Notes · Environmental ScienceUK · A-Levels

Biogeochemical cycles

This chapter examines how the chemical elements essential to life are recycled between the living and non-living world. It develops the idea of stores and fluxes, works through the carbon and nitrogen cycles in detail, including the microbial processes that drive the nitrogen cycle, and analyses how human activities have disrupted both cycles, with consequences that reach into climate change, water quality and soil fertility.

4 sections~13 min reading time3 competenciesLevel Foundation 1 · Standard 1 · Advanced 2

T·0666 / 16

Exam profile

AO1 · Describe the carbon and nitrogen cycles, their stores, fluxes and the microbial processes involvedAO2 · Apply cycle knowledge to explain the consequences of human activities and interpret flux dataAO3 · Analyse imbalances between stores and evaluate strategies to restore the cycles

Operators:describeexplainanalyseevaluateapplyinterpret

basic level

AS-Level expects you to describe the main stores and processes of the carbon and nitrogen cycles and how human activity disrupts them.

higher level

The full A-Level requires the microbial detail of the nitrogen cycle, quantitative reasoning about stores and fluxes, and the evaluation of measures to restore the cycles.

Depth

Text

Contents · 4 sections

Biogeochemical cycles

Stores and fluxes: how a biogeochemical cycle works#

FoundationAQA 7447 3.2.4

A generalised store-and-flux system

Fig. 1A store stays constant when the input flux equals the output flux; an imbalance makes it grow or shrink.

A biogeochemical cycle is the movement of a chemical element between the four spheres of the Earth: the atmosphere, hydrosphere, lithosphere and biosphere. The word captures the idea that these cycles are biological (driven partly by living organisms), geological (involving rocks and sediments) and chemical (involving reactions that change the element's form). Because matter is conserved, the same atoms are used again and again, unlike energy, which flows through once and is lost as heat. This distinction between cycling matter and one-way energy flow, met in the life-processes chapter, is fundamental.

Every cycle can be described in terms of stores and fluxes. A store, or reservoir, is a place where the element resides, such as carbon dioxide in the atmosphere or nitrogen gas in the air; a flux, or flow, is a transfer of the element from one store to another, such as photosynthesis moving carbon from the atmosphere into plants. Stores are measured as amounts and fluxes as rates (amount per unit time). Learning to read a cycle as a set of boxes (stores) joined by arrows (fluxes) makes even a complex cycle manageable.

Stores turn over at very different rates. Some fluxes are fast, such as photosynthesis and respiration, which exchange carbon between the atmosphere and living things over days to years; others are slow, such as the formation of fossil fuels and carbonate rocks, which lock carbon away for millions of years. A store connected only by slow fluxes, such as fossil fuels, is effectively removed from the active cycle until something, such as human extraction and combustion, speeds up its release. Distinguishing fast from slow cycling is central to understanding human impact.

A store is in equilibrium when its inputs equal its outputs, so its size stays constant; when inputs and outputs are unbalanced the store grows or shrinks. Human activities disturb cycles by changing fluxes, for example by burning fossil fuels, which adds a large flux into the atmospheric carbon store faster than natural processes can remove it, so that store grows. Framing human impact as an imbalance between fluxes, rather than as a vague pollution, is the analytical approach the examination rewards.

Worked example

Reasoning about an imbalance

The atmospheric carbon store receives about 9 units per year from human activities but natural processes only remove about 5 of these extra units per year. Explain what happens to the store.

Compare the fluxes
The input from human activity (9 units) exceeds the extra natural removal (5 units).
Find the imbalance
About $9 - 5 = 4$ units per year remain in the atmosphere.
Conclude
Because the input exceeds the output, the atmospheric carbon store grows year on year, raising carbon dioxide concentration.

Result: The atmospheric carbon store grows by about 4 units per year, because inputs exceed outputs.

Typical mistakes

Saying elements are used up; the atoms are recycled through the cycle, unlike energy which is lost as heat.
Confusing a store (an amount) with a flux (a rate of transfer).

Active revision

Explain, using the ideas of stores and fluxes, why burning fossil fuels increases the amount of carbon dioxide in the atmosphere.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

The carbon cycle#

StandardAQA 7447 3.2.4

The carbon cycle

Fig. 2Carbon cycles between the atmosphere, biosphere, soil, oceans and lithosphere through named fluxes.

The carbon cycle moves carbon between the atmosphere (as carbon dioxide), the biosphere (as organic molecules in living things), the soil (as organic matter), the oceans (as dissolved carbon dioxide and carbonate) and the lithosphere (as fossil fuels and carbonate rocks). The biological heart of the cycle is the balance between photosynthesis, which fixes atmospheric carbon dioxide into organic molecules in producers, and respiration, which returns carbon dioxide to the atmosphere as organisms release energy. In an undisturbed system these two great fluxes are close to balance.

