2.3 Biogeochemical cycles

Biogeochemical cycles move finite elements between organisms and abiotic stores; uptake makes an element available for life and decomposition returns it.

Follow the element, not just the organism: identify its store, the process moving it, and whether a human action speeds or blocks that flow.

Nitrogen in soil nitrate enters a plant, moves to a herbivore, returns in waste, then microbes convert it back to an inorganic form.

Label the element’s current store and the process on the arrow; this prevents confusing a transfer with a transformation.

Cycles reuse matter, but they do not mean every atom returns quickly or through the same route.

Compare total input I with output O: I>O means net accumulation (sink), O>I means net release (source), and I=O means balance.

The label describes the net flux during the chosen time interval. A store can change role when conditions or rates change.

A wetland receives 12 units of carbon and releases 9, so 3 units accumulate: it is a net sink for that interval.

Source, with net release of 3 units; do not keep the old label after rates change.

A sink is not an empty store—it is a store whose input exceeds output over the period measured.

A carbon store is a pool; residence time is how long carbon typically remains there before leaving.

Atmosphere, soil and ocean hold inorganic carbon forms; organisms and fossil fuels hold organic carbon in the guide’s classification. Residence time helps distinguish fast biological exchange from slow geological storage.

A leaf may exchange carbon within months, while carbon buried in fossil fuel can remain isolated for millions of years.

Atmosphere; limestone’s geological residence time is much longer.

A large store is not automatically a fast store; size and residence time are different properties.

Photosynthesis moves CO₂ into organic carbon; respiration returns it; feeding, death, waste and decomposition move carbon among organisms and detritus.

Name both the starting store and the process. Photosynthesis is a transformation, while feeding is mainly a transfer of carbon between organisms.

Carbon in a leaf moves to a caterpillar by feeding, then returns to the atmosphere through the caterpillar’s respiration.

Respiration (and combustion), not feeding; feeding changes organismal store but does not by itself release CO₂.

Death is not decomposition: death moves carbon to detritus, while decomposers process that material.

Sequestration removes atmospheric CO₂ into a store; its climate value depends on how much is captured and how long it stays there.

Trees capture CO₂ into biomass and soil, but fire or decomposition can return it. Geological storage is slower to reverse, so permanence and leakage risk matter.

A planted forest stores carbon for 30 years, then burns; the earlier uptake counts, but the stock is not durable unless regrowth or another store replaces it.

No. Assess the net atmospheric change over the relevant timescale and the risk of re-release.

Calling a project ‘carbon neutral’ from gross planting alone ignores later losses and displaced emissions.

Compare photosynthetic uptake P with total release R: P>R is a sink, P≈R is balanced, and R>P is a source.

Young growth can fix carbon faster than it respires; fire, drought or decay can reverse the balance. These are rate comparisons, not permanent labels for an ecosystem type.

If a forest fixes 100 units and releases 70, net uptake is 30; if fire raises release to 130, net release is 30.

No. Measure P and R for the stated period; age is only a clue, not the calculation.

Do not equate high biomass with current net uptake; a large old stock can be near balance or a source.

Burial can isolate ancient organic carbon for geological time; extraction and combustion move it to atmospheric CO₂ quickly.

A slow geological store becomes a rapid source when humans change the rate of release. The key contrast is residence time, not whether carbon is ‘natural’.

Coal formed from old plant material over long timescales, but burning a truckload releases its carbon in hours.

It transfers a long-isolated store into the atmosphere much faster than natural sinks usually remove it.

‘Natural origin’ does not mean a present-day release is balanced or harmless.

Judge a farming practice by whether it adds durable carbon to soil and biomass faster than it releases or exports it.

Cover crops, residues and reduced disturbance can increase inputs and reduce oxidation or erosion; drainage, heavy tillage and burning can increase losses. The balance is site- and time-dependent.

A cover crop adds roots before the cash crop; if residues remain in soil, carbon input rises, but measuring soil stock is still needed to verify a sink.

No. Check whole-system inputs, decomposition, erosion and whether gains persist at depth.

One practice label cannot replace a net carbon balance over a defined boundary and period.

CO₂ moves both ways between air and sea; the net direction depends on concentration difference, temperature and mixing.

A colder or CO₂-poor surface can absorb more, while warming or upwelling can favour outgassing. Oceans are a net sink overall, not a one-way pipe.

If atmospheric CO₂ rises while ocean uptake increases more slowly, ocean carbon still rises even though the ocean remains a net sink.

Yes. Sink means total uptake exceeds total release; it does not mean release is zero.

