IB DP Environmental Systems & Societies — Topic 2: Ecology
Biogeochemical
Cycles
Subtopic 2.3 — New Syllabus (First Assessment 2026)
Standard Level + Higher Level
20 SL Points
2.3.1 – 2.3.21
5 HL Extensions
2.3.12 – 2.3.16
Overview
What This Subtopic Covers
Biogeochemical cycles describe how chemical elements (carbon, nitrogen, phosphorus, water) move through biotic and abiotic components of ecosystems. Understanding these cycles is essential for analyzing human impacts on the environment.
A. The Carbon Cycle
Stores, flows, fossil fuels, ocean acidification, sequestration
Stores, flows, fossil fuels, ocean acidification, sequestration
B. The Nitrogen Cycle
Bacteria, fixation, nitrification, denitrification, human impacts
Bacteria, fixation, nitrification, denitrification, human impacts
Guiding Questions
- How do chemical elements cycle through ecosystems?
- How do human activities alter these natural cycles?
- What are the consequences of disrupting biogeochemical cycles?
2.3.1
What Are Biogeochemical Cycles?
Key UnderstandingBiogeochemical cycles ensure chemical elements continue to be available to living organisms.
Why Do Elements Need to Cycle?
- The Earth is a closed system for matter — very little new matter enters or leaves
- Life requires a finite supply of elements (C, N, P, S, H₂O, etc.)
- Without cycling, these elements would become locked up and unavailable
- Cycles ensure elements move between living (biotic) and non-living (abiotic) components
The name tells you everything: Bio (living) + Geo (Earth/rocks) + Chemical (elements) — the cycling of chemical elements through living organisms and the Earth's systems.
2.3.2
Stores, Sinks, and Sources
Key UnderstandingBiogeochemical cycles have stores, sinks, and sources.
Storage
A place where an element is held for a period of time
e.g., Carbon in atmosphere, ocean, fossil fuels, biomass
Sink
A store that is absorbing more than it releases (net uptake)
e.g., Oceans absorb more CO₂ than they release
Source
A store that is releasing more than it absorbs (net output)
e.g., Burning fossil fuels releases more CO₂ than is stored
A store can be a sink at one time and a source at another — it depends on the balance of inputs and outputs. For example, a forest is a carbon sink while growing, but becomes a carbon source when burned.
2.3.3
Carbon Stores
Key UnderstandingOrganisms, crude oil and natural gas contain organic stores of carbon. Inorganic stores can be found in the atmosphere, soils and oceans.
Organic Carbon Stores
- Biosphere: living organisms (biomass)
- Fossil fuels: crude oil, natural gas, coal
- Dead organic matter: leaf litter, detritus
- Carbon is bonded to hydrogen in complex molecules
Inorganic Carbon Stores
- Atmosphere: CO₂ (~420 ppm and rising)
- Oceans: dissolved CO₂, bicarbonates, carbonates
- Soils: carbonate minerals, limestone
- Rocks: calcium carbonate (CaCO₃)
2.3.4
Carbon Flows in Ecosystems
Key UnderstandingCarbon flows between stores in ecosystems by photosynthesis, feeding, defecation, cellular respiration, death and decomposition.
Atmosphere
CO₂
CO₂
→
Producers
Photosynthesis
Photosynthesis
→
Consumers
Feeding
Feeding
→
Decomposers
Death + Decay
Death + Decay
Key Flows
| Process | Carbon Movement |
|---|---|
| Photosynthesis | Atmosphere CO₂ → plant biomass (organic C) |
| Feeding | Producer C → consumer C (organic C transferred) |
| Defecation | Undigested C → soil (organic matter) |
| Cellular respiration | Organic C → atmosphere CO₂ |
| Death & decomposition | Dead organic C → soil → atmosphere (via decomposers) |
2.3.5
Carbon Sequestration
Key UnderstandingCarbon sequestration is the process of capturing gaseous and atmospheric carbon dioxide and storing it in a solid or liquid form.
