Ecosystems and Material Cycles

What you'll learn

This revision guide covers how living organisms interact with each other and their environment, focusing on nutrient recycling in nature. You'll master the carbon cycle, water cycle and nitrogen cycle, understanding how decomposers and microorganisms maintain life on Earth. These topics appear regularly in Paper 2 and often combine with photosynthesis, respiration and environmental change questions.

Key terms and definitions

Ecosystem — a community of organisms interacting with each other and their non-living environment (abiotic factors)

Decomposers — microorganisms (bacteria and fungi) that break down dead organic matter, releasing nutrients back into the soil

Detritivores — organisms such as earthworms and woodlice that feed on dead organic material (detritus), speeding up decomposition

Biogeochemical cycles — the natural pathways through which essential elements and compounds circulate between living organisms and the environment

Nitrification — the conversion of ammonium compounds into nitrites and then nitrates by nitrifying bacteria in aerobic soil conditions

Denitrification — the conversion of nitrates back into nitrogen gas by denitrifying bacteria under anaerobic conditions, returning nitrogen to the atmosphere

Mycorrhizae — beneficial fungi that form symbiotic relationships with plant roots, increasing nutrient absorption (particularly phosphorus)

Saprobiont — an organism that feeds on dead or decaying organic matter by secreting digestive enzymes externally (extracellular digestion)

Core concepts

The carbon cycle

Carbon constantly moves between the atmosphere, living organisms, soils and oceans through several key processes:

Carbon enters the atmosphere through:

• Respiration — all living organisms release carbon dioxide during aerobic respiration in their cells
• Combustion — burning fossil fuels (coal, oil, natural gas) and biomass releases carbon dioxide stored for millions of years
• Decomposition — microorganisms respire whilst breaking down dead material, releasing CO₂

Carbon is removed from the atmosphere through:

• Photosynthesis — plants, algae and some bacteria absorb carbon dioxide and convert it into glucose and other organic compounds
• Ocean absorption — carbon dioxide dissolves in oceans, forming carbonate compounds used by marine organisms to build shells

Carbon storage occurs in:

• Fossil fuels — formed from organisms that died millions of years ago under specific high-pressure, low-oxygen conditions
• Peat bogs — waterlogged, acidic conditions slow decomposition, allowing partially decomposed plant material to accumulate
• Living biomass — carbon stored in the tissues of organisms as proteins, lipids and carbohydrates
• Soils — organic matter and humus contain carbon compounds

Human activities have significantly increased atmospheric CO₂ levels by burning fossil fuels and deforestation (reducing photosynthesis whilst releasing stored carbon). This enhanced greenhouse effect contributes to global climate change.

The water cycle

Water continuously cycles through different states and locations on Earth:

Key processes:

• Evaporation — liquid water from oceans, lakes and rivers turns into water vapour due to heat energy from the sun
• Transpiration — water evaporates from plant leaves through stomata after being absorbed by roots
• Condensation — water vapour cools in the atmosphere, forming clouds as tiny water droplets
• Precipitation — water falls as rain, snow, sleet or hail when clouds become saturated
• Percolation — water moves through soil layers into underground aquifers and groundwater stores
• Surface runoff — water flows across land surfaces into streams, rivers and eventually oceans

Plants play a crucial role by absorbing water through roots and releasing it through transpiration, contributing approximately 10% of atmospheric water vapour. The water cycle ensures fresh water availability for terrestrial ecosystems and helps regulate global temperatures.

The nitrogen cycle

Nitrogen is essential for making amino acids and proteins, but atmospheric nitrogen gas (N₂) cannot be used directly by most organisms. The nitrogen cycle converts nitrogen between different chemical forms:

Nitrogen fixation:

• Biological fixation — nitrogen-fixing bacteria (such as Rhizobium) in root nodules of legumes (peas, beans, clover) convert N₂ into ammonium compounds
• Lightning — high energy breaks nitrogen bonds, allowing nitrogen to combine with oxygen, forming nitrates in rainwater
• Industrial fixation — the Haber process produces ammonia for artificial fertilisers

Nitrification:

A two-step process carried out by nitrifying bacteria in aerobic soil:

1. Nitrosomonas bacteria convert ammonium compounds (NH₄⁺) into nitrites (NO₂⁻)
2. Nitrobacter bacteria convert nitrites into nitrates (NO₃⁻)

Plants absorb nitrates through root hair cells via active transport and use them to synthesise amino acids and proteins.

