1. The Water Cycle

Introduction

Water is one of the most important substances on Earth. Every living organism depends on it for survival. Plants need water for photosynthesis and growth, while animals rely on it for drinking, transporting nutrients, regulating body temperature, and carrying out essential chemical reactions. Although Earth has a vast amount of water, it is constantly moving between the atmosphere, land, rivers, lakes, oceans, and living organisms.

This continuous movement of water is called the water cycle (also known as the hydrological cycle). The water cycle is powered mainly by the Sun, which provides the energy needed for evaporation, and gravity, which returns water to Earth's surface as precipitation. Because water is continually recycled, the same water molecules have been moving around our planet for billions of years.

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What Is the Water Cycle?

The water cycle is the continuous movement of water between the Earth's surface, the atmosphere, and living organisms.

Water changes between three states during the cycle:

• Solid (ice and snow)
• Liquid (water)
• Gas (water vapour)

The main stages of the water cycle are:

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Collection
• Infiltration
• Runoff

Together, these processes recycle Earth's water.

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Figure 1. The water cycle continuously moves water between the atmosphere, land, oceans, and living organisms.

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Stage 1: Evaporation

Evaporation is the process by which liquid water changes into water vapour.

The Sun heats water in:

• Oceans
• Lakes
• Rivers
• Puddles
• Soil

As water warms, some molecules gain enough energy to escape into the atmosphere as an invisible gas.

Evaporation is greatest on:

• Hot days
• Windy days
• Dry days
• Large bodies of water

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Transpiration

Plants also contribute water to the atmosphere.

Transpiration is the release of water vapour from plant leaves through tiny openings called stomata.

Together, evaporation and transpiration are sometimes called evapotranspiration.

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Figure 2. Water enters the atmosphere through evaporation from water surfaces and transpiration from plants.

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Stage 2: Condensation

As warm, moist air rises, it cools.

Cooling causes water vapour to change back into tiny liquid droplets.

This process is called condensation.

Condensation forms:

• Clouds
• Fog
• Mist
• Dew

Millions of tiny droplets combine to form visible clouds.

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Stage 3: Precipitation

When water droplets in clouds become too heavy to remain suspended, they fall back to Earth as precipitation.

Types of precipitation include:

• Rain
• Snow
• Sleet
• Hail

The type of precipitation depends mainly on the air temperature.

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Figure 3. Condensation forms clouds, and precipitation returns water to Earth's surface.

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Stage 4: Collection

After precipitation falls, water collects in:

• Oceans
• Lakes
• Rivers
• Wetlands
• Reservoirs

Some water remains on the surface, while some begins moving through the soil.

Eventually, much of it returns to the oceans, where the cycle begins again.

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Stage 5: Infiltration and Groundwater

Some water soaks into the ground.

This process is called infiltration.

Underground, water fills tiny spaces between soil particles and rocks, forming groundwater.

Groundwater:

• Supplies wells.
• Feeds springs.
• Keeps rivers flowing during dry periods.
• Provides water for many ecosystems.

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Stage 6: Runoff

Water that does not soak into the ground flows across the land.

This is called runoff.

Runoff carries water into:

• Streams
• Rivers
• Lakes
• Oceans

Runoff can also transport:

• Soil
• Nutrients
• Pollution

This is why protecting watersheds is important for maintaining clean water.

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Figure 4. Water either infiltrates into the ground or flows across the land as runoff.

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How Water Moves Through Ecosystems

Water moves continuously between living organisms and the environment.

For example:

• Plants absorb water through their roots.
• Animals drink water.
• Animals release water through breathing, sweating, and waste.
• Plants release water through transpiration.
• Decomposers return water to the soil as they break down organic matter.

This constant movement supports every ecosystem on Earth.

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Why Is the Water Cycle Important?

The water cycle is essential because it:

• Recycles Earth's freshwater.
• Supplies drinking water.
• Supports plant growth.
• Provides habitats for aquatic organisms.
• Transports nutrients.
• Regulates climate.
• Helps maintain healthy ecosystems.

Without the continuous movement of water, life on Earth could not exist.