Natural Sequestration
- Photosynthesis: plants capture CO₂ → biomass
- Ocean absorption: CO₂ dissolves into seawater
- Soil organic matter: decomposition stores C in soil
- Rock weathering: CO₂ reacts with minerals
Artificial Sequestration
- CCS: Carbon capture and storage from industrial sources
- Direct air capture: Machines that pull CO₂ from air
- BECCS: Bioenergy with carbon capture and storage
- Afforestation: Planting trees on non-forest land
2.3.6
Ecosystems as Carbon Stores, Sinks, or Sources
Key UnderstandingEcosystems can act as stores, sinks or sources of carbon.
| Ecosystem | Carbon Role | Why? |
|---|---|---|
| Healthy forest | Sink | Growth absorbs more CO₂ than respiration releases |
| Burning forest | Source | Fire releases stored carbon rapidly |
| Open ocean | Sink | Phytoplankton + dissolution absorb CO₂ |
| Wetland | Store | Waterlogged conditions slow decomposition → peat |
| Agricultural land | Source (often) | Tilling releases soil carbon; monocultures reduce storage |
Whether an ecosystem is a sink or source depends on the balance of photosynthesis/respiration and decomposition. Human activities can flip a sink into a source (e.g., deforestation).
2.3.7
Fossil Fuels: Ancient Carbon Stores
Key UnderstandingFossil fuels are stores of carbon with unlimited residence times. They were formed when ecosystems acted as carbon sinks in past eras and become carbon sources when burned.
Formation and Timescales
- Coal: formed from partially decomposed terrestrial plants (millions of years)
- Oil: formed from marine organisms (plankton, algae) in ocean sediments
- Natural gas: formed alongside oil from organic matter under heat and pressure
- Residence time: effectively infinite until extracted and burned
Key concept: Fossil fuels stored carbon that was removed from the active carbon cycle millions of years ago. Burning them releases ancient carbon into the modern atmosphere in seconds — disrupting the natural balance.
2.3.8
Agriculture and the Carbon Cycle
Key UnderstandingAgricultural systems can act as carbon stores, sources and sinks, depending on the techniques used.
Carbon Sinks (Good Practices)
- Agroforestry: trees + crops → more biomass storage
- No-till farming: soil carbon preserved
- Cover crops: continuous photosynthesis
- Composting: organic matter added to soil
Carbon Sources (Bad Practices)
- Tilling: exposes soil carbon to oxidation → CO₂ release
- Deforestation for farmland: releases tree biomass carbon
- Rice paddies: anaerobic conditions → methane emissions
- Livestock: enteric fermentation → methane
2.3.9
Oceans and Carbon
Key UnderstandingCarbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of solution.
Ocean Carbon Exchange
Dissolution equilibrium
CO₂ (atmosphere) ↔ CO₂ (dissolved in seawater) ↔ H₂CO₃ ↔ HCO₃⁻ ↔ CO₃²⁻
How Oceans Absorb CO₂
- CO₂ dissolves at the ocean surface
- Cold water absorbs more than warm water
- Phytoplankton take up dissolved CO₂ via photosynthesis
- ~30% of human CO₂ emissions absorbed by oceans
Temperature Effect
As ocean temperatures rise, water holds less dissolved CO₂ → reduced absorption capacity → more CO₂ remains in atmosphere → warming accelerates (positive feedback).
2.3.10
Ocean Acidification
Key UnderstandingIncreases in concentrations of dissolved carbon dioxide cause ocean acidification, harming marine animals.
The Chemistry
When CO₂ dissolves in seawater
CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃⁻
More H⁺ ions = lower pH = more acidic. Ocean pH has dropped from ~8.2 to ~8.1 since pre-industrial times (a 26% increase in acidity).
Who Is Affected?
- Corals: can't build calcium carbonate skeletons → bleaching + death
- Molluscs: shells dissolve or become thinner
- Plankton: shell-forming species decline → food web collapse
- Fish: sensory disruption, behavioural changes
Why It Matters
- Marine food webs depend on shell-forming organisms
- Coral reefs support 25% of marine biodiversity
- Fisheries and coastal protection at risk
- Extremely difficult to reverse
2.3.11
Alleviating Human Impacts on the Carbon Cycle
Key UnderstandingMeasures are required to alleviate the effects of human activities on the carbon cycle.
| Strategy | How It Helps | Example |
|---|---|---|
| Reduce emissions | Less CO₂ entering atmosphere | Renewable energy, electric vehicles |
| Carbon sequestration | Remove CO₂ from atmosphere | Afforestation, CCS technology |
| Protect carbon sinks | Preserve existing storage | Stop deforestation, protect peatlands |
| Restore degraded land | Re-establish carbon uptake | Reforestation, wetland restoration |
| Change agriculture | Reduce emissions + increase soil carbon | No-till, cover crops, agroforestry |
For exam essays: always link specific human activities to their effect on the carbon cycle (which store, which flow is disrupted), then propose targeted mitigation.
