Biogeochemical Cycles Reference (C, N, P, S, H2O) Cheat Sheet

A printable reference covering carbon, nitrogen, phosphorus, sulfur, and water cycle processes, reservoirs, human impacts, and key vocabulary for grades 9-12.

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Biogeochemical cycles describe how matter moves between living organisms, the atmosphere, water, soil, and rocks. This reference covers the carbon, nitrogen, phosphorus, sulfur, and water cycles, which are central to ecosystems and environmental science. Students need these cycles to understand climate change, nutrient availability, pollution, acid rain, and ecosystem health. A clear comparison helps show which processes are biological, geological, chemical, or physical.

Key Facts

In the carbon cycle, photosynthesis removes CO2 from the atmosphere using 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2.
Cellular respiration returns carbon to the atmosphere using C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy.
Combustion of fossil fuels and biomass releases stored carbon as CO2, increasing atmospheric greenhouse gas concentrations.
Nitrogen fixation converts atmospheric nitrogen gas, N2, into usable forms such as NH3 or NH4+ by bacteria or lightning.
Nitrification converts NH4+ to NO2- and then NO3-, while denitrification converts NO3- back to N2 gas.
The phosphorus cycle has no major atmospheric phase and mainly moves through rocks, soil, water, and living organisms as phosphate, PO4^3-.
The sulfur cycle includes weathering, volcanic emissions, decomposition, and fossil fuel combustion, with sulfur moving as SO2, sulfate, and sulfide compounds.
The water cycle moves H2O through evaporation, transpiration, condensation, precipitation, infiltration, runoff, and groundwater flow.

Vocabulary

Reservoir: A reservoir is a place where a chemical element or compound is stored for a period of time, such as the atmosphere, ocean, soil, rocks, or organisms.
Flux: Flux is the rate at which matter moves from one reservoir to another in a biogeochemical cycle.
Photosynthesis: Photosynthesis is the process in which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen.
Nitrogen Fixation: Nitrogen fixation is the conversion of atmospheric nitrogen gas into ammonia or ammonium that living organisms can use.
Eutrophication: Eutrophication is nutrient enrichment of water, often from nitrogen or phosphorus runoff, that can cause algal blooms and low oxygen levels.
Transpiration: Transpiration is the release of water vapor from plant leaves into the atmosphere.

Common Mistakes to Avoid

Confusing nitrogen gas with usable nitrogen is wrong because most organisms cannot use N2 directly and need nitrogen fixed into NH3, NH4+, or NO3-.
Saying the phosphorus cycle has a major atmospheric stage is wrong because phosphorus mainly cycles through rocks, soil, water, and organisms, not the air.
Treating decomposition as only part of one cycle is wrong because decomposers return carbon, nitrogen, phosphorus, sulfur, and water-related materials to the environment.
Forgetting human impacts is wrong because fertilizer use, fossil fuel burning, mining, and land clearing can greatly change natural cycle rates.
Mixing up evaporation and transpiration is wrong because evaporation is water changing to vapor from surfaces, while transpiration is water vapor released by plants.

Practice Questions

1 A power plant burns coal and releases 1,000 kg of carbon as CO2 in one hour. Which biogeochemical cycle is directly affected, and what reservoir receives the carbon?
2 A farmer applies fertilizer containing nitrate, NO3-, before heavy rain. Name one cycle affected and explain how runoff could lead to eutrophication.
3 If a forest stores 500 metric tons of carbon and a fire releases 20 percent of it, how many metric tons of carbon are released?
4 Why can the phosphorus cycle limit plant growth in many ecosystems even though nitrogen and carbon are also essential nutrients?

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