Biogeochemical Cycles: Carbon, Nitrogen & Water
Trace carbon, nitrogen and water through ecosystem reservoirs, learn why matter cycles but energy flows one way, and see how humans disrupt each cycle.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Biogeochemical Cycles: Carbon, Nitrogen & Water, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
You have already followed energy through a food web and watched it dwindle at every level. Now follow the atoms. The carbon in your exhaled breath may have been inside a fern 300 million years ago, and the nitrogen in your muscle protein was almost certainly pulled out of the air by bacteria. Unlike energy, atoms are not used up — they get repackaged and reused, over and over, on timescales from seconds to millions of years.
This lesson traces three cycles — carbon, nitrogen and water — between their major reservoirs, showing which processes move matter into living things and which release it back. You will also see why the one-way flow of energy and the cycling of matter are two sides of the same set of chemical reactions, and how burning fuel, spreading fertilizer and paving land bend these cycles out of their usual balance.
This lesson traces three cycles — carbon, nitrogen and water — between their major reservoirs, showing which processes move matter into living things and which release it back. You will also see why the one-way flow of energy and the cycling of matter are two sides of the same set of chemical reactions, and how burning fuel, spreading fertilizer and paving land bend these cycles out of their usual balance.
Matter Cycles, Energy Flows One Way
A biogeochemical cycle is the path an element takes as it moves between reservoirs — places where matter accumulates — in the biotic (living) and abiotic (non-living) parts of an ecosystem. The atmosphere, oceans, rocks, soil and bodies of organisms are all reservoirs. The movement of matter from one reservoir to another is a flux, and fluxes are almost always driven by chemical reactions or physical processes such as evaporation.
Why does matter cycle while energy does not? Because of what happens in each transfer. Atoms are conserved: the six carbon atoms in a glucose molecule are still six carbon atoms after cellular respiration breaks it apart, just rearranged into carbon dioxide. Energy is also conserved, but at every transfer a large fraction is converted to thermal energy that radiates away from the ecosystem and cannot be recaptured by producers. Sunlight enters, heat leaves — energy makes a one-way trip. Carbon has nowhere to leave to, so it is used again.
A frequent mistake is saying matter cycles "because organisms recycle it deliberately." No intent is involved — decomposers break down molecules for their own energy, and returning nitrogen and carbon to the soil and air is simply a by-product. Another error is thinking nutrients are inexhaustible. Matter cycles, but a nutrient can be locked in a slow reservoir like limestone for millions of years, which is why phosphorus or nitrogen often limits growth.
Why does matter cycle while energy does not? Because of what happens in each transfer. Atoms are conserved: the six carbon atoms in a glucose molecule are still six carbon atoms after cellular respiration breaks it apart, just rearranged into carbon dioxide. Energy is also conserved, but at every transfer a large fraction is converted to thermal energy that radiates away from the ecosystem and cannot be recaptured by producers. Sunlight enters, heat leaves — energy makes a one-way trip. Carbon has nowhere to leave to, so it is used again.
| Feature | Matter | Energy |
|---|---|---|
| Enters ecosystem as | Already present; recycled | Sunlight |
| Leaves ecosystem as | Very little leaves | Heat radiated to space |
| Path | Cyclic between reservoirs | One-way, producers to consumers |
| Effect of each transfer | Rearranged, not lost | Roughly 90 percent lost as heat |
The Carbon Cycle: Reservoirs and the Processes That Connect Them
Carbon's main reservoirs are the atmosphere (as ), the ocean (as dissolved and bicarbonate), living and dead biomass, soil organic matter, sedimentary rock such as limestone, and fossil fuels. By far the largest is rock; the atmosphere is small but fast-moving, which is why changes to it show up quickly.
Four processes do most of the work. Photosynthesis pulls out of air or water and fixes it into organic molecules — this is the only major route carbon takes into the living world. Cellular respiration, performed by producers, consumers and decomposers alike, returns to the air. Decomposition transfers carbon from dead tissue into soil and, through the respiration of decomposers, back to the atmosphere. Combustion burns biomass or fossil fuel and releases rapidly. Slow geological fluxes matter too: shells and skeletons settle and become limestone, and volcanic activity and weathering release that carbon back over long timescales.