Do not infer net flux from one local measurement or one season; compare both directions over the stated boundary.

More dissolved CO₂ changes carbonate chemistry, lowers pH and can make calcium-carbonate shell or skeleton building harder.

pH is logarithmic, so a small numerical fall represents a meaningful increase in hydrogen-ion concentration. Organism response depends on species and local conditions, but calcification can slow or weaken.

If a reef site shifts from pH 8.2 to 8.1, treat it as a real chemical change, not ‘only 0.1’, then test calcification data.

The change is chemical: carbonate availability and pH alter the reaction conditions even when the water appears normal.

Lower pH means more acidic, not ‘no carbon’; and it does not prove every species responds identically.

A robust carbon strategy cuts source flows and protects or builds stores that keep carbon out of the atmosphere for long periods.

Efficiency and low-carbon energy reduce new release; forest and soil protection reduce losses; durable capture can address residual emissions. Each action has limits and leakage or land trade-offs.

Replacing coal cuts a source, while restoring a wetland protects a store; neither alone compensates automatically for all remaining emissions.

Reduction lowers the inflow immediately; restoration cannot safely offset an unlimited or continuing source.

A tree-planting claim is not a substitute for accounting the permanence, additionality and emissions avoided.

Fossil fuels and limestone hold carbon for geological times because burial and rock cycling isolate it from rapid biological exchange.

Organic carbon is stored in coal, oil and gas; inorganic carbon is stored in carbonate rock. Their long residence time makes their natural release slow compared with combustion or mining.

A limestone deposit can retain carbon for millions of years, but heating it for cement production transfers part to CO₂ much faster.

Carbonate rock and geological isolation slow exchange; the store is not rapidly available to organisms.

Long residence time does not mean the store is irrelevant; extraction can change its flux abruptly.

Corals and molluscs build calcium-carbonate hard parts; after burial, some become part of long-lived limestone stores.

Dissolved carbon is incorporated into shell or skeleton, then death, sedimentation and lithification can transfer it to rock. This is one pathway among several, not a complete cycle by itself.

A dead mollusc shell settles on the seabed; buried carbonate may later lithify, moving carbon from a biological store to the lithosphere.

Burial and lithification; merely producing a shell does not guarantee long-term storage.

Not every shell is preserved, and limestone formation is not the same as immediate shell growth.

Coal mainly formed from buried land plants; oil and natural gas mainly formed from buried marine organic matter under suitable geological conditions.

Low oxygen, burial, pressure, heat and long time limited decomposition and transformed the material. These conditions were uneven, so not every deposit forms the same fuel.

A coal seam points to ancient plant-rich wetlands; an oil source rock points to organic-rich marine sediments rather than simply ‘old dead matter’.

Fuel type depends on original biomass and formation environment, not age alone.

Geological time is necessary but not sufficient; source material and preservation conditions matter.

Methanogens can produce methane when dead organic matter is abundant and oxygen is absent or very limited.

Anaerobic conditions occur in wetlands, flooded rice fields, landfills and ruminant guts. Organic matter alone is not enough: oxygen status and microbial conditions control production.

A waterlogged landfill cell with food waste and little oxygen is more methane-prone than a dry, aerated compost pile.

Oxygen exposure; methanogenesis is favoured when competing aerobic decomposition cannot use the material.

Methane is not produced by every decomposer or in every wet place; check substrate and anaerobic conditions.

Methane persists for roughly a decade on average, yet is a potent greenhouse gas while present; lifetime and potency are different properties.

Oxidation eventually converts methane mainly to CO₂, but the climate effect during its atmospheric lifetime matters. Cutting emissions can reduce methane forcing relatively quickly.

A short-lived methane pulse can cause strong near-term warming, while a continuous leak keeps replacing the methane removed by oxidation.

No. Compare both concentration, radiative effect and the time horizon; a continuing source prevents decline.

‘Short-lived’ does not mean ‘weak’; persistence and greenhouse potency must not be collapsed into one label.

Organic nitrogen is in proteins and other carbon-based matter; inorganic nitrogen includes N₂, ammonium, nitrite and nitrate.

Plants generally cannot use atmospheric N₂ directly, so microbial conversion to soluble forms controls biological availability. The atmosphere is the largest pool, not necessarily the most accessible.

Protein in a dead leaf is organic nitrogen; nitrate dissolved in soil water is inorganic and can be taken up by roots.

Nitrate, because roots can absorb it; N₂ requires fixation first.

Largest pool does not equal usable pool; classify by chemical form and access.