HL — 2.3.12
Carbon in the LithosphereHL
Key UnderstandingThe lithosphere contains carbon stores in fossil fuels and in rocks, such as limestone.
Fossil Fuel Stores
- Coal: terrestrial plant matter compressed over millions of years
- Oil: marine organisms buried in sedimentary rock
- Natural gas: formed with oil from organic matter under pressure
- Residence time: millions of years (until extracted)
Rock Stores
- Limestone (CaCO₃): largest inorganic carbon store
- Formed from accumulated shells and coral skeletons
- Weathering slowly releases CO₂ back to atmosphere
- Residence time: hundreds of millions of years
HL — 2.3.13
Reef-Building Organisms & LimestoneHL
Key UnderstandingReef-building corals and molluscs have hard parts that contain calcium carbonate that can become fossilized in limestone.
How Limestone Forms
Coral/Mollusc
builds CaCO₃ shell
builds CaCO₃ shell
→
Dies +
Sinks
Sinks
→
Sediment
accumulates
accumulates
→
Compression
over millions of years
over millions of years
→
Limestone
CaCO₃ rock
CaCO₃ rock
Connection: Limestone weathering is a natural carbon sink — it slowly absorbs atmospheric CO₂. But this process operates over millions of years, far too slow to offset current human emissions.
HL — 2.3.14
Fossil Fuel FormationHL
Key UnderstandingIn past geological eras, organic matter from partially decomposed plants became fossilized in coal, and partially decomposed marine organisms became fossilized in oil and natural gas held in porous rocks.
Coal Formation
- Terrestrial plants in swamp forests die
- Waterlogged conditions prevent full decomposition
- Layers accumulate → peat → lignite → coal
- Process: millions of years under heat and pressure
Oil & Gas Formation
- Marine organisms (plankton, algae) die and sink
- Buried in ocean sediment → compressed
- Heat transforms organic matter → oil and gas
- Migrate through porous rock → trapped in reservoirs
HL — 2.3.15
Methane ProductionHL
Key UnderstandingMethane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria.
Where Is Methane Produced?
| Source | Conditions | Scale |
|---|---|---|
| Wetlands | Waterlogged, anaerobic soil | Largest natural source |
| Rice paddies | Flooded fields, anaerobic | Major agricultural source |
| Ruminant digestion | Anaerobic gut fermentation | Cattle, sheep, goats |
| Landfills | Decomposition without oxygen | Growing source |
| Permafrost thaw | Frozen organic matter decomposes | Concerning feedback loop |
Methane (CH₄) is ~80× more potent as a greenhouse gas than CO₂ over a 20-year period, making it a critical climate concern.
HL — 2.3.16
Methane's Atmospheric LifetimeHL
Key UnderstandingMethane has a residence time of about 10 years in the atmosphere and is eventually oxidized to carbon dioxide.
Methane in the Atmosphere
Oxidation reaction
CH₄ + 2O₂ → CO₂ + 2H₂O
Unlike CO₂ (which persists for centuries), methane is relatively short-lived. However, while it is in the atmosphere, it is an extremely powerful greenhouse gas.
Implications
- Reducing methane emissions gives fast climate benefits
- Methane reduction is one of the most cost-effective climate strategies
- But methane sources are diverse and widespread
The Paradox
Methane is short-lived but potent. CO₂ is long-lived but less potent per molecule. Both matter — methane for near-term warming, CO₂ for long-term climate trajectory.
2.3.17
The Nitrogen Cycle: Overview
Key UnderstandingThe nitrogen cycle contains organic and inorganic stores.
