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.