Biogeochemical Cycles — Print · Grade 10 Science
Chapter: Ecosystems & Conservation DecisionsBiogeochemical Cycles
Learning goals
- I can describe how carbon, nitrogen, and water move through their biogeochemical cycles.
- I can explain why matter is recycled through ecosystems while energy flows through only once.
Readings for this unit
Before this unit
Trace atoms and separate reservoirs from transfer rates.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
Why can a conserved nutrient still become scarce in one location?
Illustrative evidence case. Material moves between reservoirs and forms; a local stock depends on usable forms, input and output.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can a conserved nutrient still become scarce in one location?
Your first model
Label reservoir amounts, annual flows, compound forms and a boundary.
Build the language from the idea
- biogeochemical cycle
- The pathway by which an element or molecule moves through living things (bio), the Earth (geo), and chemical reactions — for example, the carbon, nitrogen, and water cycles.
- carbon cycle
- The movement of carbon among the atmosphere, living things, oceans, soil, and rocks, driven largely by photosynthesis and respiration.
- nitrogen fixation
- The conversion of unusable nitrogen gas (N₂) into ammonia or nitrates that plants can absorb, carried out mainly by certain bacteria (and by lightning).
- transpiration
- The release of water vapor from plant leaves into the air, a major part of the water cycle.
- photosynthesis
- The process by which producers use sunlight to convert carbon dioxide and water into sugar and oxygen.
- cellular respiration
- The process organisms use to break down sugar with oxygen to release energy, giving off carbon dioxide and water.
- nitrification
- Microbial oxidation pathways converting ammonia/ammonium through nitrite to nitrate under suitable conditions.
- denitrification
- Microbial pathways converting oxidized nitrogen compounds to gaseous forms under suitable conditions.
Energy and matter behave very differently in an ecosystem, and the contrast is the heart of this unit. Energy streams in from the Sun and, after powering life for a while, leaves for good as heat — a one-way trip. Matter cannot leave. Earth is essentially a sealed container for atoms: no meaningful supply of new carbon, nitrogen, or water arrives from space. So the same atoms that build one generation of living things must be dismantled and reused to build the next, over and over. The looping pathways that carry elements through the air, water, rock, and living tissue are called biogeochemical cycles, and three of them — carbon, nitrogen, and water — keep the biosphere running.
The carbon cycle tracks carbon as it shuttles among four great reservoirs: the atmosphere (as carbon dioxide gas), living things (whose bodies are built on carbon skeletons), the oceans, and the rocks and fossil fuels underground. The living part of the cycle turns on two mirror-image reactions. Photosynthesis pulls carbon dioxide out of the air and locks its carbon into sugars inside producers; that carbon then travels up the food web as organisms eat one another. Cellular respiration runs the reaction backward — organisms break those sugars down for energy and exhale carbon dioxide straight back into the air. Decomposition and combustion release carbon too. Some of this loop is *fast* (days to years, through living things), and some is *slow* (millions of years, as carbon is buried in limestone and fossil fuels), which is why burning coal and oil — releasing ancient carbon in a geologic instant — matters so much.
The nitrogen cycle solves a stubborn problem. Nitrogen makes up about 78% of the air, and every living thing needs it to build amino acids, proteins, and the DNA that carries genetic information — yet almost nothing can use nitrogen gas (N₂) directly, because its two atoms are locked together by an extraordinarily strong triple bond. The gateway is nitrogen fixation: certain bacteria (including *Rhizobium*, which lives in nodules on the roots of beans, clover, and other legumes) and bolts of lightning convert N₂ into ammonia and nitrates that plants can finally absorb. Plants build that nitrogen into tissue, animals get it by eating plants, and decomposers release it back to the soil when organisms die. Other bacteria then perform *denitrification*, returning nitrogen gas to the atmosphere and closing the loop.
The water cycle moves the most familiar molecule of all. The Sun's heat evaporates water from oceans and lakes, and plants add their share through transpiration, releasing water vapor from tiny pores in their leaves — together these lift enormous volumes of water into the sky. The vapor rises, cools, and *condenses* into clouds, then returns to the surface as *precipitation* — rain or snow. From there it runs off into rivers or soaks down into groundwater, and the journey begins again. Along the way, moving water dissolves and carries minerals and nutrients, quietly stitching the water cycle into the carbon and nitrogen cycles.
