Biogeochemical cycles
Life depends on a finite set of chemical elements: carbon, nitrogen, phosphorus, and others that must be continuously recycled. Unlike energy, which flows in one direction (sunlight in, heat out), matter circulates.
A biogeochemical cycle is a natural pathway through which a chemical element moves between the biotic (living) and abiotic (non-living) components of Earth: the atmosphere, hydrosphere, lithosphere, and biosphere.
These cycles allow the same nitrogen atom that sits in your DNA today to have once formed part of a Cretaceous fern. Without continuous recycling, essential nutrients would become locked in inaccessible compartments and life would cease.
The four major Earth spheres involved in cycling:
- Biosphere: all living organisms and their interactions
- Atmosphere: the gas envelope surrounding Earth, containing N₂, O₂, CO₂, and trace gases
- Hydrosphere: all water: oceans, rivers, lakes, groundwater, and atmospheric vapour
- Lithosphere: Earth's crust, upper mantle, and soils; includes rocks and sedimentary deposits
The key cycles examined in this subtopic are the carbon cycle and (at HL) the nitrogen cycle. Both illustrate the same principle: elements are stored in compartments, flow between them, and are either transferred without changing form or transformed chemically along the way.
The sustainability of an ecosystem depends on these cycles functioning within normal ranges.
When human activity pushes inputs or outputs beyond natural limits, for example by releasing fossil carbon that took hundreds of millions of years to accumulate, the system can shift to a new, often less hospitable state.
Stores, sinks, and sources
Every biogeochemical cycle contains compartments that hold elements for varying lengths of time. The net balance between inputs and outputs determines how each compartment is classified.
A store (or storage) is a compartment where an element is held in approximate equilibrium: the rate of input roughly equals the rate of output, so the total amount remains relatively stable over time.
A sink is a compartment that accumulates more of an element than it releases, producing a net increase in the amount held there.
A source is a compartment or process that releases more of an element than it accumulates, producing a net increase in the element elsewhere in the system.
The same compartment can shift roles depending on conditions:
- A growing forest is a sink (absorbing more carbon than it releases)
- A mature, undisturbed forest is a store (inputs ≈ outputs)
- A forest cleared by fire becomes a source (releasing stored carbon rapidly)
These three terms are frequently confused in exams. The key is the word net: a sink has a net gain, a source has a net loss, and a store is approximately neutral. Always state the direction of the imbalance when using these terms.
Factors that influence how quickly elements move between compartments include weathering, decomposition rates, temperature, moisture, and human disturbance. The rate of movement determines how long an element stays in any one place: its residence time.
Carbon stores
Carbon is held in two broad categories of store, distinguished by whether it forms part of biological molecules.
Organic carbon stores contain carbon bonded to hydrogen in compounds derived from living or once-living material:
- Living organisms: plants, animals, and microorganisms store carbon in biomass (carbohydrates, proteins, lipids, nucleic acids)
- Dead organic matter: decaying plant litter and animal remains in soil
- Crude oil, coal, and natural gas: fossilised remains of ancient organisms, representing carbon removed from active cycling millions of years ago
Inorganic carbon stores contain carbon in non-biological chemical forms:
- Atmosphere: carbon dioxide (CO₂) and methane (CH₄)
- Soils: carbonate minerals
- Oceans: dissolved CO₂, carbonic acid (H₂CO₃), bicarbonate ions (HCO₃⁻), and carbonate ions (CO₃²⁻)
- Sedimentary rocks: limestone (CaCO₃) and dolomite, formed over geological timescales
Residence time is the average period that a carbon atom remains in a particular store before moving to another part of the cycle.
Residence times vary enormously:
| Carbon store | Approximate residence time |
|---|---|
| Atmosphere | Years to decades |
| Plant biomass | Decades to centuries |
| Soil organic matter | Decades to centuries |
| Deep ocean | Centuries to millennia |
| Fossil fuels | Hundreds of millions of years |
| Limestone and rocks | Hundreds of millions of years |
Without human extraction, fossil fuels would remain locked in the lithosphere essentially indefinitely. Mining and combustion short-circuit a residence time that would naturally be measured in geological eras.
A store is in equilibrium when the rate of carbon absorption equals the rate of carbon release.
Human activities (particularly fossil fuel combustion and deforestation) are disrupting this equilibrium by releasing carbon far faster than natural sinks can absorb it.