Inorganic Nitrogen Stores
- Atmosphere: N₂ gas (78% of atmosphere)
- Soil: ammonium (NH₄⁺), nitrate (NO₃⁻)
- Water: dissolved nitrates and ammonia
Organic Nitrogen Stores
- Living organisms: proteins, DNA, chlorophyll
- Dead organic matter: leaf litter, detritus
- Waste products: animal excretion (urea, ammonia)
Key challenge: Atmospheric N₂ is abundant but cannot be used directly by most organisms — it must first be "fixed" into a usable form (NH₃ or NO₃⁻).
2.3.18
Bacteria in the Nitrogen Cycle
Key UnderstandingBacteria have essential roles in the nitrogen cycle.
| Process | Bacteria Type | What Happens |
|---|---|---|
| Nitrogen fixation | Nitrogen-fixing bacteria (e.g., Rhizobium) | N₂ → NH₃ (ammonia) |
| Nitrification | Nitrifying bacteria | NH₃ → NO₂⁻ → NO₃⁻ (nitrate) |
| Ammonification | Decomposer bacteria | Organic N → NH₃ (ammonia) |
| Denitrification | Denitrifying bacteria | NO₃⁻ → N₂ (back to atmosphere) |
Without bacteria, the nitrogen cycle would stop. No other organisms can convert N₂ gas into usable forms — bacteria are the gatekeepers of this essential element.
2.3.19
Denitrification Conditions
Key UnderstandingDenitrification only happens in anaerobic conditions, such as soils that are waterlogged.
Why Anaerobic?
- Denitrifying bacteria use NO₃⁻ instead of O₂ for respiration
- Only possible when oxygen is absent
- Waterlogged soils, wetlands, flooded rice paddies
Impact
- Removes usable nitrogen from soil → reduced fertility
- Returns N₂ to atmosphere → closes the cycle
- Over-irrigation can increase denitrification → crop yield loss
2.3.20
Plants and Nitrogen Fixation
Key UnderstandingPlants cannot fix nitrogen so atmospheric dinitrogen is unavailable to them unless they form mutualistic associations with nitrogen-fixing bacteria.
The Legume–Rhizobium Mutualism
Legume
(plant)
(plant)
⇄
Rhizobium
(bacteria in root nodules)
(bacteria in root nodules)
Plant provides: carbohydrates (energy) + shelter in root nodules
Bacteria provides: fixed nitrogen (NH₃) the plant can use
Other nitrogen fixers: cyanobacteria in rice paddies, free-living soil bacteria (Azotobacter), and lightning (small abiotic contribution).
2.3.21
Nitrogen Cycle Flows
Key UnderstandingFlows in the nitrogen cycle include mineral uptake by producers, consumption, excretion, death, decomposition and ammonification.
N₂
Atmosphere
Atmosphere
→
NO₃⁻
Soil
Soil
→
Producers
Uptake
Uptake
→
Consumers
Feeding
Feeding
→
Decomposers
Ammonification
Ammonification
| Flow | Process |
|---|---|
| Mineral uptake | Producers absorb NO₃⁻ / NH₄⁺ from soil |
| Consumption | N moves up food chain when organisms eat each other |
| Excretion | Animals excrete nitrogenous waste (urea, ammonia) |
| Death | Dead organisms contain organic nitrogen |
| Decomposition | Decomposers break down organic N |
| Ammonification | Organic N → NH₃ (ammonia) in soil |
2.3.22
Human Impacts on the Nitrogen Cycle
Key UnderstandingHuman activities such as deforestation, agriculture, aquaculture and urbanization change the nitrogen cycle.
| Activity | Impact on Nitrogen Cycle |
|---|---|
| Agriculture (fertilizers) | Massive increase in reactive N (NO₃⁻) in soils and waterways |
| Deforestation | Removes vegetation that would take up nitrogen → N accumulates in soil |
| Aquaculture | Fish waste and uneaten feed release ammonia into water |
| Urbanization | Sewage, vehicle emissions (NOₓ), industry release reactive N |
| Burning fossil fuels | Produces nitrogen oxides (NOₓ) → acid rain + smog |
2.3.23
The Haber Process
Key UnderstandingThe Haber process is an industrial process that produces ammonia from nitrogen and hydrogen for use as fertilizer.