Notice the common thread running through all three cycles: the same atoms are used again and again, handed off at key junctions by producers, decomposers, and specialized bacteria. A single carbon atom in your breath this morning may once have been part of a dinosaur, a limestone cliff, and an ocean wave. This is the decisive difference between matter and energy in ecology — matter cycles endlessly within the system, while energy flows through once and dissipates. A healthy ecosystem depends on both: a steady one-way supply of energy to do work, and tight recycling of the limited matter available.
Because these cycles are so tightly balanced, human activity can throw them off with surprising force. Burning fossil fuels floods the atmosphere with carbon dioxide faster than photosynthesis and the oceans can pull it back out, tipping the carbon cycle and warming the climate. Manufacturing synthetic fertilizer has more than doubled the amount of usable nitrogen entering ecosystems; when that excess washes off farm fields into rivers and lakes, it triggers *eutrophication* — explosive algae growth that, on dying, is decomposed by bacteria that strip the water of oxygen and suffocate fish. Even the water cycle bends under deforestation and paved cities, which cut transpiration and speed runoff. Understanding these cycles, in other words, is the first step to understanding the human impacts we turn to next.
Carbon, nitrogen and water move through biological and nonliving stores. Matter cycling requires mechanisms such as photosynthesis, decomposition and microbial transformations. A reservoir's size and the rate entering or leaving it are different quantities.
Plants generally acquire nitrogen in usable compounds rather than directly using atmospheric N₂. Microbial processes can fix nitrogen, transform compounds and return nitrogen to the atmosphere. The cycle includes several pathways rather than one compulsory circle.
Human actions can change flows, such as fertilizer input or fossil-carbon release. A nutrient increase can stimulate growth and later oxygen depletion as decomposition proceeds. State the mechanism and limiting conditions, since not every added nutrient produces the same response.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Reservoirs and fluxes
A reservoir or stock is an amount stored; a flux is a transfer amount per time. Large inputs and outputs can nearly balance, leaving little net change, while small persistent imbalances accumulate. A storage model needs starting amount, boundary and time interval. It does not imply a material appears whenever a reservoir gains it; identify the source reservoir and outgoing processes. Matter cycling and useful energy flow are distinct accounts, so a closed atom budget does not make work indefinitely self-powered.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
What happens to a 100-unit stock after three years of input 8 and output 5 per year?
Visual reasoning — after your attempt
It reaches 109 units under constant-rate assumptions: 100+3×(8−5).
A biogeochemical cycle is best described with two quantities: reservoirs, which are the amounts held in the atmosphere, oceans, soil, rock, and living things, and fluxes, which are the rates at which the element moves between them. A reservoir grows whenever its inputs exceed its outputs and shrinks when they do not, so the net change is simply the difference between fluxes.
This framing makes the cycles quantitative rather than descriptive. A forest holding 800 tonnes of carbon, gaining 45 and losing 30 in a year, is growing by 15 tonnes. Stating a cycle as a diagram of arrows without rates cannot answer questions about direction or speed, which are usually the questions that matter.
Pause and explain
A reservoir's inputs and outputs are both very large but nearly equal. What does that tell you about its size over time?
Check your reasoning after answering
It stays nearly constant, because the net change is the small difference between the two large fluxes. Large fluxes mean rapid turnover, not a changing reservoir.
Part 2 · Carbon, in and out
The carbon cycle traces carbon atoms among air, organisms, soils, water and geological material. Fixation incorporates inorganic carbon into organic compounds; respiration and decomposition can return it through other transformations. Combustion transfers carbon from fuel to products rather than adding brand-new carbon to Earth. A faster release from a long-stored reservoir can alter the distribution despite conservation across a larger boundary. Rates, storage and process identity must be distinguished when linking organisms with environmental change.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why can fossil-carbon release change atmospheric amount without violating conservation?
Visual reasoning — after your attempt
It transfers existing carbon from another reservoir; conservation concerns the complete boundary, not constant amount in each store.
Carbon enters living things almost entirely through photosynthesis, which fixes atmospheric carbon dioxide into organic molecules. It returns through respiration by all organisms, through decomposition as decomposers respire dead material, and through combustion. These processes are the same chemistry met in the energy chapter, now viewed at the scale of a whole system.
