Decomposition of grass cuttings: results

The graph clearly shows that the temperature rises (bottom graph) and the oxygen level drops in a sealed bag of grass cuttings. As the oxygen level dropped, the rate of temperature rise slowed, so the bag was opened and the grass stirred to introduce more oxygen. The bag was then left open to the air. This caused a brief spike in temperature, which then dropped and thereafter continued to slowly rise.

Answers to questions

1. Without decay, waste organic matter would build up. In addition, there would be no recycling of nutrients and minerals. Eventually organic life would come to a halt.
2. Microorganisms release enzymes that break up the plant compounds, so the microorganism cells can absorb them.
3. The two main groups of microorganisms that cause decay are the “decomposers”, bacteria and fungi.
4. Microorganisms require warmth, oxygen and water to cause rapid decay.
5. Respiration is the process by which living organisms take in oxygen and release carbon dioxide.
6. In the experiment, the humidity may drop or rise initially according to the moisture content of the grass content. Wet grass releases water vapour into the atmosphere above it. Microorganisms require water to function so will take water from their surroundings. Microorganisms start to respire more as they break down the grass, releasing water vapour. Rising temperature will also cause more of the water in the grass to become a gas.
7. If the oxygen level drops, microorganisms will switch from aerobic decay (with air), to anaerobic decay (without air). Anaerobic decay is much slower and does not generate sufficient energy to heat up the plant material. Compost undergoing anaerobic decay will be wet and smell unpleasant.

Teachers’ notes

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among systems of the Earth. Together with the nitrogen cycle and the water cycle, it is essential to ensure the Earth is capable of supporting life. It describes the movement of carbon as it is recycled and reused throughout the biosphere.

Because the DrDAQ measures oxygen content and temperature to 0.1% and °C, trends are seen quickly and this experiment may be used within a lesson to demonstrate that decomposing plants consume oxygen and give out energy in the form of heat. Ensure that all plant material is at room temperature before you start, or the temperature may drop initially. It is best to carry this experiment out in warmer weather, or the rate of decay may be very slow. Alternatively, it could be used as the basis of a range of projects investigating decay for Key Stages 3 to 5. It can be carried out simply with just the temperature sensor. It could be extended to investigate the best conditions for making compost, using temperature rise as a measure of microorganism activity.

This type of decomposition is quite sensitive to moisture levels of the plant material. If the grass cuttings are collected when too wet, the experiment may not work. Higher temperature rises can be obtained from a sack of fresh grass cuttings left open to the air (make sure they are not compacted).

Credit

This experiment was written by Susan Hammond of Woking College.