A biogeochemical cycle is the route a chemical element takes between Earth’s living and non-living components. “Bio” refers to organisms, “geo” to Earth materials, and “chemical” to the element moving through the cycle.

Energy enters ecosystems and is eventually dispersed as heat. Matter, by contrast, is reused. Organisms take up elements, which later return to the air, water or soil and become available again. Carbon, for example, may pass from atmospheric $CO_2$ into plant biomass before respiration or decomposition returns it.

This recycling keeps biological molecules supplied with the elements needed to build them. Human activities may change the size of stores, as well as the rate or direction of flows. When inputs or removals become seriously unbalanced, the effects can spread to nutrient availability, productivity, biodiversity and therefore ecosystem sustainability. Human activity changes nutrient cycling, and these changes can weaken the long-term functioning of environmental systems.

A store, or storage, is a part of a system where an element is held in equilibrium with the environment. Absorption and release are balanced, so the quantity stays broadly stable even while material enters and leaves.

A sink accumulates an element because input exceeds output. A source does the reverse: output exceeds input, causing a net release. These labels describe the current balance of flows rather than a fixed identity. If conditions or management change, the same forest or soil can switch from sink to source.

Systems diagrams use rectangular boxes for stores and arrows for flows. The arrows point in the direction that matter moves, while their width may show the magnitude of the flow. By comparing total inputs with total outputs, you can determine whether a component acts as a store, sink or source.

An organic carbon store holds carbon in living organisms, their remains or materials formed from past organic matter. Organisms, crude oil and natural gas are organic stores. Biomass contains carbon in carbohydrates, lipids, proteins and nucleic acids.

An inorganic carbon store holds carbon in non-living forms, including atmospheric or dissolved $CO_2$ and carbon compounds in soils and oceans. Take care with the ESS classification: $CO_2$ and carbonates count as inorganic carbon even though they contain carbon.

Residence time is the average period that a carbon atom remains in a store. The size of the store and the rates of flow into and out of it determine this period. Without mining, fossil carbon would stay underground for hundreds of millions of years. Human extraction sharply reduces that effective residence time, transferring ancient carbon into the active carbon cycle.

The carbon cycle is a biogeochemical cycle that moves carbon between living organisms, the atmosphere, water, soil and geological stores.

A transfer moves matter from one location or store to another without altering its chemical form. For example, feeding transfers organic carbon from prey to consumer. Defecation moves carbon into waste, while death moves biomass into dead organic matter. A transformation, by contrast, changes the chemical form or state of matter. Photosynthesis transforms atmospheric $CO_2$ into organic carbon, cellular respiration transforms organic carbon into $CO_2$, and decomposition converts dead organic matter into simpler organic and inorganic forms.

The principal ecosystem flows are:

• Photosynthesis: producers convert atmospheric or dissolved $CO_2$ into organic matter.
• Feeding: organic carbon moves through food chains.
• Defecation: unabsorbed organic carbon passes into detritus or soil.
• Cellular respiration: organisms release carbon as $CO_2$.
• Death: carbon stored in biomass moves into dead organic matter.
• Decomposition: decomposers process dead material and release carbon, including through respiration.

When drawing a systems diagram, use rectangular boxes for the atmosphere, producers, consumers, dead organic matter, soil, oceans and fossil fuels. Join the boxes with directional arrows labelled by process. Check the direction of each arrow: photosynthesis must point from $CO_2$ to producers, whereas respiration points back towards atmospheric or dissolved $CO_2$.

Carbon sequestration is the process of capturing gaseous and atmospheric carbon dioxide and storing it in a solid or liquid form. It removes $CO_2$ from the atmosphere, keeping the carbon in a different store.

Trees do this naturally. During photosynthesis, they absorb $CO_2$ and convert it into biomass. Some of that carbon enters wood and soils, where it can remain for years or even centuries. Over geological time, buried organic matter may be fossilized into coal, oil and natural gas. Artificial sequestration captures $CO_2$ and deliberately places it in a long-lived liquid or solid-associated store.