Some carbon leaves circulation for long periods. Organic matter buried in sediment, and carbon incorporated into carbonate rock, can remain out of the cycle for geological spans. Burning fossil carbon returns some of that long-stored material to the atmosphere far faster than burial removes it, which changes the balance between the fluxes rather than creating new carbon.
Pause and explain
Why is burning fossil carbon described as changing a flux rather than adding new carbon?
Check your reasoning after answering
The carbon already existed, stored in a long-term reservoir. Combustion moves it into the atmosphere, increasing that inward flux far faster than burial removes carbon, so the atmospheric reservoir grows while total carbon is unchanged.
Part 3 · Nitrogen: abundant but unusable
Nitrogen fixation converts atmospheric N₂ into forms that can enter biological nitrogen use. Nitrification and denitrification are different microbial transformations; denitrification can return nitrogen to gaseous forms under suitable conditions. Many plants take up nitrogen compounds rather than directly using N₂, with some relying on microbial associations. Availability depends on pathways, soil/water conditions and transport, not only the total nitrogen nearby. A diagram should trace compound forms and mechanisms rather than assign every microbial process the word fixation.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why does 78% nitrogen in air not mean a plant can directly build proteins from that N₂?
Visual reasoning — after your attempt
The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.
The atmosphere is about 78 percent nitrogen, yet nitrogen frequently limits plant growth. The reason is chemical: atmospheric nitrogen is N₂, held together by a strong triple bond that makes it unreactive, and plants cannot use it in that form. They require nitrate or ammonium, which must be produced by nitrogen-fixing bacteria, by lightning, or industrially.
The cycle then runs through several bacterial steps: fixation produces ammonium, nitrification converts it to nitrate, plants assimilate nitrate into protein and nucleic acids, consumers obtain nitrogen by eating, decomposition returns it to the soil, and denitrification converts soil nitrogen back to atmospheric N₂. Because most steps depend on particular bacteria, the nitrogen cycle is more biologically mediated than the carbon cycle.
Pause and explain
Why does abundance in the atmosphere not make nitrogen available to plants?
Check your reasoning after answering
Availability depends on chemical form, not abundance. Atmospheric N₂ has a strong triple bond and is unreactive, and plants can take up only nitrate or ammonium, so the limiting quantity is the rate at which fixation produces usable forms.
Part 4 · When humans change a flux
An initiating factor starts a chain, while a proximate mechanism directly produces the outcome being explained. A nutrient input can support biological growth; later decomposition and respiration can increase oxygen demand, contributing to low dissolved oxygen under suitable conditions. Nutrient addition is not itself the same event as oxygen being removed by a reaction. Flow, temperature, photosynthesis and other inputs also affect the balance. Test the chain with nutrient, biomass, oxygen and control records rather than infer one universal cause from cloudy water.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why should nutrient amount and dissolved oxygen be measured as different variables?
Visual reasoning — after your attempt
They constrain different steps in the pathway; one input alone does not uniquely establish the final oxygen outcome.
Human activity rarely creates or destroys an element; it changes the rates. Industrial nitrogen fixation and fertilizer use raise the flux of usable nitrogen into soils and waters enormously; fossil-fuel combustion raises the flux of carbon into the atmosphere; land clearance reduces the flux into plant reservoirs. Framing these as flux changes keeps the accounting honest and makes the consequences traceable.
The nitrate case shows how a flux change cascades. Nitrate running into a lake removes the nutrient limit on algae, which bloom and then die in quantity; decomposers respiring that dead material consume dissolved oxygen faster than it is replaced, and fish die of oxygen shortage. The initiating change is in the nitrogen flux, but the proximate cause of the fish deaths is oxygen depletion — a distinction that matters for deciding what to measure and what to do.
Pause and explain
In nutrient runoff that kills fish, why is it important to distinguish the initiating cause from the proximate cause?
Check your reasoning after answering
The initiating cause is the increased nitrate flux, which must be reduced to prevent recurrence, while the proximate cause is oxygen depletion by decomposers, which is what to measure to detect the problem. Confusing them leads to monitoring or intervening at the wrong point.
