This unit follows carbon, nitrogen, phosphorus, and water as they move between living things, air, oceans, soil, and rock. By the end, you should be able to trace each cycle step by step, explain how water and greenhouse gases regulate Earth's temperature, and predict what happens when humans add extra carbon or nutrients to a system.
Skim the overview first, then work through each topic's notes and checkpoint question before reading the common mistakes at the bottom.
Energy often enters an ecosystem as sunlight and eventually leaves as heat. Matter cycles and can enter or leave an ecosystem. This unit tracks where four kinds of matter go and how long they stay in each place. It ends with how burning fuel, clearing forests, and spreading fertilizer change those cycles.
Honors Biology Unit 19 covers biogeochemical cycles, the paths that carbon, nitrogen, phosphorus, and water take through living and nonliving parts of Earth. You learn how photosynthesis and respiration move carbon, how bacteria fix and convert nitrogen, why phosphorus moves mostly through rock and water, and how evaporation, transpiration, and precipitation move water. The unit connects these cycles to climate through the greenhouse effect and finishes with human impacts like rising (about 280 ppm before industrialization, over 420 ppm today), ocean acidification, and eutrophication.
A reservoir is a place where an element is stored, like the atmosphere, the ocean, soil, or fossil fuels. Processes such as photosynthesis, runoff, or denitrification move atoms between reservoirs. If you can name the reservoirs and the arrows between them, you can draw any cycle in this unit.
Plants pull carbon out of the air, and nearly all organisms put it back through respiration. Bacteria and archaea carry out many nitrogen transformations, and other organisms and nonliving processes also change nitrogen's chemical form. Decomposers return carbon, nitrogen, and phosphorus from dead matter so producers can reuse them.
When more carbon enters the air than leaves it, the extra heat-trapping gas warms the planet and acidifies the ocean. When extra nitrogen or phosphorus washes into a lake, it can trigger an algal bloom. Decomposition of the bloom can then lower dissolved oxygen. Most environmental problems in this unit come down to an input that is bigger than the system's output.
Atoms don't disappear. They move from one reservoir to another. Natural cycles were roughly balanced for thousands of years, and human activity has sped up certain transfers, like moving buried carbon into the air or nitrogen from the air into rivers. Once you see each problem as too much matter in one reservoir, climate change and dead zones become predictable outcomes instead of separate facts to memorize.
Carbon, nitrogen, and phosphorus atoms get reused again and again.
Water moves among oceans, air, land, ice, and living things.
Humans have sped up the release side of several cycles faster than natural uptake can keep pace.
Carbon, nitrogen, and phosphorus atoms get reused again and again. One carbon atom can sit in the air as , get built into glucose by a maple leaf, pass into a caterpillar, and return to the air when the caterpillar respires.
The carbon cycle runs on two opposite reactions. Photosynthesis () pulls carbon out of the air, and cellular respiration reverses it. Longer-term stores include ocean water, shells and limestone (), soil, and fossil fuels.
Nitrogen makes up 78% of the air, but most organisms can't use because its triple bond is so strong. Microbes handle many of the conversions. Biological fixation turns into ammonia, nitrification turns ammonium into nitrite and then nitrate, plants absorb nitrate or ammonium and build nitrogen into organic molecules (assimilation), decomposers release ammonium from dead matter and waste (ammonification), and denitrification returns nitrogen to the air as .
Phosphorus has no major atmospheric reservoir, although trace amounts occur in gases. Weathering rocks release phosphate, which moves through soil, water, and food webs and settles into sediment that can become rock over millions of years.
Worked example. A forest plot absorbs 1,200 g of carbon per square meter per year through photosynthesis. Plants respire 550 g C/m² and decomposers respire 500 g C/m² that year. If there are no other carbon transfers, net storage is g C/m² per year, so the forest is a carbon sink. If a fire also released 300 g C/m² that year, the net would be g C/m² per year, and the forest would become a carbon source.
Check yourself: A farmer plants clover in a field before growing corn. Why does this add nitrogen to the soil? (Answer: Clover is a legume whose root nodules hold Rhizobium bacteria that fix from the air into ammonia. When clover material decomposes, some of its nitrogen can become available to the corn.)
| Feature | Carbon | Nitrogen | Phosphorus |
|---|---|---|---|
| Main atmospheric form | $CO_2$ | $N_2$ (78% of air) | No major atmospheric form |
| Largest reservoir | Rocks and ocean | Atmosphere | Rocks and sediment |
| How it enters living things | Photosynthesis | Plant uptake of nitrate or ammonium, or fixation by some microbes | Plants absorb phosphate from soil and water |
| Used to build | All organic molecules | Proteins and nucleic acids | DNA, RNA, ATP, phospholipids |
| Relative speed | Fast in living things, slow in rock | Moderate, depends on bacteria | Slowest |
Water moves among oceans, air, land, ice, and living things. Solar energy drives evaporation, while gravity drives precipitation and water flow downhill. Evaporation lifts water off oceans and lakes, and transpiration releases it through stomata in leaves. Rising air cools, vapor condenses on dust or salt particles to form clouds, and precipitation brings it back down. From there it either runs off into streams or infiltrates into soil and aquifers, where recharge can take thousands of years.