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Factors That Affect Water Movement

Several environmental factors influence how quickly water moves through the water cycle.

Temperature

Higher temperatures increase evaporation.

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Wind

Wind removes moist air above water surfaces, increasing evaporation.

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Humidity

High humidity slows evaporation because the air already contains large amounts of water vapour.

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Vegetation

Forests increase transpiration and help water infiltrate into the soil.

Removing vegetation often increases runoff and soil erosion.

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Land Surface

Impermeable surfaces such as roads and parking lots prevent infiltration.

This increases runoff and the risk of flooding.

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Figure 5. Forests encourage infiltration, while urban areas increase runoff because less water can soak into the ground.

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Worked Example

Question

Place the following stages of the water cycle in the correct order.

• Condensation
• Collection
• Evaporation
• Precipitation

Solution

Correct order:

1. Evaporation
2. Condensation
3. Precipitation
4. Collection

Water then continues through infiltration, runoff, and eventually evaporates again.

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Real-World Connection

Cities often experience increased flooding because roads, pavements, and buildings prevent rainwater from soaking into the ground. Engineers reduce this problem by installing permeable pavements, rain gardens, green roofs, and stormwater ponds that slow runoff and allow more water to infiltrate into the soil. These solutions help protect ecosystems and improve water quality.

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Did You Know?

At any given moment, about 97% of Earth's water is found in the oceans. Of the remaining 3% that is freshwater, most is locked away in glaciers and ice caps. Less than 1% of Earth's freshwater is easily available in lakes, rivers, and shallow groundwater for humans and most terrestrial ecosystems to use.

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Key Terms

Collection – The gathering of water in oceans, lakes, rivers, and other bodies of water.

Condensation – The process by which water vapour cools and changes into liquid water.

Evaporation – The process by which liquid water changes into water vapour.

Groundwater – Water stored beneath Earth's surface in soil and rock.

Infiltration – The process by which water soaks into the ground.

Precipitation – Water falling from clouds as rain, snow, sleet, or hail.

Runoff – Water flowing across the land into rivers, lakes, or oceans.

Transpiration – The release of water vapour from plant leaves.

Water cycle (Hydrological cycle) – The continuous movement of water between the atmosphere, land, oceans, and living organisms.

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Key Takeaways

• The water cycle continuously recycles water through Earth's atmosphere, land, oceans, and living organisms.
• The main stages are evaporation, transpiration, condensation, precipitation, collection, infiltration, and runoff.
• The Sun provides the energy that drives evaporation, while gravity returns water to Earth as precipitation.
• Water moves through ecosystems by passing between the atmosphere, soil, plants, animals, and bodies of water.
• Temperature, wind, humidity, vegetation, and land use all influence how quickly water moves through the water cycle.

2. The Carbon Cycle

Introduction

Carbon is one of the most important elements for life on Earth. It is found in all living organisms and forms the backbone of important molecules such as carbohydrates, proteins, fats, and DNA. Carbon is also present in the atmosphere, oceans, rocks, soil, and fossil fuels. Rather than remaining in one place, carbon is constantly moving between these different parts of the Earth.

The carbon cycle describes the continuous movement of carbon through living organisms, the atmosphere, the oceans, and the Earth's crust. This cycle helps maintain the balance of carbon dioxide in the atmosphere, supports life through photosynthesis, and plays an important role in regulating Earth's climate.

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What Is the Carbon Cycle?

The carbon cycle is the continuous movement of carbon between the:

• Atmosphere
• Living organisms
• Oceans
• Soil
• Rocks
• Fossil fuels

Carbon moves through these reservoirs by natural processes such as:

• Photosynthesis
• Respiration
• Feeding
• Decomposition
• Combustion
• Ocean exchange

Unlike energy, which flows in one direction, carbon is continually recycled.

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Figure 1. The carbon cycle continuously moves carbon between the atmosphere, living organisms, oceans, and Earth's crust.

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Carbon Reservoirs

A carbon reservoir is any place where carbon is stored.

Major carbon reservoirs include:

The Atmosphere

Carbon is mainly found as carbon dioxide (CO₂).