Apply the whole unit · Balance a carbon budget
Illustrative annual carbon fluxes for one woodland, in tonnes per year over 120 hectares. Into plants by photosynthesis: 610. Out by plant respiration: 300. Out by animal and decomposer respiration: 250. Carbon removed as harvested timber: 35. Carbon added from leaf litter blown in from outside: 12. Measured reservoir in living plants at the start of the year: 4,100 t.
Your task
Calculate the woodland's net annual carbon change and the percentage by which its living-plant reservoir would grow if the net change accumulated there, state whether the woodland is a net sink or source, and identify which single flux change would most quickly turn it into a source. Then explain why the harvested timber figure cannot be treated as a release to the atmosphere.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Total inputs: 610 by photosynthesis plus 12 blown in = 622 t.
Total outputs: 300 plant respiration plus 250 animal and decomposer respiration plus 35 harvested = 585 t.
Net change: 622 − 585 = +37 t per year, so the woodland accumulates carbon.
Percentage growth if accumulated in living plants: 37 / 4,100 × 100 ≈ 0.90 percent.
Classify: inputs exceed outputs, so the woodland is a net carbon sink this year.
Identify the sensitive flux: photosynthesis at 610 t is by far the largest input, so a reduction of about 6 percent — around 37 t, achievable through drought, defoliation, or clearance — would eliminate the surplus and make the woodland a source.
Treat the timber correctly: harvested carbon leaves the woodland but is not necessarily released; it remains stored in wood products until burned or decomposed, so its fate must be tracked separately rather than counted as an atmospheric emission.
Inputs total 622 tonnes and outputs 585, so the woodland gains 37 tonnes of carbon a year and is a net sink. If that surplus accumulated entirely in living plants it would increase that 4,100-tonne reservoir by about 0.90 percent. The budget is sensitive to one flux above all: photosynthesis supplies 610 of the 622 tonnes entering, so a reduction of roughly 6 percent — plausible in a drought year, after defoliation, or following partial clearance — would wipe out the surplus and turn the woodland into a source. The harvested timber needs separate treatment: those 35 tonnes leave the woodland's reservoir but are not released to the atmosphere at that moment, since the carbon stays in wood products until they are burned or decay. Counting it as an emission would overstate the atmospheric effect, and following it requires a second budget for the products themselves.
Scientific references and adapted source figures
Parent / teacher teaching notes
Work these cycles as reservoirs and fluxes with numbers rather than as arrow diagrams to memorize; the net-change arithmetic is what turns the cycle into something that can answer a question. The nitrogen point — abundant yet limiting — is the best single question in the unit, because it requires chemical form rather than quantity. In the eutrophication case, insist on naming both the initiating and the proximate cause, since that distinction is what makes the explanation useful rather than merely correct. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
An animated carbon cycle diagram showing carbon dioxide moving between the atmosphere, plants, animals, decomposers, and fossil fuels.
The key idea
Matter moves among organisms and environmental reservoirs through chemical and physical processes. Track each element, pathway and system boundary. Cycling does not make every local reservoir closed or guarantee endless availability; energy transfers eventually disperse as thermal energy.
Worked examples
Example 1
- The plant absorbs the carbon dioxide from the air and, through photosynthesis, locks the carbon atom into a sugar molecule in its leaf.
- A deer eats the leaf, so the carbon atom becomes part of the deer's body tissue.
- The deer breaks the sugar down through cellular respiration and exhales the carbon atom back into the air as carbon dioxide (or decomposers release it after the deer dies).
Example 2 · Worked application
- Nutrient limitation can be reduced, increasing producer growth.
- Decomposition of added organic material consumes oxygen.
- Low oxygen occurs if consumption exceeds replenishment.
Nutrients do not directly become oxygen depletion without a mechanism.
Example 3
- Nitrogen gas (N₂) has a very strong triple bond, so plants cannot break it apart or absorb it directly from the air.
- Nitrogen fixation — carried out mainly by bacteria such as Rhizobium in legume roots, and by lightning — converts N₂ into ammonia and nitrates.
- Plants can absorb those nitrates from the soil, so nitrogen becomes available only after it has been 'fixed' into a usable form.
Example 4
- Added nutrients can stimulate producer growth under limiting conditions.
