takes for the falling pH to turn the pink indicator colourless, which marks a set drop in pH.
  • Repeat at a range of temperatures set with a water bath, such as 10, 20, 30, 40 and 50 degrees Celsius.
  • The independent variable is the temperature and the dependent variable is the time for the colour to change; keep the volumes of milk, alkali and lipase the same.
  • A shorter time means a faster rate of decay, so calculate the rate as one divided by the time taken.
  • Repeat each temperature and take a mean to make the results more reliable.

Compost as a natural fertiliser

  1. Gardeners and farmers create the best conditions for rapid decay to make compost.
  2. A compost heap is kept warm, moist and well aerated, so the decomposers work quickly.
  3. The compost is used as a natural fertiliser, returning mineral ions to the soil for crops and garden plants.

Anaerobic decay and biogas

Definition

Biogas

A fuel, mainly methane, made by the anaerobic decay of waste material such as animal dung, sewage or plant matter.

  1. Where there is no oxygen, decay is anaerobic and produces methane gas.
  2. Biogas generators use the anaerobic decay of waste to produce methane, which is burned as a fuel.
  3. The waste can be animal dung, sewage or plant material, giving a renewable source of energy.
Self review
  • Name the three factors that affect the rate of decay.
  • Explain why decay slows down at very high temperatures.
  • A sample loses 40 g in 8 days; calculate the rate of decay.
  • Why do gardeners keep a compost heap moist and well aerated?
  • What gas is produced by anaerobic decay in a biogas generator?
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7.2.3 Decomposition (biology only) Revision Guide

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