An ecosystem’s role depends on the balance between its total carbon inputs and outputs. When inputs and outputs are equal, the ecosystem acts as a store in equilibrium. If inputs are greater, carbon builds up and the ecosystem acts as a sink. If outputs are greater, it acts as a source.

Photosynthesis provides a major carbon input; cellular respiration is a major output. When photosynthesis exceeds respiration, there is a net uptake of $CO_2$. When respiration exceeds photosynthesis, there is a net release.

Forest development shows this distinction clearly:

• A young forest usually gains biomass quickly. Photosynthesis exceeds respiration, so the forest acts as a sink.
• In a mature forest, photosynthesis and respiration may be close to balance, making it mainly a store.
• A forest destroyed by fire or deforestation becomes a source. Combustion, decomposition and soil disturbance release carbon, while photosynthetic uptake falls.

These labels can change over time. A large carbon store isn’t necessarily a sink; it is only a sink while the quantity of stored carbon is increasing.

Fossil fuels are carbon-rich geological materials formed from organic matter that built up when past ecosystems acted as carbon sinks. Coal, oil and natural gas keep this carbon outside the rapid biological cycle.

The guide treats these fuels as stores with unlimited residence times. Without human extraction and combustion, the carbon would remain stored on human timescales. Burning fossil fuels turns them into carbon sources: combustion moves ancient geological carbon into the atmosphere as $CO_2$, adding it far faster than slow geological processes can replace the store.

At this level, concentrate on the store-to-source relationship rather than detailed geological formation or the dates of individual fuels.

Agricultural land may store, build up or release carbon, depending on how it is managed. What matters is how much organic matter enters the soil, how quickly it decomposes, and whether existing soil carbon is exposed or removed.

Regenerative methods generally encourage soil to act as a carbon sink:

• Crop rotation changes the crops grown, helping to maintain productive roots and plant residues.
• Cover crops leave living roots in the soil between harvests and supply organic matter.
• No-till farming limits soil disturbance, so less soil organic matter is exposed to rapid decomposition.

Wetland drainage has the opposite effect. It brings oxygen into carbon-rich soils, speeding up decomposition. Monoculture may reduce the diversity and continuity of organic inputs. Heavy tillage also breaks up soil structure and exposes organic matter. As a result, these practices encourage soil to act as a carbon source.

Timescale matters, as does what happens to harvested material. Long-lived crops may accumulate carbon over many years, though harvesting moves that carbon elsewhere. Durable products store it for longer than material that is soon burned or decomposed.

Atmospheric $CO_2$ dissolves into the ocean at the air–water boundary. The reverse can also happen: dissolved $CO_2$ can leave the solution and return to the atmosphere as a gas. Exchange runs in both directions.

At present, the oceans absorb more $CO_2$ globally than they release, making them a net carbon sink. However, not every ocean region absorbs carbon all the time. Temperature, circulation and concentration gradients influence both the local direction and the rate of exchange.

Fossil-fuel combustion by humans releases inorganic carbon faster than the oceans can absorb it. As a result, atmospheric carbon keeps accumulating even though the oceans take up a substantial share of emissions. This ocean uptake slows the accumulation, but it doesn’t remove the cause of the imbalance.

Ocean acidification is a chemical change in seawater caused when the ocean absorbs additional atmospheric $CO_2$, lowering its pH. The dissolved carbon dioxide reacts with water and forms carbonic acid:

$$CO_2 + H_2O \to H_2CO_3$$

This reaction reduces the amount of carbonate available to deposit calcium carbonate, $CaCO_3$. As a result, even a small drop in pH can make it harder for molluscs and reef-building corals to form shells and skeletons. Existing calcium carbonate structures may become more vulnerable to dissolution too.

Biological effects can include slower growth, weaker shells or skeletons, reduced survival and damage to reef habitat. Reefs support complex food webs, so changes affecting calcifying organisms can spread through the wider ecosystem.

