Biology · Introductory biology · Concept
Biogeochemical cycles: carbon and nitrogen
Energy flows through an ecosystem and leaves as heat, but matter is used again and again. A biogeochemical cycle follows an element between reservoirs: the atmosphere, living things, soil, water and rock. Carbon enters living things when photosynthesis fixes CO₂ and returns to the air when organisms respire, decompose or burn. Nitrogen gas is plentiful but useless to most organisms until bacteria fix it; other bacteria turn the ammonium into nitrate, plants absorb both, and denitrifying bacteria return nitrogen to the air.
Reservoirs and fluxes
A reservoir is a place where an element is stored, such as the atmosphere, the ocean, living things, soil or rock. A flux is a movement between two reservoirs, such as photosynthesis moving carbon from the air into plants. Some reservoirs exchange quickly; others, such as fossil fuels and sedimentary rock, hold carbon for millions of years.
The carbon cycle
Photosynthesis moves CO₂ from the atmosphere into living things, and respiration by plants, animals and decomposers returns it. Carbon dioxide dissolves into the ocean and also comes back out of it. Dead matter buried before it decays can become fossil fuel over millions of years, and carbonate shells settle into ocean sediments; burning fossil fuels returns that long-stored carbon to the air within decades.
| Process | From | To |
|---|---|---|
| Photosynthesis | Atmosphere CO₂ | Plants and animals |
| Cellular respiration | Plants and animals | Atmosphere CO₂ |
| Decomposition | Plants and animals | Soil and dead matter |
| Decomposers respire | Soil and dead matter | Atmosphere CO₂ |
| Burial | Soil and dead matter | Fossil fuels |
| Combustion | Fossil fuels | Atmosphere CO₂ |
| Ocean carbon uptake | Atmosphere CO₂ | Ocean |
| Ocean carbon release | Ocean | Atmosphere CO₂ |
| Sedimentation and burial | Ocean | Rock and sediment |
The nitrogen cycle
Nitrogen gas, N₂, makes up 78% of the air, but only some bacteria and archaea can break its triple bond. Nitrogen fixation converts N₂ to ammonia, which becomes ammonium, NH₄⁺, in the soil. Nitrifying bacteria oxidize ammonium to nitrite, NO₂⁻, and then to nitrate, NO₃⁻. Plants absorb ammonium and nitrate to build proteins and nucleic acids; decomposers return the nitrogen in dead matter and wastes as ammonium, which is ammonification; and denitrifying bacteria in low-oxygen soil reduce nitrate back to N₂.
| Process | From | To |
|---|---|---|
| Nitrogen fixation | Atmosphere N₂ | Ammonium NH₄⁺ |
| Nitrification | Ammonium NH₄⁺ | Nitrite NO₂⁻ |
| Nitrification | Nitrite NO₂⁻ | Nitrate NO₃⁻ |
| Assimilation | Nitrate NO₃⁻ | Plants and animals |
| Assimilation | Ammonium NH₄⁺ | Plants and animals |
| Ammonification | Plants and animals | Ammonium NH₄⁺ |
| Denitrification | Nitrate NO₃⁻ | Atmosphere N₂ |
People change the cycles
Burning fossil fuels and clearing forests add CO₂ to the atmosphere faster than photosynthesis and the ocean take it up, so its concentration rises. Fertilizer made by the industrial Haber–Bosch process adds fixed nitrogen to farmland, and nitrate washed off fields can feed algal blooms that leave low-oxygen dead zones in lakes and coastal waters.
Energy flows, matter cycles
The same carbon atom can pass from the air into a leaf, a caterpillar and a bird, then back to the air as CO₂. Energy cannot be reused this way: each transfer loses some as heat, so an ecosystem needs a steady input of sunlight while its matter is recycled.
Common mistakes
- Saying plants take nitrogen from the air: most cannot use N₂; they absorb ammonium and nitrate from the soil, and legumes also get fixed nitrogen from bacteria in their root nodules.
- Mixing up nitrification and denitrification: nitrification turns ammonium into nitrate; denitrification turns nitrate back into N₂ gas.
- Saying energy cycles like matter: energy flows through an ecosystem and leaves as heat.
- Forgetting that respiration, not only burning, returns carbon to the air: every organism that respires releases CO₂.
Key terms
- Biogeochemical cycle
- Movement and transformation of matter among biological and nonliving reservoirs. Arrows represent specified processes, not creation or destruction of the tracked element.
- Matter reservoir
- A location or compartment storing the material tracked in a cycle. Reservoir amount is a stock, whereas a flux describes transfer.
- Material flux
- Transfer of a quantity between compartments, often measured per time. In a qualitative cycle diagram, an arrow identifies direction without necessarily specifying rate.
- Photosynthesis
- Conversion of light energy into chemical processes that support carbon fixation. Light-dependent reactions supply energy carriers used by subsequent carbon-assimilation pathways.
- Cellular respiration
- A set of reactions extracting usable energy from fuel molecules. Aerobic respiration uses oxygen as the terminal electron acceptor in its respiratory chain.
- Decomposer
- An organism, such as a fungus or bacterium, that breaks down dead material and returns its nutrients to the soil or water. The energy it uses comes from that dead material.
- Nitrogen fixation
- Conversion of molecular nitrogen, N₂, into reduced nitrogen compounds that can enter biological metabolism. Biological fixation depends on specialized organisms and energy input.
- Nitrification
- Microbial oxidation of reduced nitrogen, commonly ammonium through nitrite to nitrate. It differs from nitrogen fixation and changes nitrogen’s chemical form.
- Ammonification
- Conversion of organic nitrogen into ammonium during decomposition and related metabolism. The nitrogen is transformed from existing material rather than newly created.
- Denitrification
- Microbial reduction of oxidized nitrogen compounds toward gaseous products such as N₂. It returns some biologically available nitrogen to the atmosphere.
- Biological assimilation
- Incorporation of acquired material into an organism’s molecules. Carbon fixation specifically incorporates inorganic carbon into organic compounds; subsequent processing can involve additional pathways.
Work through an example
A nitrogen atom starts in N₂ in the air above a meadow and ends up back in the air. Trace its path through soil bacteria, a grass plant, a rabbit and the soil, naming each process and who carries it out.
Trace a nitrogen atom through the cycle →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Open the nitrogen cycle Open the carbon cycle Open worked example on a board Biogeochemical processes in Biology ReferenceYour existing work stays on this device. Examples open as editable copies.