Notice the symmetry. The equation for photosynthesis read backwards is essentially respiration, so the two processes together form the fast loop of the carbon cycle. If global photosynthesis exceeded global respiration and combustion, atmospheric would fall.
Where students go wrong: many write that animals "give carbon to plants" as a favor, or that only animals respire. Plants respire constantly, day and night. Another common error is treating fossil fuels as part of the fast cycle. Coal, oil and gas are carbon that left the fast cycle hundreds of millions of years ago because organisms were buried before decomposers could reach them; burning them injects an old reservoir into a fast one.
Four processes do most of the work. Photosynthesis pulls out of air or water and fixes it into organic molecules — this is the only major route carbon takes into the living world. Cellular respiration, performed by producers, consumers and decomposers alike, returns to the air. Decomposition transfers carbon from dead tissue into soil and, through the respiration of decomposers, back to the atmosphere. Combustion burns biomass or fossil fuel and releases rapidly. Slow geological fluxes matter too: shells and skeletons settle and become limestone, and volcanic activity and weathering release that carbon back over long timescales.
Notice the symmetry. The equation for photosynthesis read backwards is essentially respiration, so the two processes together form the fast loop of the carbon cycle. If global photosynthesis exceeded global respiration and combustion, atmospheric would fall.
Where students go wrong: many write that animals "give carbon to plants" as a favor, or that only animals respire. Plants respire constantly, day and night. Another common error is treating fossil fuels as part of the fast cycle. Coal, oil and gas are carbon that left the fast cycle hundreds of millions of years ago because organisms were buried before decomposers could reach them; burning them injects an old reservoir into a fast one.
The Nitrogen Cycle: A Cycle Run by Bacteria
Nitrogen makes up about 78 percent of the atmosphere, yet plants can starve for it. The reason is the triple bond in , which is extremely difficult to break. Plants cannot use directly; they absorb nitrogen as ammonium, , or nitrate, . Nearly every step that converts nitrogen from one usable form to another is performed by prokaryotes.
Nitrogen matters because it is in every amino acid and every nucleotide. An ecosystem short on nitrogen cannot build protein or DNA no matter how much sunlight it receives, which is why nitrogen is a classic limiting nutrient.
Two confusions are worth clearing up. First, fixation and nitrification are not the same: fixation starts from atmospheric , while nitrification starts from ammonium that is already in the soil. Second, denitrification is not a malfunction — it is the return flux that closes the cycle, sending nitrogen back to the atmosphere. Legumes such as beans and clover host fixing bacteria in root nodules, a mutualism that also explains why farmers rotate crops.
| Process | What changes | Who does it |
|---|---|---|
| Nitrogen fixation | Free-living and root-nodule bacteria; also lightning | |
| Assimilation | or into amino acids and nucleic acids | Plants, then consumers that eat them |
| Ammonification | Organic N in waste and dead tissue | Decomposing bacteria and fungi |
| Nitrification | Nitrifying bacteria | |
| Denitrification | Denitrifying bacteria in low-oxygen soil and sediment |
Two confusions are worth clearing up. First, fixation and nitrification are not the same: fixation starts from atmospheric , while nitrification starts from ammonium that is already in the soil. Second, denitrification is not a malfunction — it is the return flux that closes the cycle, sending nitrogen back to the atmosphere. Legumes such as beans and clover host fixing bacteria in root nodules, a mutualism that also explains why farmers rotate crops.
The Water Cycle and How the Three Cycles Interlock
Water's reservoirs are the oceans (over 97 percent of Earth's water), ice sheets and glaciers, groundwater, lakes and rivers, atmospheric water vapor, and living organisms. The driving fluxes are evaporation from open water, transpiration — evaporation through the stomata of plant leaves — condensation into clouds, precipitation as rain or snow, plus runoff across the surface and infiltration down into groundwater. Ecologists often combine the first two as evapotranspiration, because in a forest most water reaching the air passes through leaves.
Unlike carbon and nitrogen, water usually moves through the cycle without being chemically changed. The exception is biologically important: photosynthesis splits water molecules and respiration reassembles them, so the hydrogen in your food and the oxygen you breathe both tie the water cycle to the carbon cycle.