Although CO₂ makes up only a small fraction of the atmosphere, it is essential for photosynthesis and influences Earth's climate.

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Living Organisms

Plants, animals, fungi, and microorganisms all contain carbon in their cells.

Carbon forms part of:

• Sugars
• Proteins
• Lipids (fats)
• DNA

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Oceans

The oceans store enormous amounts of carbon.

Carbon is found:

• Dissolved in seawater
• In marine organisms
• In shells made from calcium carbonate

The oceans act as one of Earth's largest carbon reservoirs.

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Soil

Dead plants and animals add carbon to the soil as they decompose.

Healthy soils store large amounts of organic carbon.

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Rocks and Fossil Fuels

Over millions of years, carbon becomes locked inside:

• Limestone
• Coal
• Oil
• Natural gas

These are considered long-term carbon reservoirs.

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Figure 2. Carbon is stored in several major reservoirs throughout Earth's systems.

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Photosynthesis

Plants remove carbon dioxide from the atmosphere during photosynthesis.

Using sunlight, plants convert:

• Carbon dioxide
• Water

into:

• Glucose (food)
• Oxygen

This transfers carbon from the atmosphere into living organisms.

Respiration

All living organisms carry out cellular respiration.

During respiration:

• Glucose is broken down.
• Energy is released for life processes.
• Carbon dioxide is produced.

Plants, animals, fungi, and microorganisms all return carbon dioxide to the atmosphere through respiration.

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Figure 3. Photosynthesis removes carbon dioxide from the atmosphere, while respiration returns it.

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Feeding and Food Webs

Carbon moves through ecosystems whenever organisms eat one another.

For example:

Grass → Rabbit → Fox

• Grass absorbs carbon dioxide during photosynthesis.
• The rabbit obtains carbon by eating the grass.
• The fox obtains carbon by eating the rabbit.

In this way, carbon moves through food chains and food webs.

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Decomposition

When organisms die or produce waste, decomposers break down the organic matter.

Examples of decomposers include:

• Bacteria
• Fungi

During decomposition:

• Carbon is returned to the soil.
• Carbon dioxide is released into the atmosphere through respiration.

Some carbon remains in the soil for long periods.

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Fossil Fuels and Combustion

Millions of years ago, some dead organisms became buried underground.

Over time, heat and pressure transformed this organic matter into:

• Coal
• Oil
• Natural gas

These are called fossil fuels.

When fossil fuels are burned (combustion):

• Carbon combines with oxygen.
• Carbon dioxide is released into the atmosphere.

Because fossil fuels formed over millions of years, burning them releases stored carbon much faster than natural processes can remove it.

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Figure 4. Burning fossil fuels rapidly transfers carbon stored underground back into the atmosphere as carbon dioxide.

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Human Impacts on the Carbon Cycle

Human activities have significantly altered the natural carbon cycle.

Burning Fossil Fuels

Power stations, factories, and vehicles release large amounts of carbon dioxide.

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Deforestation

Cutting down forests reduces the number of plants available to remove carbon dioxide through photosynthesis.

Burning forests also releases stored carbon.

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Agriculture

Some farming practices release carbon from soils into the atmosphere.

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Cement Production

Manufacturing cement releases additional carbon dioxide.

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These activities increase atmospheric carbon dioxide, strengthening the greenhouse effect and contributing to climate change.

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Figure 5. Human activities have increased the amount of carbon dioxide in the atmosphere, altering the natural carbon cycle.

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Why the Carbon Cycle Is Important

The carbon cycle is essential because it:

• Recycles carbon needed for life.
• Supports photosynthesis.
• Helps regulate Earth's climate.
• Maintains the balance of atmospheric carbon dioxide.
• Connects living organisms with Earth's atmosphere, oceans, and rocks.

Without the carbon cycle, ecosystems and life as we know them could not exist.

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Worked Example

Question

Identify whether each process removes carbon dioxide from the atmosphere or adds carbon dioxide to the atmosphere.