Haber Process
N₂ + 3H₂ → 2NH₃ (high temperature, high pressure, iron catalyst)
Why It Matters
- Made synthetic nitrogen fertilizer possible
- Enabled the Green Revolution → food for billions
- Without it, Earth could only support ~4 billion people
- ~50% of nitrogen in human bodies came via Haber process
The Environmental Cost
- Massive increase in reactive nitrogen in environment
- Runoff → eutrophication of waterways
- N₂O emissions → potent greenhouse gas
- Soil acidification
Fritz Haber won the Nobel Prize for this invention — but it has been called both "bread from air" and a "dual-use" technology, as the same process was used to make explosives in WWI.
2.3.24
The Nitrogen Planetary Boundary: CROSSED
Key UnderstandingIncreases in nitrates in the biosphere from human activities have led to the planetary boundary for the nitrogen cycle being crossed, making irreversible changes to Earth systems likely.
What Happens When the Boundary Is Crossed?
- Eutrophication: excess nitrogen → algal blooms → oxygen depletion → dead zones
- Biodiversity loss: nitrogen-loving species outcompete others → species decline
- Groundwater contamination: nitrates in drinking water → health risks
- Acid rain: NOₓ emissions → soil and water acidification
- Climate change: N₂O is 298× more potent than CO₂ as a greenhouse gas
The scale of the problem: Humans now fix more nitrogen industrially (Haber process) than all natural terrestrial processes combined.
2.3.25
Global Collaboration on Nitrogen
Key UnderstandingGlobal collaboration is needed to address the uncontrolled use of nitrogen in industrial and agricultural processes and bring the nitrogen cycle back within planetary boundaries.
What Needs to Happen
- Precision agriculture: apply fertilizer only where and when needed
- Organic farming: reduce synthetic fertilizer dependence
- Wastewater treatment: remove nitrogen before discharge
- Dietary shifts: less meat = less nitrogen-intensive agriculture
- International agreements: coordinated targets for nitrogen reduction
- Cover crops and crop rotation: reduce nitrogen fertilizer needs
The 2023 UNEP resolution on nitrogen called for halving nitrogen waste by 2030 — similar in ambition to the Paris Agreement for climate. Know this as a key policy example.
Key Terms Glossary
Essential Vocabulary
| Term | Definition |
|---|---|
| Biogeochemical cycle | Cycling of chemical elements through biotic and abiotic components |
| Store | Place where an element is held in a cycle |
| Sink | Store absorbing more than it releases (net uptake) |
| Source | Store releasing more than it absorbs (net output) |
| Carbon sequestration | Capturing atmospheric CO₂ and storing it in solid/liquid form |
| Ocean acidification | Decrease in ocean pH from dissolved CO₂ |
| Methane (CH₄) | Greenhouse gas produced by methanogenic bacteria in anaerobic conditions |
| Nitrogen fixation | Conversion of N₂ gas to NH₃ by bacteria |
| Nitrification | Conversion of NH₃ to NO₃⁻ by bacteria |
| Denitrification | Conversion of NO₃⁻ back to N₂ in anaerobic conditions |
| Ammonification | Decomposition of organic N → NH₃ |
| Haber process | Industrial production of NH₃ from N₂ and H₂ |
| Eutrophication | Nutrient enrichment of water → algal blooms → oxygen depletion |
| Residence time | Average time an element spends in a particular store |
Exam Preparation
Key Takeaways & Exam Tips
Must-Know Processes
Carbon Cycle
Photosynthesis ↔ Respiration ↔ Decomposition ↔ Fossil fuel burning
Nitrogen Cycle
Fixation → Nitrification → Uptake → Ammonification → Denitrification
Common Exam Patterns
- Trace a cycle: follow an element through all stores and flows
- Explain human disruption: which store/flow is affected and how
- Ocean acidification: know the chemistry and biological consequences
- Nitrogen bacteria: match each type to its process (fixation, nitrification, denitrification, ammonification)
- Haber process: benefits vs environmental costs
- Compare carbon and nitrogen cycles: both involve bacteria, but carbon also involves photosynthesis/respiration
- Planetary boundaries: know which cycles have been crossed and consequences
For 9-mark essays: Use specific examples, name the bacteria/processes, and always link human activities to specific disruptions in the cycle. Show you understand cause → effect → consequence chains.
You've covered all 25 syllabus points ✅
2.3.1 – 2.3.21 (SL) + 2.3.12 – 2.3.16 (HL)