Decomposition and ammonification:

Decomposers (bacteria and fungi) break down proteins in dead organisms and animal waste, releasing ammonium compounds into the soil. This process requires appropriate moisture, temperature and oxygen levels to proceed efficiently.

Denitrification:

Denitrifying bacteria in waterlogged, anaerobic soils convert nitrates back into nitrogen gas, which returns to the atmosphere. This process reduces soil fertility, which is why farmers often improve drainage to maintain aerobic conditions.

Assimilation:

Plants use nitrates to build amino acids, which are assembled into proteins. Animals obtain nitrogen compounds by consuming plants or other animals, breaking down proteins into amino acids and reassembling them into animal proteins.

Factors affecting decomposition rate

Decomposition is essential for nutrient recycling, but its rate varies significantly depending on environmental conditions:

Temperature:

• Decomposers work fastest between 25-40°C when enzyme activity is optimal
• Below 5°C, microbial activity slows dramatically
• Above 50°C, enzymes denature and decomposition stops
• Tropical rainforests show rapid decomposition; Arctic regions show very slow rates

Oxygen availability:

• Aerobic decomposition (with oxygen) is faster and more complete than anaerobic decomposition
• Waterlogged soils lack oxygen, slowing decomposition and leading to peat formation
• Well-aerated soils support larger populations of aerobic bacteria and fungi

Water content:

• Moisture is essential for metabolic processes in decomposers
• Dry conditions prevent enzyme secretion and slow decomposition
• Excessively wet conditions exclude oxygen, slowing aerobic decomposition
• Optimal moisture content supports maximum decomposer activity

pH levels:

• Most decomposers prefer neutral to slightly acidic conditions (pH 6-7)
• Very acidic peat bogs (pH 3-4) slow decomposition significantly
• Alkaline soils may also reduce decomposer efficiency

Understanding these factors helps gardeners optimise composting and allows ecologists to predict decomposition rates in different ecosystems.

Human impacts on material cycles

Human activities increasingly interfere with natural biogeochemical cycles:

Carbon cycle disruption:

• Fossil fuel combustion releases approximately 35 billion tonnes of CO₂ annually
• Deforestation reduces carbon fixation and releases stored carbon
• Agriculture and livestock produce methane (CH₄), a potent greenhouse gas
• Ocean acidification occurs as excess atmospheric CO₂ dissolves in seawater

Nitrogen cycle disruption:

• Artificial fertilisers add excess nitrates to agricultural systems
• Eutrophication occurs when nitrates leach into waterways, causing algal blooms
• Algae block sunlight, killing aquatic plants; dead algae deplete oxygen when decomposed
• Fish and other aerobic organisms suffocate in oxygen-depleted water
• Livestock farming produces ammonia emissions affecting air quality

Water cycle disruption:

• Deforestation reduces transpiration, affecting regional rainfall patterns
• Urbanisation increases surface runoff and reduces groundwater recharge
• Irrigation depletes underground aquifers faster than natural replenishment
• Climate change alters precipitation patterns globally

Mitigation strategies:

• Reforestation and afforestation increase carbon fixation
• Renewable energy reduces fossil fuel dependence
• Precision agriculture minimises fertiliser overuse
• Wetland restoration filters nutrients before reaching waterways
• Sustainable water management preserves aquifer levels

Practical investigations: decomposition

GCSE students should understand how to investigate factors affecting decomposition rate:

Investigating temperature effects:

• Use milk, bread or agar cubes inoculated with microorganisms
• Measure mass loss or pH change over time at different temperatures
• Control variables: oxygen availability, moisture, surface area, initial mass
• Safety: wear gloves, disinfect surfaces, seal samples to prevent contamination

Investigating oxygen effects:

• Compare decomposition in sealed (anaerobic) versus open (aerobic) containers
• Measure gas production or pH changes
• Use indicator solutions to detect acidic decomposition products

Key skills:

• Identifying independent, dependent and control variables
• Plotting and interpreting graphs showing decomposition rates
• Calculating rates of change from results tables
• Evaluating experimental limitations and suggesting improvements

Worked examples

Example 1: Carbon cycle (4 marks)

Question: Explain how carbon dioxide is removed from the atmosphere and how it is returned. Include the names of processes in your answer.