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Real-World Connection

Planting trees is one way to help reduce atmospheric carbon dioxide because growing trees absorb CO₂ through photosynthesis and store carbon in their trunks, branches, roots, and leaves. However, reducing emissions from burning fossil fuels is equally important because newly planted forests cannot absorb carbon as quickly as large amounts are released by human activities.

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Did You Know?

The oceans absorb about one-quarter of the carbon dioxide released by human activities each year. This helps slow the increase of atmospheric CO₂, but it also makes seawater more acidic. Ocean acidification can make it difficult for organisms such as corals, oysters, and some plankton to build and maintain their calcium carbonate shells and skeletons.

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Key Terms

Atmosphere – The layer of gases surrounding Earth.

Carbon cycle – The continuous movement of carbon through Earth's atmosphere, living organisms, oceans, soil, and rocks.

Carbon reservoir – A place where carbon is stored.

Carbon dioxide (CO₂) – A gas involved in photosynthesis, respiration, and the greenhouse effect.

Combustion – The burning of a substance, releasing energy and carbon dioxide.

Decomposition – The breakdown of dead organic matter by decomposers.

Fossil fuels – Coal, oil, and natural gas formed from ancient organisms over millions of years.

Photosynthesis – The process by which producers use sunlight to convert carbon dioxide and water into glucose and oxygen.

Respiration – The process by which cells break down glucose to release energy, producing carbon dioxide and water.

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Key Takeaways

• The carbon cycle continuously recycles carbon between Earth's atmosphere, living organisms, oceans, soil, and rocks.
• Photosynthesis removes carbon dioxide from the atmosphere and stores carbon in living organisms.
• Respiration, decomposition, and combustion return carbon dioxide to the atmosphere.
• Carbon is stored in major reservoirs such as the atmosphere, oceans, forests, soils, rocks, and fossil fuels.
• Human activities, especially burning fossil fuels and deforestation, have altered the natural carbon cycle and contributed to climate change.

3. The Nitrogen Cycle

Introduction

Nitrogen is one of the most abundant elements on Earth. In fact, about 78% of the Earth's atmosphere is made up of nitrogen gas (N₂). Despite its abundance, most living organisms cannot use nitrogen gas directly. Instead, nitrogen must first be converted into forms that plants and other organisms can absorb and use.

The nitrogen cycle is the continuous movement and transformation of nitrogen between the atmosphere, soil, living organisms, and decomposers. Tiny microorganisms, especially bacteria, play a crucial role in this cycle by converting nitrogen into different chemical forms. Without the nitrogen cycle, plants could not grow, food chains would collapse, and life on Earth would not be possible.

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Why Is Nitrogen Important?

Nitrogen is essential for all living organisms because it is needed to make:

• Proteins
• Amino acids
• DNA
• RNA
• Chlorophyll (in plants)

Without nitrogen:

• Plants cannot grow properly.
• Animals cannot build or repair tissues.
• Cells cannot reproduce normally.

Nitrogen is therefore one of the most important nutrients in every ecosystem.

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What Is the Nitrogen Cycle?

The nitrogen cycle is the continuous movement and transformation of nitrogen between:

• The atmosphere
• Soil
• Plants
• Animals
• Decomposers

Unlike the carbon cycle, nitrogen changes into several different chemical forms as it moves through ecosystems.

The major stages are:

• Nitrogen fixation
• Nitrification
• Assimilation
• Consumption
• Ammonification (decomposition)
• Denitrification

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Figure 1. The nitrogen cycle transforms nitrogen into different chemical forms as it moves through ecosystems.

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Stage 1: Nitrogen Fixation

Although nitrogen gas (N₂) is abundant in the atmosphere, plants cannot use it directly.

Nitrogen fixation is the process that converts nitrogen gas into usable nitrogen compounds.

This is carried out mainly by nitrogen-fixing bacteria that live:

• In the soil
• Inside the root nodules of legumes (such as peas, beans, and clover)

Some nitrogen is also fixed naturally by lightning, which converts atmospheric nitrogen into nitrogen compounds that are washed into the soil by rain.

The bacteria convert nitrogen gas into ammonia (NH₃) or ammonium ions (NH₄⁺).