Only about 3% of Earth's water is fresh, and most of that is frozen in ice caps and glaciers. Groundwater is the largest supply of fresh water people can actually reach.
Water also helps set Earth's temperature. Greenhouse gases like water vapor, , and methane absorb infrared heat given off by the surface. That natural greenhouse effect keeps the global average near 15°C instead of about −18°C. Oceans store huge amounts of heat because water has a high specific heat, which keeps coastal climates milder than inland ones. Forests affect rainfall too, since transpiration in the Amazon supplies a large share of the region's own moisture.
Worked example. A watershed gets 1,000 mm of rain in a year. Evapotranspiration returns 600 mm to the air, and 250 mm leaves as runoff. If the remaining water all recharges the aquifer, recharge is mm, or 15% of the rain. If paving raised runoff to 400 mm while evapotranspiration stayed at 600 mm, the same model would give mm of recharge.
Check yourself: Why does a city with lots of pavement often flood more and have lower groundwater than a forested area with the same rainfall? (Answer: Pavement blocks infiltration, so more rain becomes runoff and less soaks in to recharge aquifers, and there are fewer plants returning water through transpiration.)
Humans have sped up the release side of several cycles faster than natural uptake can keep pace. Burning coal, oil, and gas moves carbon that was buried for millions of years into the air. Deforestation removes trees that would have absorbed and releases the carbon stored in their wood. Atmospheric has climbed from about 280 ppm before the Industrial Revolution to over 420 ppm today.
The ocean takes up about a quarter of human-caused emissions. Some of that reacts with water to form carbonic acid, which lowers pH. That makes it harder for corals, clams, and some plankton to build calcium carbonate shells. Warmer water also causes coral bleaching.
Human activities, including fertilizer production, fossil fuel burning, and cultivation of nitrogen-fixing crops, have roughly doubled the amount of reactive nitrogen added to land ecosystems each year. Extra nitrogen and phosphorus wash into lakes and coastal waters and trigger eutrophication. Algae bloom, die, and sink, and decomposers use up dissolved oxygen as they break the algae down, creating dead zones like the one in the Gulf of Mexico. Phosphorus can linger for years because it binds to lake sediment instead of escaping as a gas.
Worked example. Ocean pH fell from about 8.2 to 8.1. Because pH is logarithmic, the hydrogen ion concentration rose by a factor of , about a 26% increase in acidity (often rounded to 30%). The ocean is still basic, with a pH above 7, but it is moving toward acidic.
Check yourself: A town stops dumping phosphate-rich sewage into a lake, but algae blooms continue for several more summers. Why? (Answer: Phosphorus stored in lake sediment can be released back into the water and feed algae. For example, low oxygen near the bottom can make some sediment release phosphate.)
| Human activity | Cycle affected | Main result |
|---|---|---|
| Burning fossil fuels | Carbon | More atmospheric $CO_2$, warming, ocean acidification |
| Deforestation | Carbon and water | Less $CO_2$ uptake, stored carbon released, and potentially less local rainfall |
| Synthetic nitrogen fertilizer | Nitrogen | Eutrophication, dead zones, $N_2O$ emissions |
| Phosphate fertilizer and sewage | Phosphorus | Eutrophication and phosphorus buildup in sediment |
| Paving and over-pumping wells | Water | More runoff and less infiltration from paving; lower groundwater levels from over-pumping |
| Term | Definition |
|---|---|
| Biogeochemical cycle | The path an element or compound like carbon, nitrogen, phosphorus, or water takes between living organisms and Earth's air, water, and rock. |
| Reservoir | A place where an element is stored in a cycle, such as the atmosphere, ocean, soil, or fossil fuels. |
| Carbon sink | A reservoir or ecosystem that absorbs more carbon than it releases over a given time. |
| Ammonification | The release of ammonium by decomposers as they break down proteins and other nitrogen compounds in dead matter and waste. |
| Haber-Bosch process | An industrial method that fixes atmospheric nitrogen into ammonia to make synthetic fertilizer. |
| Evapotranspiration | The combined water vapor that enters the air from evaporation off surfaces and transpiration from plants. |
| Groundwater recharge | The process of water seeping down through soil and rock to refill an aquifer. |
| Coral bleaching | The loss of a coral's symbiotic algae when water gets too warm, which leaves the coral white and short on food. |
Matter is recycled, but energy is not. Sunlight enters as light, and energy is released as heat at each step in a food chain. Carbon atoms in your sandwich can end up back in a plant, while energy in the sandwich can pass to other organisms before it leaves as heat.
Without the natural greenhouse effect, Earth would average about −18°C and be frozen. The concern is the enhanced greenhouse effect, where extra and methane from human activity trap more heat than before. Also, the ozone hole is a separate issue about UV radiation and isn't the main cause of warming.
Both are done by bacteria and both start with N, so they blur together. Fixation pulls out of the air and turns it into ammonia. Nitrification starts with ammonium already in the soil and converts it to nitrate, so it never touches gas.
Living algae produce oxygen during photosynthesis, but they also consume oxygen through respiration. After a bloom dies, decomposers can use large amounts of dissolved oxygen as they break down the dead algae. Remember the order: nutrients, bloom, die-off, decomposition, low oxygen.