Key Concepts of Biogeochemical Cycles

Why This Matters

Biogeochemical cycles are the foundation for understanding how ecosystems function as interconnected systems. On the AP Biology exam, you're being tested on your ability to explain how matter cycles through trophic levels while energy flows in one direction—a fundamental distinction that appears repeatedly in multiple-choice questions and FRQs. These cycles demonstrate the conservation of matter in living systems and connect producers, consumers, and decomposers in ways that maintain ecosystem stability.

The key insight here is that each cycle has specific reservoirs (where elements are stored) and processes (how elements move between reservoirs). You'll need to recognize how photosynthesis, cellular respiration, decomposition, and human activities drive these cycles and how disruptions cascade through ecosystems. Don't just memorize the steps—know which organisms perform each transformation and why the cycle matters for building biological molecules like proteins, nucleic acids, and ATP.

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Cycles with Atmospheric Reservoirs

These cycles feature a significant gaseous phase, meaning elements can move rapidly through the atmosphere and are globally distributed. The atmospheric reservoir allows for relatively fast cycling and widespread availability of these elements.

Carbon Cycle

• Photosynthesis and respiration are the primary biological drivers—producers fix atmospheric $CO_2$ into organic molecules, while all organisms release it through cellular respiration
• Decomposers complete the cycle by breaking down dead organic matter and returning carbon to the atmosphere and soil
• Human combustion of fossil fuels releases carbon that was sequestered for millions of years, disrupting the balance between carbon fixation and release

Nitrogen Cycle

• Nitrogen fixation converts atmospheric $N_2$ to ammonia ($NH_3$)—only certain bacteria (including Rhizobium in root nodules) can break nitrogen's triple bond
• Nitrification transforms ammonia to nitrates ($NO_3^-$) through bacterial action, making nitrogen available for plant uptake and incorporation into amino acids and nucleic acids
• Denitrification returns nitrogen to the atmosphere as $N_2$, completing the cycle and performed by anaerobic bacteria in waterlogged soils

Oxygen Cycle

• Photosynthesis is the primary source of atmospheric oxygen—the light reactions split water molecules, releasing $O_2$ as a byproduct
• Cellular respiration consumes oxygen as the final electron acceptor in the electron transport chain, linking oxygen directly to ATP production
• Phytoplankton produce roughly half of Earth's oxygen, making ocean ecosystems critical to maintaining atmospheric composition

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Cycles Without Gaseous Phases

The phosphorus cycle is unique among major biogeochemical cycles because it lacks an atmospheric component. This means phosphorus moves much more slowly and is often a limiting nutrient in ecosystems.

Phosphorus Cycle

• Rock weathering is the primary source—phosphate ($PO_4^{3-}$) is released from rocks over geological timescales and enters soil and water
• Phosphorus is essential for DNA, RNA, ATP, and phospholipids—without it, cells cannot store genetic information or transfer energy
• No atmospheric reservoir means local depletion is possible—agricultural runoff creates excess phosphorus in aquatic systems, causing eutrophication and algal blooms

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The Universal Solvent Cycle

Water connects all other biogeochemical cycles by serving as the medium for chemical reactions and transport of dissolved nutrients. The hydrologic cycle is driven primarily by solar energy and gravity.

Water Cycle (Hydrologic Cycle)

• Evaporation and transpiration move water to the atmosphere—transpiration from plants accounts for a significant portion of water vapor over land
• Precipitation returns water to terrestrial and aquatic ecosystems, where it becomes available for biological processes and dissolves nutrients for uptake
• Water is the medium for all metabolic reactions—its polarity allows it to dissolve ionic compounds and transport materials through organisms and ecosystems

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Cycles Connecting Geology and Biology

These cycles operate on longer timescales and demonstrate how geological processes provide the raw materials that sustain life. Understanding these connections helps explain nutrient availability and ecosystem productivity.

Sulfur Cycle

• Sulfur is essential for protein structure—the amino acids cysteine and methionine contain sulfur, enabling disulfide bonds that stabilize protein folding
• Volcanic activity and decomposition release sulfur compounds—bacteria in anaerobic environments also produce hydrogen sulfide ($H_2S$)
• Chemosynthetic bacteria at hydrothermal vents oxidize $H_2S$ to produce ATP, supporting entire ecosystems without sunlight

Rock Cycle

• Weathering releases mineral nutrients including phosphorus, calcium, and potassium that are essential for plant growth and ecosystem productivity
• Sedimentation can sequester carbon for millions of years in limestone and fossil fuels, representing a long-term carbon reservoir
• Soil formation depends on rock breakdown—the mineral component of soil determines nutrient availability and ecosystem type

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Quick Reference Table

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Self-Check Questions

1. Which two cycles both require bacterial action to convert atmospheric gases into biologically usable forms, and how do the specific bacteria involved differ in their ecological niches?

2. Compare and contrast the carbon and phosphorus cycles in terms of their reservoirs, cycling speed, and why one is more likely to cause rapid climate effects when disrupted.

3. If an ecosystem shows signs of eutrophication, which cycles have been disrupted, and how would you determine whether nitrogen or phosphorus is the primary cause?

4. Explain how the oxygen cycle is directly dependent on the carbon cycle, referencing the specific metabolic processes that connect them.

5. An FRQ asks you to describe how matter cycles but energy flows through ecosystems. Using the carbon cycle as your example, explain why carbon atoms can be recycled indefinitely while the energy they carry cannot.