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Figure 2. Nitrogen-fixing bacteria living in the root nodules of legumes convert atmospheric nitrogen into forms plants can use.

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Stage 2: Nitrification

Other soil bacteria convert ammonia into forms that plants can absorb.

This process is called nitrification.

It occurs in two main steps:

1. Ammonia is converted into nitrites (NO₂⁻).
2. Nitrites are converted into nitrates (NO₃⁻).

Nitrates are the main form of nitrogen absorbed by plants.

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Stage 3: Assimilation

Assimilation occurs when plants absorb nitrates from the soil through their roots.

Plants use nitrogen to produce:

• Proteins
• DNA
• Chlorophyll

Animals obtain nitrogen by eating plants or by eating other animals.

In this way, nitrogen moves through food chains.

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Figure 3. Plants absorb nitrates through their roots and use them to build important biological molecules.

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Stage 4: Consumption

Nitrogen moves through ecosystems as organisms feed on one another.

For example:

Grass → Rabbit → Fox

• Grass absorbs nitrates from the soil.
• The rabbit obtains nitrogen by eating the grass.
• The fox obtains nitrogen by eating the rabbit.

Nitrogen is transferred from one trophic level to the next through feeding relationships.

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Stage 5: Ammonification (Decomposition)

When plants and animals die or produce waste, decomposers break down the organic matter.

This process is called ammonification.

Decomposers such as bacteria and fungi convert organic nitrogen into:

• Ammonia (NH₃)
• Ammonium ions (NH₄⁺)

These compounds return nitrogen to the soil, where it can be reused.

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Stage 6: Denitrification

Some bacteria convert nitrates back into nitrogen gas.

This process is called denitrification.

The nitrogen gas is released into the atmosphere, completing the cycle.

Without denitrification, nitrogen would gradually build up in soils and water.

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Figure 4. Different groups of bacteria drive the major stages of the nitrogen cycle.

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Human Impacts on the Nitrogen Cycle

Human activities have changed the natural nitrogen cycle.

Fertiliser Use

Farmers add nitrogen-rich fertilisers to increase crop growth.

However, excess fertiliser may be washed into rivers and lakes.

This can lead to:

• Algal blooms
• Reduced oxygen levels
• Fish deaths

This process is known as eutrophication.

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Burning Fossil Fuels

Vehicle engines and power stations release nitrogen oxides (NOₓ) into the atmosphere.

These gases contribute to:

• Air pollution
• Acid rain
• Smog

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Deforestation

Removing forests reduces plant uptake of nitrogen and can increase soil erosion and nutrient loss.

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Wastewater and Agriculture

Animal waste and untreated sewage release additional nitrogen compounds into waterways, affecting aquatic ecosystems.

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Figure 5. Excess nitrogen from fertilisers can cause eutrophication, harming aquatic ecosystems.

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Why the Nitrogen Cycle Is Important

The nitrogen cycle is essential because it:

• Supplies plants with usable nitrogen.
• Supports the production of proteins and DNA.
• Maintains soil fertility.
• Supports food chains.
• Recycles nitrogen between living organisms and the environment.

Without this cycle, life on Earth would quickly run out of usable nitrogen.

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Worked Example

Question

Match each stage of the nitrogen cycle with its description.

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Real-World Connection

Many farmers rotate crops with legumes such as beans, peas, and clover because these plants contain nitrogen-fixing bacteria in their root nodules. The bacteria naturally enrich the soil with nitrogen, reducing the need for artificial fertilisers and helping to improve soil fertility for future crops.

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Did You Know?

Although the air around us is nearly 78% nitrogen, humans and other animals cannot use nitrogen gas directly. We obtain all of the nitrogen needed to build our proteins and DNA by eating plants or by eating animals that have eaten plants. In other words, nearly all usable nitrogen enters food chains through plants.

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Key Terms

Ammonification – The process in which decomposers convert organic nitrogen into ammonia or ammonium.

Assimilation – The uptake of nitrates by plants to build proteins, DNA, and other molecules.

Denitrification – The process in which bacteria convert nitrates back into nitrogen gas.