- Decomposition of added organic matter consumes oxygen faster than it is replenished.
Example 5 · Find and fix the mistake
- Test the claim against the stated conditions: Nutrients do not directly become oxygen depletion without a mechanism.
- Most plants need usable nitrogen compounds; microbes contribute transformations including fixation.
Nutrients do not directly become oxygen depletion without a mechanism.
Example 6 · Reason from the visual evidence
- Nitrogen fixation converts atmospheric N₂ into forms that can enter biological nitrogen use. Nitrification and denitrification are different microbial transformations; denitrification can return nitrogen to gaseous forms under suitable conditions. Many plants take up nitrogen compounds rather than directly using N₂, with some relying on microbial associations. Availability depends on pathways, soil/water conditions and transport, not only the total nitrogen nearby. A diagram should trace compound forms and mechanisms rather than assign every microbial process the word fixation.
- The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.
Original selected nitrogen-route model, omitting several compounds and microbial pathways; it is not a full balanced nitrogen-cycle chemistry chart.
Avoid these mistakes
- Most plants do not directly turn atmospheric nitrogen gas into usable nitrogen compounds.
Check your understanding
Explain how excess nutrients can lead to low dissolved oxygen in a water body.
Discuss the answer
Nutrient enrichment can promote blooms followed by oxygen depletion, depending on conditions.
Why is a nutrient stock different from its annual input rate?
Discuss the answer
A stock is an amount stored; annual input is amount per time. Net change needs input minus output and the interval, combined with starting stock.
Return to the phenomenon
Return to the initial case. Why can a conserved nutrient still become scarce in one location? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Conservation does not mean equal distribution or immediate availability; microbial and physical pathways determine transfers and local biological effects.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. Why can a conserved nutrient still become scarce in one location? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Conservation does not mean equal distribution or immediate availability; microbial and physical pathways determine transfers and local biological effects.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
A nitrogen store begins at 50 units and receives 12 while losing 9 each year for four years. Find its final amount and distinguish that result from the annual input.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.
Words to know
- biogeochemical cycle
- The pathway by which an element or molecule moves through living things (bio), the Earth (geo), and chemical reactions — for example, the carbon, nitrogen, and water cycles.
- carbon cycle
- The movement of carbon among the atmosphere, living things, oceans, soil, and rocks, driven largely by photosynthesis and respiration.
- nitrogen fixation
- The conversion of unusable nitrogen gas (N₂) into ammonia or nitrates that plants can absorb, carried out mainly by certain bacteria (and by lightning).
- transpiration
- The release of water vapor from plant leaves into the air, a major part of the water cycle.
- photosynthesis
- The process by which producers use sunlight to convert carbon dioxide and water into sugar and oxygen.
- cellular respiration
- The process organisms use to break down sugar with oxygen to release energy, giving off carbon dioxide and water.
- nitrification
- Microbial oxidation pathways converting ammonia/ammonium through nitrite to nitrate under suitable conditions.
- denitrification
- Microbial pathways converting oxidized nitrogen compounds to gaseous forms under suitable conditions.
Photosynthesis and respiration: one reaction, run both ways
The carbon cycle's living engine is really a single chemical reaction read in two directions. Photosynthesis combines carbon dioxide and water, using light energy, to build glucose and release oxygen. Cellular respiration does the exact reverse: it combines glucose with oxygen to release energy, giving off carbon dioxide and water. The outputs of one are the inputs of the other, which is why the two are taught side by side — together they shuttle the same carbon and oxygen atoms endlessly between the air and living things.
The chemist who learned to feed billions — and disrupt a cycle
In the early 1900s, chemist Fritz Haber worked out how to pull nitrogen straight from the air and forge it into ammonia; Carl Bosch scaled it into industry, and the Haber-Bosch process was born. It now fixes more nitrogen than all the world's natural processes combined, and roughly half the nitrogen atoms in your body have passed through it. The upside is staggering — synthetic fertilizer feeds billions of people. The downside is that it has more than doubled the reactive nitrogen flooding into ecosystems, driving pollution and dead zones far downstream.