Carbon passes along food chains from producers to consumers as organic molecules, and returns to the atmosphere by respiration at every trophic level. When organisms die, decomposers break down their remains, respiring some carbon back to the atmosphere and adding the rest to the soil as organic matter; soil is a large and important carbon store. Where dead material is buried and does not fully decompose, it may over geological time be compressed into fossil fuels, or, in the sea, the shells and skeletons of marine organisms form carbonate rocks such as limestone. These are the slow parts of the cycle.

The oceans are the largest active store and exchange carbon dioxide with the atmosphere at the surface: carbon dioxide dissolves in cold water and is released from warm water, and marine organisms and the sinking of carbon to the deep ocean form the biological carbon pump. Combustion, whether the natural burning of vegetation or the human burning of fossil fuels, is a flux that returns carbon rapidly to the atmosphere. Being able to name each flux (photosynthesis, respiration, feeding, decomposition, combustion, sedimentation, dissolving) and the store it connects is exactly what structured questions test.

Among the stores, the distribution of carbon is instructive: soils hold more carbon than the vegetation growing on them, and both are dwarfed by the oceans and by the vast geological store of carbonate rock and fossil fuels. This matters because disturbing a large slow store, by burning fossil fuels or by releasing soil carbon through ploughing or drainage, transfers carbon into the fast atmospheric store and unbalances the cycle, linking directly to climate change.

Comparing land carbon stores

Fig. 3The land stores compared (illustrative values, gigatonnes of carbon): soil holds more carbon than vegetation or the atmosphere. The oceans and geological rocks are far larger still.

Worked example

Tracing a carbon atom

Describe how a carbon atom in atmospheric carbon dioxide could become part of an animal and then return to the atmosphere.

Fixation
The carbon dioxide is absorbed by a plant and fixed into glucose and other organic molecules by photosynthesis.
Transfer
The plant is eaten by an animal, so the carbon becomes part of the animal's organic molecules (feeding).
Return
The animal respires, releasing carbon dioxide back to the atmosphere; alternatively, when it dies, decomposers respire the carbon back.

Result: Carbon moves atmosphere to plant (photosynthesis) to animal (feeding) to atmosphere (respiration or decomposition).

Typical mistakes

Forgetting that plants respire as well as photosynthesise, and so return carbon to the atmosphere.
Overlooking the soil and ocean as major carbon stores and treating the atmosphere as the largest.

Active revision

Describe the route taken by a carbon atom from the atmosphere, through a food chain, and back to the atmosphere, naming the processes involved.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

The nitrogen cycle#

AdvancedAQA 7447 3.2.4

The nitrogen cycle

Fig. 4The nitrogen cycle is driven by microbes: fixation, nitrification, ammonification and denitrification.

Nitrogen is essential to life because it is a component of proteins and nucleic acids, yet the vast store of nitrogen gas in the atmosphere is unusable by most organisms because the triple bond in the nitrogen molecule is very stable. The nitrogen cycle is therefore largely a story of microbes that convert nitrogen between forms plants can and cannot use. Four microbial processes are central, and knowing which type of organism carries out each is the key to this cycle.

Nitrogen fixation converts atmospheric nitrogen gas into ammonium or ammonia that organisms can use. It is carried out by nitrogen-fixing bacteria, some free-living in the soil and some living in the root nodules of legumes in a mutualistic relationship; lightning fixes a smaller amount, and industrially the Haber process fixes nitrogen to make fertiliser. Ammonification (decomposition) is the release of ammonium from the breakdown of dead organisms and waste by decomposers (bacteria and fungi), returning nitrogen to the soil in a usable form.

Nitrification is the oxidation of ammonium to nitrite and then to nitrate by nitrifying bacteria; it requires oxygen, so it occurs in well-aerated soils. Nitrate is the main form in which plants absorb nitrogen, which they assimilate into proteins; animals then obtain nitrogen by eating plants. Denitrification is the reverse of fixation: denitrifying bacteria, active in waterlogged, oxygen-poor soils, convert nitrate back to nitrogen gas, returning it to the atmosphere. A well-drained soil favours nitrification and so keeps nitrogen available to plants, whereas a waterlogged soil favours denitrification and loses nitrogen, which is why drainage improves fertility.

Putting the processes together gives the cycle: nitrogen gas is fixed to ammonium, ammonium is nitrified to nitrate, nitrate is assimilated into plant and then animal protein, decomposition returns nitrogen as ammonium, and denitrification returns nitrogen gas to the atmosphere. The examination frequently gives the name of a process and asks for the organism and the change in form, or gives a soil condition and asks how it affects the cycle, so it is worth being able to move fluently between process, organism, form and condition.

