Biogeochemical cycles describe how matter moves through living organisms, the atmosphere, water, rocks, and soil. This reference helps students track the major pathways of water, carbon, nitrogen, and phosphorus in ecosystems. These cycles are important because organisms need these elements and compounds to build cells, transfer energy, and maintain stable environments.
Understanding the cycles also helps explain climate change, fertilizer runoff, and ecosystem health.
The most important idea is that matter is recycled, but energy flows through ecosystems and is not recycled in the same way. Reservoirs store materials, while fluxes move materials between reservoirs through processes such as evaporation, photosynthesis, respiration, decomposition, and weathering. Carbon moves between CO2, organic molecules, fossil fuels, and carbonates, while nitrogen must often be converted by bacteria before plants can use it.
Phosphorus has no major atmospheric phase and usually moves through rocks, soil, water, and organisms.
Key Facts
- A biogeochemical cycle is the movement of matter through biological, geological, chemical, and physical parts of Earth.
- In the water cycle, evaporation and transpiration add water vapor to the atmosphere, condensation forms clouds, and precipitation returns water to Earth.
- Photosynthesis removes carbon dioxide from the atmosphere using the overall equation 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2.
- Cellular respiration returns carbon dioxide to the atmosphere using the overall equation C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP energy.
- Nitrogen fixation converts atmospheric nitrogen gas, N2, into ammonia, NH3, or ammonium, NH4+, that can enter food webs.
- Nitrification converts NH4+ into nitrite, NO2-, and then nitrate, NO3-, which plants can absorb from soil.
- Denitrification converts nitrate, NO3-, back into nitrogen gas, N2, returning nitrogen to the atmosphere.
- The phosphorus cycle mainly moves phosphate, PO4^3-, through rocks, soil, water, and organisms, with little to no gaseous atmospheric stage.
Vocabulary
- Reservoir
- A reservoir is a place where matter is stored for a short or long time, such as the atmosphere, ocean, soil, rock, or living tissue.
- Flux
- A flux is the movement or transfer of matter from one reservoir to another within a cycle.
- Transpiration
- Transpiration is the release of water vapor from plant leaves into the atmosphere.
- Nitrogen fixation
- Nitrogen fixation is the process that converts nitrogen gas into ammonia or ammonium compounds that organisms can use.
- Assimilation
- Assimilation is the process in which organisms take in nutrients such as nitrate, ammonium, or phosphate and build them into biological molecules.
- Eutrophication
- Eutrophication is excessive nutrient enrichment of water that can cause algal blooms, oxygen depletion, and harm to aquatic organisms.
Common Mistakes to Avoid
- Confusing energy flow with matter cycling is wrong because energy enters ecosystems mostly as sunlight and leaves as heat, while atoms such as carbon and nitrogen are reused.
- Saying plants get most of their mass from soil is wrong because much of plant biomass comes from carbon dioxide fixed during photosynthesis.
- Treating atmospheric N2 as directly usable by most plants is wrong because plants usually absorb nitrogen as nitrate or ammonium after bacterial conversion.
- Forgetting decomposition is wrong because decomposers return carbon, nitrogen, and phosphorus from dead organisms and waste back to soil, water, and air.
- Adding an atmospheric gas phase to the phosphorus cycle is wrong because phosphorus mainly cycles through rocks, soil, water, and organisms rather than the atmosphere.
Practice Questions
- 1 A plant absorbs 18 molecules of CO2 during photosynthesis. Using 6CO2 + 6H2O -> C6H12O6 + 6O2, how many glucose molecules can it produce?
- 2 A fertilizer sample adds 30 grams of nitrogen to a field. If 40% runs off into nearby water, how many grams of nitrogen enter the water?
- 3 In a pond, algae grow rapidly after phosphate levels increase from 0.02 mg/L to 0.10 mg/L. By how many mg/L did phosphate concentration increase?
- 4 Explain why cutting down a forest can affect both the carbon cycle and the water cycle, even if no pollution is directly added.
Understanding Biogeochemical Cycles Reference
A useful way to study any cycle is to follow one atom or molecule over time. A carbon atom in a leaf may become part of a sugar, move into an insect, enter a bird, and later return to the air when organisms respire or decay. The same atom can be stored for very different lengths of time.
It may remain in a plant for weeks, in soil organic matter for decades, or in sedimentary rock for millions of years. This difference in storage time matters.
Fast pathways can change conditions within a season. Slow pathways shape Earth over geological time.
Water links every other cycle because substances often travel while dissolved in water. Rain can carry nitrate from farm fields into streams. Flowing water can move phosphate released by weathering.
Water entering soil may be taken up by roots, stored underground, or carried to rivers. Students should distinguish water that soaks into the ground from water that runs across the surface. Groundwater moves slowly, so pollution below ground can persist for a long time.
Plants influence local water movement through transpiration. Large-scale removal of forests can reduce this transfer of moisture to the air and change nearby rainfall patterns.
Nitrogen shows why chemical form is important. Most organisms cannot use nitrogen gas directly, even though it makes up much of the air. Microbes carry out several conversion steps, and each step depends on conditions in the soil or water.
In oxygen-rich soil, certain bacteria commonly produce nitrate. In waterlogged soil with little oxygen, other bacteria can release nitrogen gas. Decomposers are central because they break down dead material and waste, returning nitrogen compounds to the soil.
Farmers use fertilizers because crops need available nitrogen, but excess nitrate can wash into waterways. There it can support rapid algae growth. When the algae die, decomposition uses oxygen, which can harm fish and other aquatic organisms.
Phosphorus is often a limiting nutrient because its supply comes mainly from slow rock weathering. In a lake, a small increase in phosphate can have a large biological effect. Phosphate can attach to soil particles, then enter water when erosion carries sediment away.
This is why bare soil near a stream creates a problem even without a visible chemical spill. Human activities alter carbon pathways too. Burning coal, oil, and natural gas transfers carbon from long-term underground storage to the atmosphere very quickly.
When reading cycle diagrams, pay attention to arrow direction, arrow thickness, and the conditions needed for each process. A diagram is a model, not a complete map. Real ecosystems have seasonal changes, disturbances, and feedbacks that can speed up or slow down the pathways.