Ecosystems and biogeochemical cycles: WACE Year 12 Earth and Environmental Science
“Explain the carbon, nitrogen and water cycles and how they sustain ecosystem services”
Biogeochemical cycles move matter such as carbon, nitrogen and water between reservoirs in the atmosphere, hydrosphere, geosphere and biosphere through transfers and transformations. These cycles sustain ecosystem services including soil fertility, clean water and climate regulation, and human activity can disrupt the balance of fluxes between reservoirs.
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.
Reservoir: a store where matter resides, for example carbon in limestone. Flux: the rate of transfer between reservoirs. Residence time: the average time a unit of matter stays in a reservoir. Sink: a reservoir that takes in more than it releases.
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.
Although nitrogen gas is abundant, the strong triple bond makes it inert. Only nitrogen-fixing bacteria and lightning break that bond naturally, so biological fixation is the gateway that lets nitrogen enter food webs.
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.
Set up the scenario
A patch of native bushland is cleared for cropping. Trace what happens to carbon, nitrogen and water.
Carbon
Standing biomass and soil organic carbon are exposed and oxidised; respiration and any burning transfer carbon from biosphere and soil reservoirs to the atmosphere. The removed plants no longer fix carbon dioxide, so a sink is lost.
Nitrogen
The nitrogen held in vegetation and litter is no longer cycled tightly. Tillage speeds decomposition, releasing nitrate; without deep-rooted plants to take it up, nitrate leaches into groundwater and streams.
Water
Removing transpiring vegetation cuts the return of water to the atmosphere and increases recharge to the water table. In the WA wheatbelt this has raised saline groundwater, causing dryland salinity.
Synthesise
One land-use change perturbs all three cycles at once, showing why SCSA frames them as an interacting system rather than separate diagrams.
- Treating cycles as separate
- The carbon, nitrogen and water cycles interact; water carries dissolved nutrients and drives weathering.
- Forgetting the geosphere store
- Carbon locked in limestone and fossil fuels is part of the carbon cycle on long timescales.
- Saying plants use nitrogen gas directly
- Most plants absorb nitrate from soil; nitrogen gas must be fixed first.
- Listing services without linking them to a cycle
- SCSA wants the connection, for example nutrient cycling supporting soil fertility.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 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 2020-style6 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.