Three factors affect the rate of decomposition: temperature, water availability and oxygen levels. Free GCSE AQA notes with key terms and 11 questions answered.
Updated 20 Mar 2026 · Revise Right Team
Factors Affecting Decomposition and Rate Calculations is part of Organisation of an ecosystem in GCSE AQA Higher Biology.
3 min
Reading time
36
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GCSE AQA Higher
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Reviewed by Revise Right
Content review
What this lesson covers
Three factors affect the rate of decomposition: temperature, water availability and oxygen levels. Free GCSE AQA notes with key terms and 11 questions answered.
Explain how temperature, water and oxygen availability affect the rate of decay of biological material
Calculate rate changes in the decay of biological material using appropriate units
Plot and interpret graphs showing decay rates with appropriate scales
Translate information between numerical and graphical forms for decomposition data
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Use this lesson to stay inside Organisation of an ecosystem while connecting the detail back to the wider Ecology topic.
Ecology
Organisation of an ecosystem
GCSE AQA Higher Biology
Revision notes
Read the core explanations from Factors Affecting Decomposition and Rate Calculations before testing yourself from memory.
1. Why Decomposition Matters
Dead leaves pile up in autumn, but by spring they've vanished. Where did they go? This disappearing act is decomposition - and without it, we'd be buried under centuries of dead material.
Decomposition is the breakdown of dead organic matter by microorganisms. It's essential for recycling nutrients back into ecosystems. But here's what's interesting - the rate varies dramatically depending on conditions.
Trap: Many students think decomposition just 'happens automatically'. Actually, it requires specific conditions and living organisms called decomposers to break down the material.
2. The Three Key Factors
Three environmental factors control how fast decomposition happens: temperature, water availability, and oxygen levels. Change any of these, and you change the rate dramatically.
Temperature: Higher temperatures increase the rate of decomposition. This happens because enzymes in decomposer organisms work faster at higher temperatures, speeding up the breakdown process. Think of a compost heap - it gets warm as decomposition accelerates.
Water availability: Decomposers need water to survive and function. In dry conditions, decomposition slows dramatically. This is why mummies preserved in dry deserts can last thousands of years.
Oxygen availability: Most decomposer organisms need oxygen for respiration. In waterlogged soils or at the bottom of ponds, low oxygen levels slow decomposition. This is why peat bogs preserve organic matter for centuries.
3. Calculating Rates of Decay
Measuring decomposition rate means calculating how much mass is lost over time. The formula is straightforward, but getting the units right is crucial for exam marks.
Worked example: Calculating Rate of Decay
A 50g sample of dead leaves loses 30g of mass over 20 days. Calculate the rate of decay. [3 marks]
$\text{Rate of decay} = \frac{\text{Change in mass}}{\text{Time taken}}$ — [M1] for correct formula
Change in mass = 30g
Time taken = 20 days — Always include units with your values
$\text{Rate of decay} = \frac{30\text{ g}}{20\text{ days}} = 1.5\text{ g/day}$ — [M1] for substitution, [A1] for answer with units
Answer: 1.5 g/day
Units matter: Rate of decay is always mass per unit time. Common units include g/day, kg/week, or tonnes/year. Match your units to the time scale of the experiment.
4. Graphs and Data Analysis
Decomposition data often appears as graphs in exams. You need to plot graphs with appropriate scales and interpret the patterns you see.
When plotting decomposition graphs, the x-axis typically shows time (days, weeks, months) and the y-axis shows either mass remaining or percentage mass lost. The curve usually starts steep then levels off as decomposition slows.
Worked example: Plotting a Decomposition Graph
Plot a graph showing mass remaining over time using this data: Day 0: 100g, Day 10: 75g, Day 20: 60g, Day 30: 50g, Day 40: 45g
x-axis: 0-40 days (5 days per square)
y-axis: 0-100g (10g per square) — Scales should use most of the graph paper
x-axis: Time (days)
y-axis: Mass remaining (g) — Always include units in axis labels
Mark each coordinate with a small cross or dot — Use a ruler to ensure accuracy
Connect points with a smooth curve (not straight lines) — Decomposition follows a curved pattern, not linear
Answer: A smooth curve showing decreasing mass over time, starting steep then levelling off
5. Exam Technique and Common Mistakes
Decomposition questions test your ability to calculate rates, interpret graphs, and explain factors. Here's how to maximise your marks.
Exam depth extension: In factors affecting decomposition and rate calculations, connect why decomposition matters to the three key factors using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Command word focus: 'Calculate' questions need the formula, substitution, and answer with units. 'Explain' questions need scientific reasons using 'because'. 'Describe' questions want what you observe, not why it happens.
Common mistakes to avoid: Don't confuse decomposition with photosynthesis or respiration. Don't forget that decomposers are living organisms that need suitable conditions. Don't assume decomposition happens at a constant rate - it usually slows down over time.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Decomposition is the breakdown of dead organic matter by microorganisms such as bacteria and fungi, measured as mass lost per unit time. It is essential for recycling nutrients back into ecosystems.
What factors affect the rate of decomposition?
Three factors affect the rate of decomposition: temperature, water availability and oxygen levels. Higher temperatures make decomposer enzymes work faster, decomposers need water to survive, and most decomposers need oxygen for respiration, so dry or waterlogged conditions slow decay.
How do you calculate the rate of decay?
Rate of decay = change in mass ÷ time taken. If a sample of dead leaves loses 30g of mass over 20 days, the rate of decay is 30 ÷ 20 = 1.5 g/day. Rate of decay is always mass per unit time, such as g/day, kg/week or tonnes/year.
Why does temperature affect decomposition?
Higher temperatures increase the rate of decomposition because enzymes in decomposer organisms work faster at higher temperatures, speeding up the breakdown process. A compost heap gets warm as decomposition accelerates.
Why do peat bogs preserve dead material?
Peat bogs preserve organic matter for centuries because they are waterlogged, so oxygen levels are low and most decomposer organisms, which need oxygen for respiration, cannot work. In the same way, mummies last thousands of years in dry deserts because decomposers need water.
What does a decomposition graph look like?
A decomposition graph usually has time on the x-axis and mass remaining on the y-axis, with a smooth curve that starts steep then levels off as decomposition slows. For example, mass going from 100g on day 0 to 45g on day 40 shows decomposition does not happen at a constant rate.
Why does oxygen affect decomposition?
Oxygen affects decomposition because most decomposer organisms need oxygen for respiration. In waterlogged soils or at the bottom of ponds, low oxygen levels slow decomposition, which is why peat bogs preserve organic matter for centuries.
Why does water affect decomposition?
Water affects decomposition because decomposers need water to survive and function. In dry conditions decomposition slows dramatically, which is why mummies preserved in dry deserts can last thousands of years.
What units is rate of decay measured in?
Rate of decay is always mass per unit time, in units such as g/day, kg/week or tonnes/year. The units should match the time scale of the experiment, for example 30g lost in 20 days is 1.5 g/day.
Why does a compost heap get warm?
A compost heap gets warm because decomposition speeds up and decomposer organisms give out energy as heat. Higher temperatures then increase the rate of decomposition, because enzymes in decomposers work faster.
Does decomposition happen at a constant rate?
Decomposition does not usually happen at a constant rate; it slows down over time. A decomposition graph of mass remaining starts steep then levels off, so it should be drawn as a smooth curve, not straight lines.