Eutrophication – The enrichment of water with nutrients, leading to excessive algal growth and oxygen depletion.

Nitrogen cycle – The continuous movement and transformation of nitrogen through the atmosphere, soil, living organisms, and decomposers.

Nitrogen fixation – The conversion of atmospheric nitrogen gas into ammonia or ammonium by bacteria or lightning.

Nitrogen-fixing bacteria – Bacteria that convert atmospheric nitrogen into forms that plants can use.

Nitrates (NO₃⁻) – The main form of nitrogen absorbed by plants.

Nitrification – The bacterial conversion of ammonia into nitrites and then nitrates.

Root nodules – Structures on the roots of legumes that contain nitrogen-fixing bacteria.

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Key Takeaways

• The nitrogen cycle converts atmospheric nitrogen into forms that living organisms can use.
• Nitrogen-fixing bacteria are essential because they convert nitrogen gas into usable nitrogen compounds.
• Plants absorb nitrates from the soil and use them to make proteins, DNA, and chlorophyll.
• Nitrogen moves through ecosystems as organisms feed on one another and is recycled by decomposers and bacteria.
• Human activities, especially fertiliser use and fossil fuel combustion, can disrupt the nitrogen cycle and damage ecosystems.

4. Decomposition and Nutrient Recycling

Introduction

Every day, plants, animals, and microorganisms die or produce waste. If this organic matter simply accumulated, ecosystems would soon become buried under dead material. Fortunately, nature has an efficient recycling system. Tiny organisms called decomposers break down dead plants, animals, and waste, returning valuable nutrients to the environment where they can be used again.

Decomposition is one of the most important ecological processes because it continually recycles essential nutrients. Without decomposers, ecosystems would eventually run out of the nutrients needed for plant growth, food chains would collapse, and life as we know it could not continue.

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What Is Decomposition?

Decomposition is the process by which dead organisms and organic waste are broken down into simpler substances.

During decomposition:

• Dead plants and animals are broken apart.
• Waste materials are decomposed.
• Nutrients are released back into the environment.
• Organic matter becomes part of the soil.

This process allows nutrients to be used repeatedly rather than being permanently locked inside dead organisms.

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Figure 1. Decomposition returns nutrients from dead organisms to the soil, where they can be reused by plants.

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Decomposers

Decomposers are organisms that obtain energy by breaking down dead organic matter.

The two main groups are:

Bacteria

Bacteria are microscopic organisms that:

• Break down dead plants and animals.
• Decompose waste.
• Recycle nutrients rapidly.

Bacteria are especially important in moist environments.

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Fungi

Fungi include:

• Mushrooms
• Moulds
• Mildews

Fungi release digestive enzymes onto dead material before absorbing the nutrients.

They are particularly effective at breaking down wood and fallen leaves.

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Detritivores

Some animals also help break down dead material.

These organisms are called detritivores.

Examples include:

• Earthworms
• Woodlice
• Millipedes
• Dung beetles

Unlike decomposers, detritivores physically break dead material into smaller pieces, making it easier for bacteria and fungi to complete decomposition.

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Figure 2. Decomposers and detritivores work together to recycle organic matter.

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How Nutrients Are Recycled

When an organism dies:

1. Detritivores shred the dead material.
2. Bacteria and fungi chemically break it down.
3. Nutrients are released into the soil.
4. Plant roots absorb the nutrients.
5. Animals obtain the nutrients by eating plants.
6. The cycle repeats.

Important recycled nutrients include:

• Nitrogen
• Phosphorus
• Potassium
• Carbon
• Calcium
• Magnesium

Unlike energy, which flows in one direction, nutrients are continually recycled through ecosystems.

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Figure 3. Nutrients move continuously between organisms, decomposers, and the environment.

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Why Is Decomposition Important?

Without decomposition:

• Dead organisms would accumulate.
• Nutrients would remain locked inside dead tissues.
• Soil fertility would decrease.
• Plant growth would slow.
• Food chains would eventually collapse.

Decomposition is essential because it:

• Recycles nutrients.
• Maintains healthy soils.
• Supports plant growth.
• Removes dead organic matter.
• Sustains ecosystem productivity.