Fixation makes ammonia from N₂; nitrification makes nitrate from ammonia; denitrification returns N₂ from nitrate; ammonification makes ammonium from organic nitrogen.

Name the starting and ending chemical forms before choosing the process. Microbes control access to nitrogen by changing form, not by creating atoms.

Nitrate in water becomes atmospheric N₂ under anoxic denitrification; dead protein becoming ammonium is ammonification.

Nitrification; it is oxidation through nitrite to nitrate.

Do not use ‘fixation’ for every nitrogen change; fixation specifically starts with atmospheric N₂.

Waterlogging reduces oxygen diffusion, favouring denitrification and leaching that remove plant-available nitrogen.

Low oxygen lets denitrifiers use nitrate and return N₂; moving water can carry nitrate away. Some insect-eating plants supplement nitrogen but still photosynthesize for carbon and energy.

After heavy rain, a saturated field may show falling nitrate and poorer crop growth even though total nitrogen remains in the wider system.

It changes oxygen and water movement, which changes denitrification and leaching rates.

Waterlogging does not destroy nitrogen; it changes chemical form or moves it out of root access.

Legumes house nitrogen-fixing microbes in root nodules: bacteria receive sugars and habitat, while the plant receives fixed nitrogen compounds.

The partnership matters where usable nitrogen limits growth. The plant still obtains carbon energy through photosynthesis; bacteria provide access to atmospheric nitrogen by fixation.

Beans may grow better in nitrogen-poor soil when nodules are active, but the benefit depends on compatible microbes and conditions.

Both partners gain: sugars and shelter are exchanged for fixed nitrogen.

The plant does not fix N₂ alone; do not confuse hosting bacteria with absorbing nitrate.

Label a nitrogen arrow by what moves and where: uptake enters producers, consumption enters consumers, death and waste enter detritus, and decomposition returns inorganic forms.

Photosynthesis builds carbon compounds but does not itself name a nitrogen flow; mineral uptake and ammonification identify the nitrogen-specific steps.

Nitrate → plant protein is mineral uptake; dead plant protein → ammonium is ammonification.

Excretion/defecation transfers nitrogen to waste; it is not decomposition until microbes process that waste.

Do not label every arrow ‘decomposition’; reserve it for breakdown of dead organic material.

Deforestation, fertilizer, aquaculture, sewage and combustion add or redirect reactive nitrogen, which can cause eutrophication and air or water pollution.

Identify the activity, nitrogen form and receiving store. Excess inputs can exceed plant uptake, move downstream or deposit from air; effects depend on dose and pathway.

Fertilizer nitrate not taken up by crops leaches into a lake, stimulating algae and later oxygen depletion.

A pathway from excess nitrogen to algal growth, decomposition and reduced oxygen, with timing and evidence.

Correlation with farming is not enough; trace the nitrogen form and mechanism to the impact.

The Haber process makes ammonia from nitrogen and hydrogen, increasing plant-available nitrogen but carrying energy and pollution costs.

Fertilizer can raise yields where nitrogen limits growth. Energy use and excess application can add emissions, leaching, runoff and eutrophication, so evaluate the whole balance.

Applying 100 kg N raises yield only if crop uptake increases; the unused fraction can leave the field and harm water quality.

No. Include production energy, nitrogen-use efficiency and downstream losses.

‘Synthetic’ is not the same as ‘always harmful’; judge dose, efficiency, source energy and receiving ecosystem.

A nitrogen boundary is pressured when reactive nitrogen enters ecosystems faster than they can safely process, causing linked soil, water and atmospheric effects.

Separate the measurable input from the boundary judgement. Fertilizer, combustion and waste can increase reactive forms; impacts vary by region and require current evidence.

A river’s nitrate rises after fertilizer application and downstream oxygen falls; the paired observations support a nitrogen pathway, not a universal global rate.

Human pressure exceeds a proposed safe operating level; it is not a claim that every ecosystem has the same threshold or outcome.

Do not present a boundary estimate as a timeless local measurement; keep scale, uncertainty and evidence visible.

Effective nitrogen management coordinates farms, wastewater, combustion and monitoring because nitrogen crosses boundaries between systems.

Combine better fertilizer timing and efficiency with nutrient recovery, sewage control, NOₓ reduction and measurements of losses. Trade-offs include yield, cost, access and who bears pollution.

A farm cuts fertilizer loss, a city recovers nitrogen from wastewater and a regulator monitors river nitrate; together they reduce the same downstream load.

Nitrogen moves downstream and through air, so uncoordinated sources can cancel local improvements.

A single technology is not a system solution; verify the complete pathway and distribution of benefits and costs.