Biogeochemical Cycles
Understand how carbon, nitrogen, and water cycle through the Earth's systems -- and how human activities are disrupting these essential processes.
The Carbon Cycle
Carbon is the building block of all organic molecules. The carbon cycle describes how carbon atoms move between the atmosphere, biosphere, hydrosphere, and lithosphere through biological, chemical, and geological processes.
Carbon Cycle Pathways
Photosynthesis
Plants absorb CO2 from the atmosphere and convert it to glucose using sunlight.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Cellular Respiration
Organisms break down glucose, releasing CO2 back into the atmosphere.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Combustion
Burning fossil fuels and biomass rapidly releases stored carbon as CO2 into the atmosphere.
Decomposition
Decomposers break down dead organic matter, returning carbon to the soil and atmosphere.
Ocean Absorption
Oceans dissolve atmospheric CO2. Marine organisms use dissolved carbon for shells (CaCO3), which can form limestone sediments over millions of years.
Key insight: Fossil fuels (coal, oil, natural gas) represent carbon that was locked away over millions of years. Burning them in decades releases this carbon far faster than natural sinks can absorb it, driving the enhanced greenhouse effect.
The Nitrogen Cycle
Nitrogen (N2) makes up 78% of the atmosphere, yet most organisms cannot use it directly. The nitrogen cycle converts atmospheric nitrogen into biologically useful forms through a series of microbial transformations.
Stages of the Nitrogen Cycle
Nitrogen Fixation
N2 → NH3 (ammonia) by bacteria (e.g. Rhizobium) or lightning
Nitrification
NH3 → NO2− → NO3− by nitrifying bacteria
Assimilation
Plants absorb NO3− or NH4+ to make amino acids and proteins
Ammonification
Decomposers convert organic N back to NH3 from dead matter
Denitrification
NO3− → N2 returned to atmosphere by anaerobic bacteria
Human impact: The Haber-Bosch process industrially fixes nitrogen for fertilisers. Excess nitrogen runoff causes eutrophication -- algal blooms that deplete oxygen in waterways, creating dead zones.
The Water Cycle and Human Impacts
The hydrological (water) cycle is driven primarily by solar energy and gravity. Water moves between the atmosphere, surface water, groundwater, and living organisms through evaporation, condensation, precipitation, and transpiration.
Water Cycle Processes
Evaporation & Transpiration
Solar energy heats surface water (oceans, lakes), converting liquid to vapour. Plants release water vapour through stomata (transpiration). Together these add moisture to the atmosphere.
Condensation & Precipitation
Water vapour cools as it rises, condensing into cloud droplets. When droplets grow large enough, water falls as rain, snow, or hail, replenishing surface and ground water.
Human Impacts on Biogeochemical Cycles
Carbon cycle: Fossil fuel combustion and deforestation increase atmospheric CO2, intensifying the greenhouse effect and driving climate change.
Nitrogen cycle: Synthetic fertilisers and burning fossil fuels introduce excess reactive nitrogen, leading to eutrophication, acid rain, and N2O (a potent greenhouse gas).
Water cycle: Urbanisation increases impervious surfaces (reducing infiltration), deforestation reduces transpiration, and climate change alters precipitation patterns and increases evaporation rates.
Key Vocabulary
Carbon Sink
A reservoir that absorbs and stores more carbon than it releases (e.g. oceans, forests, soil).
Nitrogen Fixation
The conversion of atmospheric N2 into biologically available forms such as ammonia (NH3), carried out by specific bacteria.
Eutrophication
Excessive nutrient enrichment of water bodies causing algal blooms, oxygen depletion, and ecosystem collapse.
Transpiration
The release of water vapour from plant leaves through stomata, a key process linking the water cycle and biosphere.
Worked Examples
Trace a carbon atom from the atmosphere through one complete cycle back to the atmosphere.
Step 1: A CO2 molecule in the atmosphere is absorbed by a plant during photosynthesis and incorporated into a glucose molecule.
Step 2: A herbivore eats the plant. The carbon in glucose is used in cellular respiration for energy.
Step 3: Respiration releases the carbon atom as CO2 back into the atmosphere.
Alternative pathway: If the organism dies, decomposers break down its remains, releasing carbon as CO2 -- or the remains may be buried and form fossil fuels over millions of years.
Explain why legumes are valuable in crop rotation with reference to the nitrogen cycle.
Step 1: Legumes (e.g. clover, soybeans) have a mutualistic relationship with Rhizobium bacteria in their root nodules.
Step 2: These bacteria perform nitrogen fixation, converting atmospheric N2 into NH3, which the plant converts to ammonium (NH4+).
Step 3: When the legume crop is ploughed back into the soil, the nitrogen-rich organic matter is decomposed, increasing soil nitrate levels for the next crop -- reducing the need for synthetic fertilisers.
Explain how deforestation disrupts both the carbon and water cycles.
Carbon cycle: Trees are significant carbon sinks. Removing them stops CO2 absorption via photosynthesis. Burning or decomposing the cleared biomass releases stored carbon, increasing atmospheric CO2.
Water cycle: Trees return water to the atmosphere through transpiration. Deforestation reduces transpiration, lowering local humidity and rainfall. Exposed soil also increases surface runoff while reducing groundwater infiltration.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which process removes carbon dioxide from the atmosphere and converts it into organic molecules?
Question 2
In the nitrogen cycle, which process converts atmospheric N2 into ammonia (NH3)?
Question 3
What is the primary consequence of excess nitrogen runoff entering waterways?
Question 4
Which of the following is a long-term carbon store (geological reservoir)?
Question 5
How does deforestation affect the water cycle in a region?
Key Concepts Summary
- ●The carbon cycle moves carbon through the atmosphere, biosphere, hydrosphere, and lithosphere via photosynthesis, respiration, combustion, and decomposition.
- ●The nitrogen cycle converts atmospheric N2 into usable forms through fixation, nitrification, assimilation, ammonification, and denitrification.
- ●The water cycle is driven by solar energy: evaporation, transpiration, condensation, and precipitation redistribute water globally.
- ●Human activities (fossil fuel combustion, fertiliser use, deforestation) are disrupting these cycles at rates that exceed natural buffering capacity.
- ●Understanding these cycles is essential for managing climate change, water quality, and soil fertility.