Effective management has two jobs: reduce the flow of new carbon into the atmosphere and increase secure uptake. Since no single measure tackles every source, several approaches must work together.

• Low-carbon technologies cut demand for fossil-fuel combustion, particularly when they run on low-carbon electricity.
• Reduced fossil-fuel burning directly slows the transfer of carbon from geological stores into the atmosphere.
• Reduced soil disruption protects soil organic carbon and may allow more to accumulate.
• Reduced deforestation prevents emissions from combustion and decomposition while maintaining photosynthetic uptake.
• Reforestation increases biological sequestration in biomass and soils. However, uptake takes time, and fire or clearance can release the stored carbon again.
• Artificial sequestration captures $CO_2$ and transfers it to a longer-lived store. It requires infrastructure and energy, as well as reliable long-term containment.

Each measure should be judged by its net effect across the whole system. A technology isn’t genuinely low-carbon if manufacturing it and supplying its energy simply shift large emissions elsewhere.

The lithosphere is Earth’s solid, rocky outer layer. It stores carbon in fossil fuels and in rocks such as limestone, which contains calcium carbonate, $CaCO_3$.

Under natural conditions, carbon enters and leaves lithospheric stores extremely slowly. Its residence time can reach hundreds of millions of years. As a result, these stores remain largely separate from the rapid carbon exchanges between the atmosphere, organisms, soils and surface waters—until extraction or other human disturbance speeds up its release.

Reef-building corals deposit calcium carbonate in their skeletons; molluscs deposit it in their shells. After the organisms die, some of these hard parts accumulate and become buried, eventually contributing to limestone.

Limestone is the largest carbon store in Earth systems. But not every limestone deposit consists entirely of fossilized animal remains. It can form through biological or non-biological processes, though the chemical and geological details of these pathways aren’t required here.

The sequence to know is: carbon in dissolved inorganic compounds → incorporation into calcium carbonate hard parts → burial → long-term storage in limestone.

During past geological eras, some dead organic matter didn’t decompose completely. Partially decomposed terrestrial plants became fossilized in coal, while partially decomposed marine organisms contributed to oil and natural gas that accumulated in porous rocks.

The conditions needed to form these fuels were especially common during particular geological eras. Even so, significant stores took tens of millions of years to accumulate. We now extract fossil fuels enormously faster than they form, so they’re non-renewable on human timescales.

Anaerobic conditions are environments where molecular oxygen is absent or extremely limited. Methanogenic bacteria are anaerobic microorganisms that produce methane from dead organic matter through a process called methanogenesis.

These conditions are found in waterlogged swamps and rice paddies because oxygen cannot diffuse easily into saturated material. They also occur inside the digestive systems of cattle and other ruminants. In both settings, abundant organic matter combined with a lack of oxygen allows methane, $CH_4$, to be produced.

Methane remains in the atmosphere for about 10 years. It doesn’t stay unchanged forever; atmospheric oxidation eventually converts it to $CO_2$.

As a greenhouse gas, methane absorbs outgoing infrared radiation in the atmosphere and contributes to atmospheric warming. It is potent, so methane released today can cause strong warming during its comparatively short atmospheric lifetime. Once converted to $CO_2$, its carbon remains in the broader carbon cycle even though the methane itself has been removed.

The nitrogen cycle is a biogeochemical cycle that moves nitrogen between the atmosphere, organisms, dead organic matter, soil and water.

Proteins and other nitrogenous carbon compounds in living organisms and dead organic matter form the organic nitrogen stores. Inorganic stores include atmospheric nitrogen, ammonia, ammonium, nitrites and nitrates in soil and water.

Atmospheric dinitrogen, $N_2$, forms the largest atmospheric store, although most plants can’t use it directly. Bacteria transform it into biologically available compounds. Producers then incorporate these compounds into organic matter, while food-web transfers move nitrogen through organisms.

For a systems diagram, draw boxes for atmospheric $N_2$, soil ammonium, soil nitrates, producers, consumers and dead organic matter. Connect them with labelled arrows showing fixation, nitrification, uptake, consumption, excretion, death, decomposition, ammonification and denitrification.