A dead zone the size of a small state
Every summer, fertilizer and animal waste washing down the Mississippi River fuel enormous algae blooms where the river meets the Gulf of Mexico. As the algae die and decompose, bacteria strip the oxygen from the water, creating a 'dead zone' — often thousands of square miles — where fish, shrimp, and crabs cannot survive and must flee or die. It's eutrophication written across an entire coastline, and it's a direct, large-scale consequence of overloading the nitrogen cycle upstream.
The fast cycle and the slow cycle
Carbon actually moves on two very different clocks at once. The fast carbon cycle shuttles carbon among the air, living things, and soil in days to years through photosynthesis and respiration. The slow carbon cycle locks carbon into limestone, seashells, and fossil fuels for hundreds of thousands to millions of years, releasing it only through slow processes like volcanic eruptions. Burning fossil fuels short-circuits this: it takes carbon the slow cycle spent millions of years burying and dumps it back into the fast cycle in a geological instant.
The ocean's hidden bill: acidification
The oceans quietly absorb roughly a quarter of the extra carbon dioxide humans release, which sounds helpful until you follow the chemistry. Dissolved CO₂ reacts with seawater to form carbonic acid, gradually lowering the ocean's pH — a process called ocean acidification. More acidic water makes it harder for corals, clams, oysters, and tiny plankton to build and keep their calcium carbonate shells and skeletons, threatening the base of marine food webs. It's a reminder that a 'carbon sink' isn't free; the carbon cycle sends the bill elsewhere.
In the real world
Farmers have used the nitrogen cycle for centuries without knowing the chemistry: rotating in legume crops like beans and clover, whose root bacteria fix nitrogen, naturally replenishes soil nitrogen and reduces the need for synthetic fertilizer.
Exit check
How can nitrogen fertilizer contribute to oxygen depletion in water?
Check your explanation after answering
Added nutrients can stimulate production; decomposition of increased organic material uses dissolved oxygen through respiration. The sequence depends on local conditions.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
- Photosynthesis
- Nitrogen fixation
- Combustion
- Cellular respiration
Answer and explanation
Photosynthesis
Photosynthesis absorbs carbon dioxide and builds it into sugars inside producers.
Problem 2
- breaks nitrates down into nitrogen gas
- converts nitrogen gas into a form plants can absorb
- releases carbon dioxide into the atmosphere
- turns liquid water into vapor
Answer and explanation
converts nitrogen gas into a form plants can absorb
Nitrogen fixation, done mainly by bacteria, converts unusable N₂ gas into ammonia and nitrates that plants can take up.
Problem 3
Answer and explanation
390 mL
500 × 0.78 = 390 mL of nitrogen gas.
Problem 4
- condensation
- precipitation
- transpiration
- runoff
Answer and explanation
transpiration
Transpiration is water evaporating from plant leaves; it is a major source of atmospheric water vapor.
Problem 5
Answer and explanation
The fertilizer adds extra nitrogen and phosphorus to the lake, which fuels an explosive bloom of algae. When the algae die, decomposer bacteria break them down and use up much of the dissolved oxygen in the water. With the oxygen depleted, fish and other aquatic animals suffocate. This process is called eutrophication.
Problem 6
Answer and explanation
Nutrient enrichment can promote blooms followed by oxygen depletion, depending on conditions.
Added nutrients can stimulate producer growth under limiting conditions.
Decomposition of added organic matter consumes oxygen faster than it is replenished.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 7
Hints
Nutrients do not directly become oxygen depletion without a mechanism.
Answer and explanation
The reservoir is a stored amount; the input is a rate. They cannot be compared as the same quantity.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
Atoms are reused among stores, while usable energy disperses and must be replenished.
Problem 9
Answer and explanation
The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.
Nitrogen fixation converts atmospheric N₂ into forms that can enter biological nitrogen use. Nitrification and denitrification are different microbial transformations; denitrification can return nitrogen to gaseous forms under suitable conditions. Many plants take up nitrogen compounds rather than directly using N₂, with some relying on microbial associations. Availability depends on pathways, soil/water conditions and transport, not only the total nitrogen nearby. A diagram should trace compound forms and mechanisms rather than assign every microbial process the word fixation.
The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.
Response criteria
- Read the structures, axes, directions and units correctly.
- Connect the evidence with the biological mechanism.
- Respect stated model assumptions and distinguish an observation from a further inference.
Problem 10
Answer and explanation
The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.
The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.