Temperature changes decomposition rate

1. Decomposition depends on enzyme-controlled reactions in bacteria and fungi, so environmental conditions affect how quickly dead material is broken down.
2. At low temperatures, enzyme and substrate molecules have less kinetic energy and fewer successful collisions occur each second.
3. Digestion, respiration and microbial growth are slower, so decomposition proceeds slowly.
4. As temperature increases towards a suitable optimum, molecules move faster and successful collisions become more frequent.
5. More enzyme-substrate complexes form per second, increasing the rate of reactions involved in decomposition.
6. Above a suitable temperature range, enzymes can denature, changing the shape of their active sites so substrates no longer fit effectively.
7. High temperatures can also kill microbial cells, reducing the number of decomposers able to act on the material.
8. Different microorganisms have different temperature tolerances, so a mixed decomposer community does not have one universal optimum.

Moisture and oxygen support decay

1. Microorganisms need water for cellular reactions and for dissolved substances to move to and from their cells.
2. In dry material, fewer substances can dissolve and microbial growth is restricted, so decomposition is slower.
3. Adding water to dry material can therefore increase decomposition when other conditions are suitable.
4. Aerobic conditions contain oxygen, allowing decomposers to release energy through aerobic respiration.
5. This energy supports cellular processes and growth, allowing a larger active microbial population to break down material.
6. When material becomes waterlogged, water fills the air spaces and the supply of oxygen can become limited.
7. Oxygen moves through water much more slowly than through air, so it may be used faster than it is replaced.
8. Anaerobic conditions lack oxygen, although microorganisms capable of functioning without oxygen may still decompose material.
9. Decomposition is often slower in oxygen-poor conditions, so partially decomposed material can accumulate.
10. Increasing moisture therefore does not always increase decay: very dry conditions restrict activity, but waterlogging can restrict oxygen.

Decay can be encouraged or slowed

1. A compost heap decomposes plant material efficiently when it remains moist, has a suitable temperature and receives enough oxygen.
2. Turning or mixing compost admits air, while avoiding excessive watering helps keep air spaces open.
3. Respiration by decomposers releases energy that warms the heap, although excessive heating can reduce the activity of some microorganisms.
4. Compost supplies organic material and mineral nutrients to soil as decomposition continues.
5. Refrigeration slows microbial reactions, while freezing also reduces the availability of liquid water.
6. Freezing does not reliably kill every microorganism, so microbial activity can increase again when food thaws.
7. Drying reduces available water and therefore slows the reactions and growth needed for decay.
8. Removing oxygen slows aerobic decay, but does not prevent every form of decomposition because some microorganisms can function anaerobically.
9. In canning, heating kills microorganisms and subsequent sealing prevents new microorganisms entering; sealing alone does not make contaminated food safe.

Microbial activity can change milk acidity

1. Some bacteria use the sugar in milk and produce lactic acid, causing the milk’s $\mathrm{pH}$ to decrease.
2. A faster decrease in $\mathrm{pH}$ over a fixed interval can indicate faster microbial activity when the starting conditions are controlled.
3. This is an indirect indicator of decomposition and does not measure dry mass loss.
4. Because $\mathrm{pH}$ is a logarithmic scale, equal changes in $\mathrm{pH}$ do not represent equal amounts of acid produced.

Calculate rates and percentage loss

1. Use the same time interval when comparing decomposition rates, and use dry mass when changing water content could affect the result.
2. Calculate the loss using $mass\; lost=initial\; dry\; mass-final\; dry\; mass$.
3. The mean rate is $mean\; rate\; of\; mass\; loss=\frac{mass\; lost}{time\; taken}$.
4. If mass is measured in grams and time in days, report the rate in $\mathrm{g}\mathrm{da}{\mathrm{y}}^{-1}$.
5. To compare samples with different starting masses, calculate $percentage\; mass\; lost=\frac{mass\; lost}{initial\; dry\; mass}\times 100\%$.
6. A mean rate over a whole investigation may hide changes during that period, so compare shorter intervals when the data allow this.

Interpret decomposition results

1. On a graph of mass remaining against time, a more steeply falling line indicates faster mass loss.
2. On a graph of mass lost against time, a more steeply rising line indicates faster mass loss.
3. A curve that becomes less steep shows that the rate is slowing, which may happen as the more easily decomposed material is used up.
4. Use repeat measurements to calculate a mean and inspect their spread before deciding that a small difference between treatments is convincing.
5. Check unusual results against the method and repeat the measurement where possible rather than deleting them simply because they do not match the expected pattern.
6. A conclusion should identify the observed relationship and support it with comparable values, followed by the biological explanation that fits the tested conditions.