The cycles are not independent. Water carries dissolved nitrate through soil and into streams, so a rainstorm moves nitrogen as well as water. Dissolved in the ocean links the carbon and water reservoirs. Drought lowers photosynthesis, which slows the fast carbon loop and reduces the amount of organic matter available for decomposers.
A misconception worth naming: students often draw the water cycle as ocean to cloud to land to ocean, leaving out plants and groundwater. In many terrestrial ecosystems transpiration is a major flux, and groundwater is a large but slow reservoir. Another error is treating residence time as irrelevant. A water molecule may spend about nine days in the atmosphere but thousands of years in deep groundwater or ice — same cycle, wildly different speeds.
Unlike carbon and nitrogen, water usually moves through the cycle without being chemically changed. The exception is biologically important: photosynthesis splits water molecules and respiration reassembles them, so the hydrogen in your food and the oxygen you breathe both tie the water cycle to the carbon cycle.
The cycles are not independent. Water carries dissolved nitrate through soil and into streams, so a rainstorm moves nitrogen as well as water. Dissolved in the ocean links the carbon and water reservoirs. Drought lowers photosynthesis, which slows the fast carbon loop and reduces the amount of organic matter available for decomposers.
A misconception worth naming: students often draw the water cycle as ocean to cloud to land to ocean, leaving out plants and groundwater. In many terrestrial ecosystems transpiration is a major flux, and groundwater is a large but slow reservoir. Another error is treating residence time as irrelevant. A water molecule may spend about nine days in the atmosphere but thousands of years in deep groundwater or ice — same cycle, wildly different speeds.
Human Disruption: Changing the Size of the Fluxes
Human activity rarely destroys a cycle. It changes how fast matter moves between reservoirs, and that imbalance is what causes trouble.
Burning fossil fuels transfers carbon from a reservoir that was sealed off for hundreds of millions of years into the atmosphere in seconds. Deforestation compounds this twice over: it releases stored carbon when trees are burned or decay, and it removes the photosynthesis that would have pulled back out. Rising atmospheric traps more heat, and the extra dissolving in seawater lowers ocean pH, making it harder for corals and shellfish to build calcium carbonate structures.
The nitrogen cycle has been altered even more dramatically. Industrial fertilizer production fixes nitrogen on a scale comparable to all natural fixation combined. Excess nitrate runs off fields into rivers and coastal waters, where it fuels algal blooms. When the algae die, decomposers consume oxygen as they break the biomass down, producing hypoxic dead zones where fish cannot survive. This chain — fertilizer, runoff, bloom, decomposition, oxygen depletion — is called eutrophication, and it is worth being able to explain step by step rather than just naming.
Water cycle disruption is mostly physical. Paving land replaces infiltration with runoff, so groundwater is not recharged and floods become flashier. Pumping aquifers faster than they refill depletes a slow reservoir. Removing forest cover cuts transpiration, which can reduce downwind rainfall.
The pattern to carry forward: identify which reservoir gains, which loses, and which flux humans sped up or blocked. Answers that only say "pollution is bad" miss the mechanism your teacher is looking for.
Burning fossil fuels transfers carbon from a reservoir that was sealed off for hundreds of millions of years into the atmosphere in seconds. Deforestation compounds this twice over: it releases stored carbon when trees are burned or decay, and it removes the photosynthesis that would have pulled back out. Rising atmospheric traps more heat, and the extra dissolving in seawater lowers ocean pH, making it harder for corals and shellfish to build calcium carbonate structures.
The nitrogen cycle has been altered even more dramatically. Industrial fertilizer production fixes nitrogen on a scale comparable to all natural fixation combined. Excess nitrate runs off fields into rivers and coastal waters, where it fuels algal blooms. When the algae die, decomposers consume oxygen as they break the biomass down, producing hypoxic dead zones where fish cannot survive. This chain — fertilizer, runoff, bloom, decomposition, oxygen depletion — is called eutrophication, and it is worth being able to explain step by step rather than just naming.
Water cycle disruption is mostly physical. Paving land replaces infiltration with runoff, so groundwater is not recharged and floods become flashier. Pumping aquifers faster than they refill depletes a slow reservoir. Removing forest cover cuts transpiration, which can reduce downwind rainfall.
The pattern to carry forward: identify which reservoir gains, which loses, and which flux humans sped up or blocked. Answers that only say "pollution is bad" miss the mechanism your teacher is looking for.