Worked example

Linking soil condition to the cycle

A farmer notices poor crop growth on a badly drained field. Explain, using the nitrogen cycle, why waterlogging reduces the nitrate available to the crop.

Effect on nitrification
Nitrification needs oxygen, so in a waterlogged, oxygen-poor soil the conversion of ammonium to nitrate slows, reducing nitrate production.
Effect on denitrification
Waterlogged, anaerobic conditions favour denitrifying bacteria, which convert existing nitrate to nitrogen gas that is lost to the air.
Conclude
Less nitrate is made and more is lost, so nitrate becomes scarce and crop growth suffers; draining the soil restores aeration, favouring nitrification and retaining nitrate.

Result: Waterlogging slows nitrification and speeds denitrification, so nitrate is lost; draining the soil reverses both effects.

Typical mistakes

Confusing nitrification (ammonium to nitrate, needs oxygen) with denitrification (nitrate to nitrogen gas, in waterlogged soil).
Saying plants absorb nitrogen gas or ammonium as their main source; they mainly absorb nitrate.

Active revision

Explain why a waterlogged soil tends to become deficient in nitrate and how draining it would improve its fertility.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Human disruption of the cycles and its consequences#

AdvancedAQA 7447 3.2.4

Eutrophication

Fig. 5The sequence of eutrophication: nutrient enrichment leads through an algal bloom to oxygen depletion and death.

Human activity has substantially altered both cycles by adding large new fluxes. In the carbon cycle, the burning of fossil fuels transfers carbon from a slow geological store into the atmosphere far faster than natural processes remove it, and deforestation both releases stored carbon and removes a sink for future uptake; the result is a growing atmospheric carbon store and the enhanced greenhouse effect. Framing this as an imbalance of fluxes, rather than simply as pollution, connects the cycle directly to climate change.

In the nitrogen cycle, the industrial fixation of nitrogen by the Haber process to make fertilisers, together with the combustion of fossil fuels, has roughly doubled the rate at which nitrogen is fixed compared with natural processes. Much of this reactive nitrogen escapes from farmland into water and air. The consequences include eutrophication of rivers, lakes and coastal waters, the acidification of soils and waters, and the emission of nitrous oxide, itself a potent greenhouse gas and an ozone-depleting substance, so a single disruption has several distinct effects.

Eutrophication is the most examined consequence and follows a clear sequence. Excess nitrate and phosphate wash from fields into water; the added nutrients cause a rapid growth of algae, an algal bloom, at the surface; the bloom blocks light so that submerged plants die; the dead plants and algae are decomposed by bacteria, which multiply and use up the dissolved oxygen in their respiration; and the resulting lack of oxygen kills fish and other aerobic organisms. The chain from nutrient enrichment to oxygen depletion to death is a favourite structured question, and each step should be linked to the next by cause and effect.

Restoring the cycles means reducing the disturbing fluxes and, where possible, strengthening the natural sinks. For carbon, this means cutting fossil-fuel use, preventing deforestation and restoring forests and soils; for nitrogen, it means applying fertiliser more precisely and at the right time, using buffer strips to intercept runoff, and restoring wetlands that remove nitrate by denitrification. Evaluating such measures, weighing their effectiveness against their cost and practicality, is the higher-level skill, and it recurs in the chapters on agriculture, water and sustainability.

Worked example

Explaining a fish kill

Fertiliser runs off a field into a slow-moving river, and weeks later many fish are found dead. Explain the connection using the process of eutrophication.

Enrichment and bloom
The runoff adds nitrate and phosphate, so algae grow rapidly and form a bloom that covers the surface.
Death and decomposition
The bloom blocks light, so submerged plants die; bacteria decompose the dead plants and algae and multiply.
Oxygen depletion
The multiplying bacteria respire aerobically and use up the dissolved oxygen, so the fish, which need oxygen, suffocate and die.

Result: Nutrient enrichment caused a bloom whose decomposition by bacteria removed the dissolved oxygen, killing the fish.

Typical mistakes

Saying algae use up the oxygen directly; it is the bacteria decomposing the dead algae and plants that deplete the oxygen.
Treating nitrogen pollution as a single effect; it causes eutrophication, acidification and nitrous-oxide emission.

Active revision

A lake near intensively farmed land develops an algal bloom and then a fish kill. Explain the sequence of events, linking each step to the next.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Biogeochemical cycles

4 sections~13 min reading time3 competenciesLevel Foundation 1 · Standard 1 · Advanced 2

Biogeochemical cycles

Key points

Reasoning about an imbalance

Key points

Tracing a carbon atom

Key points

Linking soil condition to the cycle

Key points

Explaining a fish kill

Sources

Biogeochemical cycles

Key points

Reasoning about an imbalance

Key points

Tracing a carbon atom

Key points

Linking soil condition to the cycle

Key points

Explaining a fish kill

Sources