Bacteria carry out transformations that make atmospheric nitrogen available to organisms. Eventually, they return it to the atmosphere.

Nitrogen fixation is conversion of nitrogen from the atmosphere into ammonia. During this process, nitrogen-fixing bacteria transform $N_2$ into $NH_3$.

Nitrification is conversion of ammonia to nitrates. This happens in stages: ammonium is first converted to nitrites, then to nitrates. You don't need to know the detailed chemical reactions.

Denitrification is conversion of nitrates to nitrogen. Denitrifying bacteria return nitrogen to the atmospheric $N_2$ store.

Decomposition is the conversion of amino acids into ammonium. Decomposers break down the organic nitrogen found in dead material and waste. In Ammonification, decomposers transform organic nitrogen into ammonia or ammonium.

These processes are easy to mix up, so track the form of nitrogen. Atmosphere to ammonia is fixation. Ammonia or ammonium moving toward nitrate is nitrification, while nitrate returning to atmospheric nitrogen is denitrification. Organic nitrogen changing to ammonium is decomposition and ammonification.

Denitrification only occurs under anaerobic conditions. When soil becomes waterlogged, water fills its pores and greatly slows oxygen diffusion. This creates suitable conditions for denitrifying bacteria.

These bacteria convert nitrates to nitrogen gas, which is then lost from the soil. Nitrates can also be removed by leaching, the downward transport of dissolved substances through soil water. As less nitrogen is available, plant growth slows or may stop altogether.

Some insectivorous plants have adapted to these nitrogen-poor habitats. Rather than relying only on mineral nitrogen from the soil, they trap and digest insects, using the insects’ tissues as an additional nitrogen source. This allows them to obtain nitrogen where ordinary root uptake is strongly limited.

Plants can’t fix nitrogen themselves. Atmospheric dinitrogen is therefore unavailable to them unless they form a mutualistic association with nitrogen-fixing bacteria.

Mutualism is an interspecific relationship in which both species benefit. Leguminous plants, including peas and clover, provide nitrogen-fixing bacteria with sugars and a protected habitat inside root nodules. In return, the bacteria fix atmospheric $N_2$ into nitrogen compounds that the plant can ultimately use to make amino acids and proteins.

This association gives legumes a competitive advantage where available nitrogen limits plant growth. Even when soil nitrate is scarce, they can still obtain usable nitrogen, allowing greater growth, survival or reproduction than plants that depend entirely on the soil nitrate store.

Nitrogen moves through ecosystems in transfer and transformation flows.

Transfer flows move nitrogen from one place or organism to another without necessarily changing its chemical form. Examples include mineral uptake by producer roots, consumption along food chains, excretion and death. They also include the movement of dead material to decomposers.

Transformation flows change nitrogen’s chemical form. As producers grow through photosynthesis, they use absorbed mineral nitrogen to make amino acids and proteins. Decomposition breaks complex organic material into simpler substances, while ammonification converts organic nitrogen into ammonia or ammonium. Other transformations—fixation, nitrification and denitrification—link atmospheric, soil and biological stores.

A complete systems diagram of the nitrogen cycle should include mineral uptake, photosynthesis and organic synthesis in producers, consumption, excretion, death, decomposition and ammonification, as well as the three named bacterial conversions.

Human activities can alter nitrogen stores, as well as the rates and directions of nitrogen flows.

Deforestation

When vegetation is removed, less organic nitrogen remains stored in biomass. The exposed soil becomes more vulnerable to erosion and leaching, so its nitrogen stores decline. Dead residues may also decompose and release nitrogen compounds.

Agriculture

Inorganic fertilizer raises soil nitrogen above natural fixation rates. Harvesting crops removes nitrogen, while runoff and leaching carry excess nitrates into aquatic systems, where they can contribute to eutrophication. Farming activities that kill soil organisms or simplify habitats may also change the rates of decomposition, nitrification and denitrification.