Every ecosystem depends on decomposers to remain healthy.

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Factors That Affect Decomposition

The rate of decomposition depends on environmental conditions.

Temperature

Warm temperatures increase the activity of decomposers.

Cold temperatures slow decomposition.

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Moisture

Decomposers require water.

Moist conditions speed decomposition.

Very dry conditions slow it dramatically.

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Oxygen

Most decomposers require oxygen.

Well-aerated soils usually have faster decomposition than waterlogged soils.

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Availability of Decomposers

More bacteria, fungi, and detritivores generally result in faster decomposition.

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Type of Organic Material

Some materials decompose quickly.

Examples:

• Fruit
• Grass
• Soft leaves

Others decompose slowly.

Examples:

• Wood
• Bones
• Thick bark

These contain tougher materials that are more difficult to break down.

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Figure 4. Temperature, moisture, oxygen, and the type of organic material all influence decomposition rates.

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Nutrient Movement Through Ecosystems

Nutrients constantly cycle between living organisms and the environment.

For example:

• Plants absorb nutrients from the soil.
• Herbivores eat plants.
• Carnivores eat herbivores.
• Organisms produce waste.
• Organisms die.
• Decomposers recycle nutrients back into the soil.

This continuous movement is called nutrient cycling.

Healthy nutrient cycling supports healthy ecosystems.

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Decomposition and Composting

Composting is a practical example of decomposition.

A compost pile contains:

• Food scraps
• Leaves
• Grass clippings
• Garden waste

Bacteria, fungi, worms, and other decomposers break these materials down into compost, a nutrient-rich material that improves soil quality.

Composting:

• Reduces waste sent to landfill.
• Improves soil fertility.
• Reduces the need for chemical fertilisers.

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Figure 5. Composting uses natural decomposition to recycle organic waste into nutrient-rich compost.

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Worked Example

Question

A fallen tree lies on the forest floor.

Describe how its nutrients eventually become available to another young tree.

Solution

1. The tree dies and begins to decay.
2. Detritivores break the wood into smaller pieces.
3. Fungi and bacteria decompose the wood.
4. Nutrients are released into the soil.
5. The roots of a young tree absorb these nutrients.
6. The nutrients are used to build new plant tissues.

This demonstrates nutrient recycling within an ecosystem.

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Real-World Connection

Gardeners and farmers often add compost to their soil because it improves nutrient availability, increases water retention, and supports beneficial microorganisms. Healthy soils rich in decomposers reduce the need for synthetic fertilisers while helping crops grow more successfully.

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Did You Know?

Some fungi produce thread-like structures called hyphae that can spread through soil for many kilometres. Together, these hyphae form a network called a mycelium, which breaks down dead organic matter and helps recycle nutrients. In some forests, fungal networks also connect the roots of different trees, allowing nutrients and chemical signals to move between plants. Because of this, mycelium is sometimes nicknamed the "Wood Wide Web."

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Key Terms

Compost – Decomposed organic matter used to improve soil.

Decomposer – An organism, such as a bacterium or fungus, that chemically breaks down dead organic matter.

Decomposition – The breakdown of dead organisms and organic waste into simpler substances.

Detritivore – An organism that feeds on dead organic matter by ingesting it and breaking it into smaller pieces.

Hyphae – Thread-like filaments that make up the body of a fungus.

Mycelium – A network of fungal hyphae that spreads through soil or organic matter.

Nutrient cycling – The continuous movement and reuse of nutrients through living organisms and the environment.

Organic matter – Material that comes from living or once-living organisms.

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Key Takeaways

• Decomposition breaks down dead organisms and waste into simpler substances.
• Bacteria and fungi are the main decomposers, while detritivores help by physically breaking down dead material.
• Decomposition recycles nutrients back into the soil, making them available for plants.
• Temperature, moisture, oxygen, decomposer activity, and the type of organic material all affect decomposition rates.
• Nutrient recycling is essential for maintaining fertile soils, healthy ecosystems, and the continued growth of living organisms.

5. Human Impacts on Biogeochemical Cycles