Key terms
- Biogeochemical cycle.
- The repeated movement of a chemical element between living organisms and the non-living reservoirs of air, water, soil and rock.
- Reservoir.
- A place where a large amount of an element accumulates, such as the atmosphere, ocean, soil, rock or biomass; fluxes are the transfers between reservoirs.
- Nitrogen fixation.
- Conversion of atmospheric into ammonia or ammonium, carried out mainly by bacteria (including root-nodule bacteria in legumes) and to a small extent by lightning.
- Nitrification.
- Bacterial oxidation of ammonium into nitrite and then nitrate, the form of nitrogen most plants absorb most readily.
- Denitrification.
- Bacterial conversion of nitrate back into atmospheric in low-oxygen soils and sediments, completing the nitrogen cycle.
- Transpiration.
- Loss of water vapor from plant leaves through stomata; a major flux returning water to the atmosphere in forests and grasslands.
- Eutrophication.
- Nutrient enrichment of water, often from fertilizer runoff, that triggers algal blooms whose decomposition depletes dissolved oxygen and creates dead zones.
- Residence time.
- The average length of time a molecule or atom stays in a particular reservoir before moving on; short in the atmosphere, long in rock and deep groundwater.
Worked example
A carbon atom is part of a molecule in the air above a meadow in spring. Trace one complete path this atom could take that ends with it back in the atmosphere, naming the process at each transfer. Then explain why the energy that entered the meadow along with this atom's journey cannot make the same round trip.
Step 1: Fixation into biomass. A grass plant takes the in through a stoma and uses photosynthesis to build glucose. The carbon atom is now in an organic molecule inside a producer. Reservoir change: atmosphere to biomass.
Step 2: Transfer to a consumer. A grasshopper eats the grass and digests it, absorbing the glucose-derived carbon and incorporating some of it into its own tissue through biosynthesis. Reservoir: producer biomass to consumer biomass.
Step 3: Death and decomposition. The grasshopper dies. Bacteria and fungi carry out decomposition, breaking its molecules down for their own use, so the carbon atom briefly becomes part of soil organic matter and then of a decomposer's cells.
Step 4: Return to the atmosphere. The decomposer performs cellular respiration, oxidizing the organic molecule and exhaling the carbon as . Reservoir: soil or decomposer biomass back to atmosphere. The cycle is closed.
Alternative endings worth knowing: the grasshopper could have respired the carbon itself in Step 2, returning it directly; or the grass could have been buried before decomposition, sending the atom into a slow reservoir on its way to becoming fossil fuel.
Step 5: The energy comparison. The sunlight captured in Step 1 was stored as chemical potential energy in glucose. At every step after that — grasshopper digestion, movement, decomposer metabolism — most of that energy was converted to thermal energy and radiated out of the meadow. No organism can convert that heat back into a chemical bond. So the same atom returns to the atmosphere ready for reuse, while the energy that came with it has left the ecosystem permanently. Matter cycles; energy flows through once.
Step 2: Transfer to a consumer. A grasshopper eats the grass and digests it, absorbing the glucose-derived carbon and incorporating some of it into its own tissue through biosynthesis. Reservoir: producer biomass to consumer biomass.
Step 3: Death and decomposition. The grasshopper dies. Bacteria and fungi carry out decomposition, breaking its molecules down for their own use, so the carbon atom briefly becomes part of soil organic matter and then of a decomposer's cells.
Step 4: Return to the atmosphere. The decomposer performs cellular respiration, oxidizing the organic molecule and exhaling the carbon as . Reservoir: soil or decomposer biomass back to atmosphere. The cycle is closed.
Alternative endings worth knowing: the grasshopper could have respired the carbon itself in Step 2, returning it directly; or the grass could have been buried before decomposition, sending the atom into a slow reservoir on its way to becoming fossil fuel.
Step 5: The energy comparison. The sunlight captured in Step 1 was stored as chemical potential energy in glucose. At every step after that — grasshopper digestion, movement, decomposer metabolism — most of that energy was converted to thermal energy and radiated out of the meadow. No organism can convert that heat back into a chemical bond. So the same atom returns to the atmosphere ready for reuse, while the energy that came with it has left the ecosystem permanently. Matter cycles; energy flows through once.