Aquaculture

Cultured animals excrete ammonia. Uneaten feed and faecal material also decompose, adding more biologically available nitrogen. If a system is poorly flushed, this nitrogen builds up and degrades water quality.

Urbanization

Clearing habitats removes nitrogen stored in biomass. At the same time, concentrated sewage carries ammonium and nitrates into rivers, lakes and coastal waters. Greater urban demand for food also moves nitrogen through agricultural production, transport, consumption and waste disposal.

For each case, trace the nitrogen. Identify its original store, the flow that changed and the store receiving it. This shows how a local activity can produce downstream effects.

The Haber process is an industrial process that produces ammonia from nitrogen and hydrogen for use as fertilizer. By industrializing nitrogen fixation, it greatly increases the supply of reactive nitrogen available for growing crops.

The main advantage is a higher crop yield. Nitrogen often limits plant growth, so adding fertilizer can boost growth and help produce food for a large human population. Farming also becomes possible in places where natural nitrogen inputs or recycling wouldn’t provide enough.

There are disadvantages throughout the system. Ammonia production uses substantial amounts of energy, often from fossil fuels, so the manufacturing stage adds to greenhouse-gas emissions. Crops don’t absorb all the fertilizer applied. Nitrate may leach or run off into water, where it promotes eutrophication and biodiversity loss. Using too much fertilizer causes pollution and wastes resources.

Haber-process fertilizer has played a central role in food security, but uncontrolled or inefficient use is environmentally unsustainable. More precise application, nutrient recycling and reduced losses can keep the yield benefits while limiting the damage.

A planetary boundary is a proposed limit for a major Earth-system process. Beyond it, the risk of large-scale and potentially irreversible environmental change rises sharply.

Humans have increased nitrate availability far above natural rates by fixing and applying nitrogen. The cited boundary for intentionally fixed nitrogen applied to agriculture is $62\ \text{Tg N year}^{-1}$, yet estimated anthropogenic input is about $190\ \text{Tg N year}^{-1}$. Human input is more than three times the boundary value.

The numerical overshoot is only part of the evidence. Excess reactive nitrogen moves from agricultural and urban land into rivers and coastal waters. There, it contributes to eutrophication and oxygen depletion, as well as biodiversity loss. Nitrogen pollution also interacts with climate change and biosphere integrity.

Global crop production relies heavily on inorganic fertilizers. At this scale, the Haber process is the main mechanism used to convert atmospheric nitrogen into fertilizer, so this dependence is a principal cause of the boundary being crossed. That doesn’t mean every change becomes irreversible immediately. It means the likelihood of severe and irreversible Earth-system change has become unacceptably high.

Nitrogen crosses political borders in rivers, groundwater and air, as well as through food trade and marine systems. As a result, one country may face costs that arise partly from production or consumption elsewhere. Local controls alone aren’t enough; countries need to coordinate their actions.

Measures include:

• setting national targets for fertilizer losses and nitrogen emissions;
• regulating excessive fertilizer use, with stronger enforcement;
• using precision application to match fertilizer supply to crop need;
• safely recycling nutrients from manure, sewage and organic waste;
• improving soil management and introducing crop rotations with nitrogen-fixing plants;
• cutting ammonia and nitrogen losses from intensive livestock production;
• collecting comparable data and sharing national action plans;
• reducing food waste and demand for nitrogen-intensive livestock products through dietary change;
• backing international commitments that reduce excess nutrients entering the environment.

Global agreements can set common goals. Their success, however, depends on monitoring, finance, technology transfer and fair implementation. Lower-income producers may need support so they can improve nutrient efficiency without sacrificing food security. Farmers aren’t solely responsible: consumers, businesses and governments also shape the system.

To bring the cycle back within planetary boundaries, new reactive nitrogen inputs must fall, while useful nitrogen must be prevented from turning into pollution. The strongest strategy combines efficient production and nutrient recovery with changed consumption and enforceable international cooperation.