Mark scheme answer:

• Carbon dioxide is removed by photosynthesis (1 mark)
• Plants/algae convert CO₂ into glucose/organic compounds (1 mark)
• Carbon dioxide is returned by respiration in all living organisms (1 mark)
• Carbon dioxide is also returned by combustion of fossil fuels/biomass (1 mark)

Example 2: Nitrogen cycle (6 marks)

Question: Describe the role of bacteria in making nitrogen available to plants.

Mark scheme answer:

• Nitrogen-fixing bacteria convert nitrogen gas into ammonium compounds/ammonia (1 mark)
• Found in root nodules of legumes/in soil (1 mark)
• Decomposers/saprobionts break down dead organisms/waste (1 mark)
• Releasing ammonium compounds through decomposition (1 mark)
• Nitrifying bacteria convert ammonium compounds into nitrites then nitrates (1 mark)
• Plants absorb nitrates through their roots/by active transport (1 mark)

Example 3: Decomposition investigation (6 marks)

Question: A student investigated how temperature affects decomposition of bread. She placed equal masses of bread in containers at 5°C, 20°C and 35°C. After one week, she measured the percentage mass lost.

Results: 5°C = 2%, 20°C = 18%, 35°C = 31%

Explain these results.

Mark scheme answer:

• Higher temperatures increase enzyme activity in decomposers (1 mark)
• Microorganisms/bacteria/fungi respire faster at higher temperatures (1 mark)
• At 35°C decomposers metabolise fastest/optimum temperature close to 35°C (1 mark)
• At 5°C enzyme activity is very slow/microorganisms nearly inactive (1 mark)
• Mass loss occurs because decomposition releases gases/carbon dioxide/water vapour (1 mark)
• Organic compounds are broken down into simpler substances (1 mark)

Common mistakes and how to avoid them

• Confusing nitrogen fixation with nitrification — nitrogen fixation converts N₂ gas into ammonium compounds; nitrification converts ammonium into nitrates. Remember "fixation fixes gas into usable form."

• Stating plants photosynthesise carbon — plants photosynthesise using carbon dioxide, converting it into glucose. Carbon is an element; it doesn't move on its own.

• Forgetting decomposers respire — students often mention decomposition releasing nutrients but forget decomposers release CO₂ through respiration. Both processes occur simultaneously.

• Writing that denitrification is beneficial — while denitrification is natural, it reduces soil fertility by converting useful nitrates back to unusable nitrogen gas. Farmers try to prevent it by ensuring good drainage.

• Mixing up detritivores and decomposers — detritivores (earthworms, woodlice) physically break down material; decomposers (bacteria, fungi) chemically break it down using enzymes. Both contribute to decomposition.

• Vague descriptions of eutrophication — follow the sequence: excess nitrates → algal bloom → blocks light → plants die → bacteria decompose dead material → oxygen depleted → fish/animals die.

Exam technique for "Ecosystems and Material Cycles"

• Command words matter: "State" requires simple facts; "Describe" needs details about what happens; "Explain" demands reasons why things happen using scientific principles. For cycles, "Describe" often means name processes and substances; "Explain" means give reasons for changes.

• Use process names precisely: Name specific processes (photosynthesis, respiration, decomposition, nitrification, nitrogen fixation, denitrification). Generic terms like "plants use it" or "bacteria do things" won't earn marks.

• Follow cycles logically: When describing material cycles, track the element/compound through different reservoirs. State what form it's in at each stage and name the process causing the change.

• Extended response structure: For 6-mark questions, use the mark scheme ratio of 2 marks per well-developed point. Include: process name, organisms involved, substances transformed, and reason/effect where appropriate.

Quick revision summary

Material cycles move essential elements between living organisms and the environment. Carbon cycles through photosynthesis (removing CO₂) and respiration/combustion (releasing CO₂). Water cycles through evaporation, transpiration, condensation and precipitation. Nitrogen cycles through fixation (N₂ to ammonium), nitrification (ammonium to nitrates), assimilation (nitrates to proteins) and denitrification (nitrates to N₂). Decomposers recycle all nutrients by breaking down dead material. Temperature, oxygen, water and pH affect decomposition rates. Human activities disrupt these cycles through fossil fuel use, deforestation and excessive fertiliser application.