Unit 03 · Biogeochemical Cycles
Matter doesn’t leave Earth — it cycles. This unit follows carbon, nitrogen, phosphorus, and water as they move between reservoirs — the atmosphere, oceans, soil, and living things — through the fluxes that link them. You’ll see where each element is stored, what moves it, and how humans have bent every cycle: burning fossil carbon, fixing nitrogen industrially through the Haber–Bosch process, mining phosphate for fertilizer, and rerouting water. Mastery means you can trace an atom through a cycle and explain how a human input throws it off balance.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| The carbon cycle | Thinks carbon just sits in living things. | Names photosynthesis and respiration but misses the ocean and fossil reservoirs. | Traces carbon through photosynthesis, respiration, oceans, and fossil fuels, and explains how combustion adds to the atmosphere. |
| The nitrogen cycle | Cannot say why nitrogen gas is unusable to most life. | Names fixation but not the microbes or the industrial route. | Explains nitrogen fixation (biological and Haber–Bosch), nitrification, and denitrification, and how fertilizer floods the cycle. |
| The phosphorus & water cycles | Treats these cycles as identical to carbon’s. | Describes the water cycle but has no phosphorus reservoir in mind. | Contrasts the phosphorus cycle (rock-bound, no atmospheric step) with the water cycle and its evaporation–precipitation loop. |
| Reservoirs, fluxes & human disruption | Confuses a reservoir (a store) with a flux (a flow). | Labels reservoirs and fluxes but cannot quantify a change. | Distinguishes reservoirs from fluxes and predicts how a human input — combustion, fertilizer runoff — shifts a cycle out of balance. |
| Field technique (cycle-flux from data) | Cannot read a nitrate or dissolved-oxygen result from a test kit. | Records water-test data but cannot tie it to a cycle. | Runs water-quality tests (nitrate, dissolved O₂) and uses the readings to estimate a flux and infer human disruption. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The stream below the farm read high in nitrate and low in dissolved oxygen — fertilizer nitrogen is entering the water and feeding decomposition. That nitrogen was pulled from the air by the Haber–Bosch process; humans now fix more nitrogen than all the microbes combined. The reservoir is the same — we’ve cranked the flux.”
“Nitrogen goes around in a circle somehow. The water was kind of dirty. I’m not sure what the numbers on the kit mean.”
You demonstrate this unit through water-quality testing — nitrate and dissolved oxygen — and by tracing a cycle’s fluxes from field or public data aloud, not on a multiple-choice test. A criterion counts as mastered only when your readings support a claim about how humans have perturbed the cycle. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.