Practice questions
Nitrogen gas makes up most of the atmosphere, yet nitrogen often limits plant growth. Which statement best explains why?
- Plants cannot break the triple bond in and must absorb nitrogen as ammonium or nitrate produced by bacteria
- Nitrogen gas is too heavy to reach plant roots, so it stays high in the atmosphere
- Plants absorb through their stomata but cannot transport it to the roots
- Nitrogen is destroyed by denitrifying bacteria before any plant can use it
Answer: Plants cannot break the triple bond in and must absorb nitrogen as ammonium or nitrate produced by bacteria
The chemistry is the whole story: the triple bond requires far more energy to break than plant enzymes can supply, so plants depend on nitrogen-fixing bacteria (and nitrifiers that follow) to deliver or . The claim about weight is not how gas mixing works, plants do not take in for nutrition at all, and denitrification returns nitrogen to the atmosphere rather than destroying it — a return flux, not a loss from the cycle.
A farming region increases fertilizer use sharply. Two years later, a coastal bay downstream develops a large area where dissolved oxygen is too low for fish. Explain the sequence of events connecting the fertilizer to the low-oxygen zone, and identify which reservoirs gained and lost nitrogen.
Answer: Nitrate from fertilizer runs off fields into streams and reaches the bay, where the added nitrogen relieves the nutrient limit on algae and triggers a bloom. The algae are short-lived; when they die, bacterial decomposers multiply and consume dissolved oxygen while breaking the biomass down, driving oxygen below the level fish need. Nitrogen has moved from an industrially fixed, soil-applied reservoir into water and then into aquatic biomass, while soil nitrogen is depleted by runoff.
The step that students most often skip is the decomposition step — many stop at "algae block sunlight." Shading matters, but the oxygen crash comes from decomposers respiring aerobically on a huge sudden supply of dead algae. Framing the answer as reservoirs and fluxes also makes the human role precise: industrial fixation added new usable nitrogen to the fast cycle, and runoff was the flux that carried it to a reservoir where it caused harm.
Explain why burning coal affects atmospheric carbon dioxide far more than a forest fire of the same mass of wood does over the following century.
Answer: Coal carbon comes from a slow geological reservoir that was removed from the fast carbon cycle hundreds of millions of years ago, and nothing returns it there on a human timescale, so burning it is a one-way addition to the atmosphere. Wood carbon was fixed from recent atmospheric , and if the forest regrows, photosynthesis pulls a comparable amount back out within decades.
This question is really about reservoir residence time. Both fires release through combustion, so the chemistry is identical; the difference is whether a return flux exists. Note the condition in the answer: if the burned forest is converted to pavement or cropland instead of regrowing, the recapture never happens and the wood carbon behaves much more like fossil carbon.
FAQ
- Why does matter cycle but energy flow in one direction?
- Atoms are conserved and stay within the ecosystem, so the same carbon or nitrogen atom can be reassembled into new molecules indefinitely. Energy is also conserved overall, but each transfer converts most of it into thermal energy that radiates away into space, and no organism can convert that heat back into chemical bonds. Sunlight enters, heat exits, and matter keeps looping.
- What is the difference between nitrogen fixation, nitrification and ammonification?
- Fixation converts atmospheric into ammonia or ammonium and is the only step that brings new nitrogen in from the air. Ammonification converts organic nitrogen in dead tissue and waste into ammonium during decomposition. Nitrification converts that ammonium into nitrite and then nitrate. Fixation starts in the air; the other two start with nitrogen already in the soil.
- Where is most of Earth's carbon actually stored?
- In sedimentary rock such as limestone, by an enormous margin, followed by the deep ocean. The atmosphere holds a small fraction of the total. That is exactly why relatively modest transfers out of fossil and geological reservoirs can change atmospheric concentration noticeably — a small reservoir is easy to shift.
- Does the water cycle involve any chemical change?
- Mostly no. Evaporation, condensation, precipitation, runoff and infiltration are physical changes of state or location. The important exception is biological: photosynthesis splits water molecules apart and cellular respiration produces water again, which links the water cycle directly to the carbon cycle.
Learn this with a teacher, not a page
The Crimsora tutor teaches Biogeochemical Cycles: Carbon, Nitrogen & Water live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.