AP Biology · Lesson 3 of 7 · Unit 8

Biogeochemical cycles: water, carbon, nitrogen and phosphorus

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While energy flows one way through an ecosystem and dissipates as heat, matter is conserved and endlessly recycled between living organisms and the physical environment. On the AP Biology exam (explicitly added to Topic 8.2 in the 2025–26 CED), you will trace water, carbon, nitrogen, and phosphorus through visual models and predict how human or natural disruptions alter biotic and abiotic reservoirs.

By the end you’ll be able to

Describe the ecological levels of organization: populations, communities, ecosystems and biomes.
Explain how energy flows through ecosystems while matter cycles between abiotic and biotic reservoirs, conserving matter in interdependent cycles.
Describe the reservoirs and processes of the water cycle and the carbon cycle, including photosynthesis, cellular respiration, decomposition and combustion.

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Core ideas (continued)
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What the exam asks

Multiple choice (Section I): Stimulus sets frequently feature box-and-arrow cycle diagrams or experimental data on soil nutrients. You will identify missing reservoirs or processes, or predict how removing a specific organism (like nitrifying bacteria or mycorrhizal fungi) impacts primary productivity.
Free response (Section II): Expect this topic in Question 4 (Conceptual Analysis) or Question 5 (Analyze Model or Visual Representation). You may be asked to describe how an element moves from an abiotic to a biotic reservoir, draw arrows on a model to represent matter transfer, or predict and justify the downstream ecological effects of fertilizer runoff or deforestation.

Core ideas

Ecological levels and the conservation of matter

Ecology scales from populations (interbreeding individuals of one species in an area) to communities (all interacting populations in an area), ecosystems (the community plus its abiotic environment), and biomes (large regions defined by climate and dominant vegetation).

At the ecosystem level, energy flows (entering as sunlight, passing through trophic levels, and exiting as metabolic heat), whereas matter cycles. According to the law of conservation of matter, atoms are neither created nor destroyed; they move continuously between abiotic reservoirs (atmosphere, water, soil, rock, fossil fuels) and biotic reservoirs (living biomass and detritus) through interdependent biogeochemical cycles.

Summary of the four biogeochemical cycles

Cycle	Largest Abiotic Reservoir	Biologically Available Form(s)	Key Processes Driving the Cycle	Biological Role (Unit 1 Link)
Water ( $\text{H}_2\text{O}$ )	Oceans	Liquid water ( $\text{H}_2\text{O}$ )	Evaporation, transpiration, condensation, precipitation, runoff, percolation	Solvent for biochemical reactions; source of electrons in photosynthesis
Carbon ( $\text{C}$ )	Sedimentary rock & fossil fuels (fast exchange: atmosphere & oceans)	Atmospheric/dissolved $\text{CO}_2$ , organic carbon (e.g., glucose)	Photosynthesis (fixes $\text{C}$ ), cellular respiration, decomposition, combustion (releases $\text{CO}_2$ )
Nitrogen ( $\text{N}$ )	Atmosphere ( $\text{N}_2$ gas, $\thicksim 78\%$ )	Ammonium ( $\text{NH}_4^+$ ) and nitrate ()
Phosphorus ( $\text{P}$ )	Sedimentary rock of marine origin	Inorganic phosphate ( $\text{PO}_4^{3-}$ )	Weathering of rock, assimilation, decomposition, excretion, sedimentation	Nucleic acids (sugar-phosphate backbone), ATP, phospholipids