How do biogeochemical cycles move matter through ecosystems and Earth systems?
Explain the carbon, nitrogen and water cycles and how they sustain ecosystem services
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on biogeochemical cycles. Covers the carbon, nitrogen and water cycles, reservoirs and fluxes, ecosystem services and human disruption, with Australian examples.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SCSA wants you to trace matter through reservoirs and explain why balanced cycling underpins ecosystem services. A strong answer names reservoirs and the processes that move matter between them, then links the cycle to a service and a human impact.
Reservoirs and fluxes
A reservoir is a store of matter, such as carbon dioxide in the atmosphere or nitrogen in soil. A flux is a flow between reservoirs, such as photosynthesis moving carbon from air to plants. Cycles are driven by energy, mainly solar energy and Earth's internal heat. When inputs and outputs to a reservoir are balanced, the system is in steady state.
The carbon cycle
Carbon moves between the atmosphere, oceans, living things and rocks.
- Photosynthesis transfers carbon dioxide from the atmosphere into plant tissue.
- Respiration and decomposition return carbon to the atmosphere.
- Dissolving moves carbon dioxide into the oceans, where it can precipitate as carbonate sediment and form limestone, a slow geosphere store.
- Combustion of fossil fuels rapidly transfers ancient carbon from the geosphere back to the atmosphere.
Oceans and forests are major carbon sinks. The carbon cycle regulates atmospheric carbon dioxide and therefore climate, which is the link Unit 4 builds on.
The nitrogen cycle
Nitrogen gas makes up most of the atmosphere but is unusable by most organisms until it is fixed.
- Nitrogen fixation by bacteria converts nitrogen gas into ammonium; lightning fixes a small amount.
- Nitrification converts ammonium to nitrites then nitrates, the form plants absorb.
- Assimilation builds nitrogen into proteins and nucleic acids.
- Decomposition returns nitrogen to soil as ammonium.
- Denitrification returns nitrogen gas to the atmosphere.
Nitrogen availability often limits plant growth, so this cycle controls soil fertility.
The water cycle
Water cycles through evaporation, transpiration, condensation, precipitation, runoff, infiltration and storage in oceans, ice, groundwater and the atmosphere. It is powered by solar energy and gravity. The water cycle distributes fresh water, shapes landscapes through erosion, and links the other cycles by transporting dissolved nutrients.
Ecosystem services
Ecosystem services are benefits people gain from healthy ecosystems sustained by these cycles.
- Supporting services: soil formation and nutrient cycling.
- Provisioning services: food, fresh water, timber.
- Regulating services: climate regulation, flood control, water purification.
- Cultural services: recreation and cultural value.
Human disruption
Human activity shifts fluxes and unbalances cycles.
- Burning fossil fuels adds carbon dioxide faster than sinks remove it.
- Fertiliser use and runoff add excess nitrogen and phosphorus to waterways, causing algal blooms and eutrophication, a problem in parts of the Swan-Canning river system in Western Australia.
- Land clearing and over-extraction of groundwater disrupt the water cycle.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20227 marksA study of a Western Australian wheatbelt catchment recorded rising nitrate concentrations in a creek downstream of fertilised paddocks, followed by a dense algal bloom and a fish kill. Using your knowledge of the nitrogen cycle, explain the sequence of processes linking fertiliser use to the fish kill.Show worked answer →
A 7 mark answer must trace a causal chain through the nitrogen cycle to the ecological outcome.
- Fertiliser input
- Nitrogen fertiliser adds large amounts of nitrate and ammonium to the soil, beyond what crops assimilate.
- Transport
- Rainfall and irrigation cause runoff and leaching, carrying soluble nitrate into the creek (a flux from soil reservoir to water reservoir).
- Eutrophication
- The excess nitrate removes the nutrient limit on algal growth, triggering a rapid algal bloom.
- Decomposition and oxygen loss
- When the bloom dies, decomposer bacteria respire as they break down the organic matter, consuming dissolved oxygen.
- Fish kill
- Dissolved oxygen falls below the level fish need, causing the fish kill (hypoxia).
Markers reward the ordered chain: surplus nitrate, runoff or leaching, bloom, decomposition consuming oxygen, hypoxia killing fish.
WACE 20206 marksExplain how the carbon, nitrogen and water cycles interact, and discuss why managing one cycle in isolation is rarely effective.Show worked answer →
A 6 mark answer needs concrete links between cycles plus a management judgement.
Interactions. Water transports dissolved carbon (as bicarbonate) and dissolved nitrogen (as nitrate) through ecosystems, so the water cycle physically moves matter in the other two cycles. Photosynthesis simultaneously fixes carbon and requires nitrogen to build proteins, coupling the carbon and nitrogen cycles in plant growth. Decomposition returns both carbon and nitrogen to soil and atmosphere together.
Why isolation fails. Because the cycles are coupled, an intervention in one shifts the others. For example, adding nitrogen fertiliser to boost plant carbon uptake also increases nitrate runoff and can change soil and water chemistry; clearing vegetation releases carbon and disrupts the water cycle and nutrient retention at once. Effective management must treat the cycles as an interacting system.
Markers reward at least two specific cross-cycle links and a reasoned point that coupling makes single-cycle management ineffective.