QCE Biology - Unit 3 - Functioning ecosystems and succession

Water, Carbon and Nitrogen Cycles | QCE Biology

Learn how matter is transferred and transformed through the water, carbon and nitrogen cycles in ecosystems.

Updated 2026-05-18 - 6 min read

Water, carbon and nitrogen cycles is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

Matter cycles

Unlike energy, matter is recycled. Atoms move between living biomass, the atmosphere, water, soil, rocks and waste products through biological, geological and chemical processes.

Water cycle

Water moves through evaporation, transpiration, condensation, precipitation, runoff and infiltration. Water availability can become an abiotic limiting factor for distribution and carrying capacity.

Carbon cycle

Carbon enters food webs through photosynthesis and returns through respiration, decomposition and combustion. Biomass production stores carbon temporarily in living organisms.

Nitrogen cycle

Nitrogen fixation converts atmospheric nitrogen into biologically usable forms. Nitrification, assimilation, ammonification and denitrification move nitrogen through soil, organisms and the atmosphere.

Process detail students are expected to use

| Cycle | Process | What moves or changes | Biological significance | | --- | --- | --- | --- | | Water | Evaporation | Liquid water becomes water vapour | Transfers water from oceans, lakes and soil to the atmosphere | | Water | Transpiration | Water vapour leaves plant stomata | Links plant physiology to local water movement | | Water | Infiltration and runoff | Water enters soil or flows over land | Affects soil moisture, erosion and aquatic habitats | | Carbon | Photosynthesis | Carbon dioxide becomes organic carbon | Producers make biomass and store chemical energy | | Carbon | Respiration | Organic carbon becomes carbon dioxide | Cells release usable energy and return carbon to air or water | | Carbon | Decomposition | Detritus is broken down by decomposers | Returns carbon compounds and nutrients to soil and water | | Carbon | Combustion | Stored carbon becomes carbon dioxide | Fire and fossil fuel burning rapidly release carbon | | Nitrogen | Nitrogen fixation | N2 becomes ammonia or ammonium | Makes atmospheric nitrogen available to food webs | | Nitrogen | Nitrification | Ammonium becomes nitrite then nitrate | Produces nitrate that plants can absorb | | Nitrogen | Assimilation | Plants absorb nitrogen compounds | Nitrogen enters proteins and nucleic acids | | Nitrogen | Ammonification | Organic nitrogen becomes ammonium | Decomposers recycle nitrogen from waste and dead matter | | Nitrogen | Denitrification | Nitrate becomes nitrogen gas | Returns nitrogen to the atmosphere |

Nitrogen is often a limiting nutrient because organisms need it for amino acids, proteins, ATP and nucleic acids, but most organisms cannot use atmospheric nitrogen gas directly. This is why changes in fertiliser use, legume abundance, runoff or decomposer activity can strongly affect productivity.

Human activity can disrupt cycles. Clearing reduces transpiration and carbon storage. Combustion increases atmospheric carbon dioxide. Excess fertiliser can increase nitrate runoff, causing algal blooms and then oxygen depletion as decomposers break down dead algae.

Linked cycle disruptions

Cycles interact. Deforestation can reduce carbon storage, reduce transpiration, increase runoff and increase erosion at the same time. A fertiliser pulse can alter the nitrogen cycle, increase algal biomass, reduce light penetration and then reduce dissolved oxygen when decomposers respire while breaking down dead algae.

Eutrophication is the enrichment of a water body with nutrients, especially nitrate or phosphate. It often begins with runoff from fertiliser, sewage or animal waste. Algae grow rapidly, then die in large amounts. Decomposers break down the dead algae and use oxygen during respiration. Dissolved oxygen falls, which can kill fish and other aerobic organisms.

Carbon can be stored in short-term reservoirs such as living biomass and long-term reservoirs such as fossil fuels, carbonate rocks and deep ocean sediments. Nitrogen can be stored in atmospheric nitrogen gas, soil ammonium, nitrate, organic matter and living biomass. Water can be stored in oceans, ice, groundwater, soil, organisms and the atmosphere.

In data questions, identify the reservoir and the process. Saying "nitrogen increases" is weaker than saying "nitrate concentration increased in the stream after fertiliser runoff."

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: Biogeochemical Cycles
• Khan Academy: Biogeochemical cycles
• Biology LibreTexts: Biogeochemical cycles/03%3A_Ecology/3.03%3A_Biogeochemical_Cycles)