Lesson 02: Biogeochemical Cycles

Warm-Up

The carbon atom in your last breath may once have been inside a dinosaur, a limestone cliff, a drop of seawater, and a lump of coal. Matter doesn't flow one way and leave like energy does — it cycles, endlessly, between living things and the air, water, rock, and soil. Environmental science is largely the study of how humans have grabbed the steering wheel on these ancient cycles: pulling carbon out of the ground faster than it can be buried, flooding the nitrogen cycle with synthetic fertilizer, mining phosphorus that took eons to form. Learn where each element is stored and what moves it, and you can predict almost every pollution problem in the course.

Core Concept

A biogeochemical cycle traces how an element moves through the living (bio) and nonliving (geo) parts of Earth. Two key vocabulary words: a reservoir (or sink) is where a large amount of the element sits (the ocean, the atmosphere, rock); a flux is the movement between reservoirs (photosynthesis, combustion).

The carbon cycle

Carbon is the backbone of all organic molecules and the key climate element.

• Reservoirs: atmosphere (CO₂), oceans (dissolved CO₂, carbonate), living biomass, soil, and the largest — sedimentary rock and fossil fuels.
• Fluxes into the atmosphere: cellular respiration, decomposition, combustion (fires and burning fossil fuels), ocean release.
• Fluxes out of the atmosphere: photosynthesis and dissolving into the ocean.

[DIAGRAM: Carbon cycle. Atmosphere CO₂ in center. Arrows out via photosynthesis to plants; arrows back via respiration, decomposition, combustion. Plants → animals (feeding) → decomposition → soil. Dead matter buried → fossil fuels (slow) → combustion returns C to atmosphere (fast, human-driven). Ocean exchanges CO₂ with atmosphere; marine organisms → sedimentary rock (limestone).]

Human disruption: Burning fossil fuels transfers carbon locked away for millions of years into the atmosphere in decades, raising atmospheric CO₂ and driving climate change (Unit 9). Deforestation removes a carbon sink.

The nitrogen cycle

Nitrogen is needed for proteins and DNA. The atmosphere is 78% N₂ gas, but that triple-bonded N₂ is useless to most organisms until it's "fixed."

Key steps (mostly done by bacteria): - Nitrogen fixation: N₂ → ammonia (NH₃/NH₄⁺), by nitrogen-fixing bacteria (often in legume root nodules) or lightning. - Nitrification: ammonia → nitrites (NO₂⁻) → nitrates (NO₃⁻), by nitrifying bacteria. Nitrate is the form plants absorb most easily. - Assimilation: plants take up nitrates/ammonium and build proteins; animals get N by eating plants. - Ammonification: decomposers convert dead organic N back to ammonium. - Denitrification: nitrates → N₂ gas, returning nitrogen to the atmosphere (by denitrifying bacteria in low-oxygen soils).

[DIAGRAM: Nitrogen cycle. Atmospheric N₂ at top. Fixation arrow down to NH₃/NH₄⁺ (bacteria/lightning). Nitrification NH₄⁺→NO₂⁻→NO₃⁻. Plants assimilate NO₃⁻. Animals eat plants. Decomposers → ammonification back to NH₄⁺. Denitrification NO₃⁻ → N₂ back to atmosphere.]

Human disruption: The Haber-Bosch process industrially fixes N₂ into fertilizer, roughly doubling the nitrogen entering ecosystems. Excess runoff causes eutrophication (Unit 8) and, burned in engines, nitrogen oxides cause smog and acid rain (Unit 7).

The phosphorus cycle

Phosphorus is needed for DNA, ATP, and bones/teeth. The defining feature for the exam:

The phosphorus cycle has NO significant atmospheric (gas) phase. Phosphorus moves through rock, soil, water, and organisms — slowly.

• Reservoir: phosphate rock. Weathering releases phosphate (PO₄³⁻) into soil and water.
• Plants absorb phosphate; it moves up the food chain; decomposition returns it to soil; it eventually settles into sediment and, over geologic time, forms new rock.
• Because there's no atmospheric reservoir and rock weathers slowly, phosphorus is often the limiting nutrient in ecosystems.

Human disruption: Mining phosphate for fertilizer and detergents; runoff drives freshwater eutrophication (phosphorus usually limits freshwater; nitrogen usually limits marine systems).

The hydrologic (water) cycle

• Processes: evaporation (and transpiration from plants — together, evapotranspiration), condensation, precipitation, infiltration (water soaking into soil to recharge groundwater/aquifers), and runoff (surface flow to rivers and oceans).
• Reservoirs: oceans (~97% of Earth's water), ice caps/glaciers, groundwater, lakes/rivers, atmosphere.

[DIAGRAM: Water cycle. Ocean → evaporation up; plants → transpiration up; both form clouds (condensation); precipitation falls; some infiltrates to groundwater/aquifer, some runs off to rivers back to ocean.]

Human disruption: Paving land with impervious surfaces increases runoff and reduces infiltration; over-pumping aquifers lowers water tables; deforestation reduces transpiration.

Why this matters

Nearly every pollution unit is a cycle knocked out of balance. Eutrophication = too much N and P. Climate change = too much C in the atmosphere. Groundwater depletion = disrupted water cycle. Know the cycles and the later units become applications.

Worked Examples

Example 1 (easy): Identify the process

A farmer plants clover (a legume) to enrich the soil with usable nitrogen without fertilizer. What process makes this work?

Solution: Legume roots host nitrogen-fixing bacteria that convert atmospheric N₂ into ammonia the plant can use — nitrogen fixation. When the crop is plowed under, that fixed nitrogen enriches the soil.

Interpretation: This is why crop rotation with legumes is a classic sustainable-agriculture answer (Unit 5).

Example 2 (medium): Contrast two cycles

Why can nitrogen recover from a local disturbance faster than phosphorus?

Solution: Nitrogen has a huge atmospheric reservoir (78% of air) and fast bacterial fluxes (fixation, denitrification) that can move it in and out quickly. Phosphorus has no atmospheric phase and depends on slow rock weathering, so once phosphorus is lost from a system (e.g., washed to deep sediment), replacing it takes geologic time.

Interpretation: "No gas phase → slow → often limiting" is the phosphorus headline.

Example 3 (AP-style): Trace human impact

Explain the chain of events by which burning coal at a power plant affects the carbon cycle and, separately, contributes to acid rain via the nitrogen cycle.

Solution: - Carbon: Coal is a carbon reservoir formed over millions of years. Combustion oxidizes that carbon to CO₂, a rapid flux into the atmosphere, increasing atmospheric CO₂ far faster than photosynthesis or ocean uptake can remove it. - Nitrogen: High combustion temperatures fix atmospheric N₂ into nitrogen oxides (NOₓ), which react with water in the atmosphere to form nitric acid, a component of acid deposition.

Interpretation: One smokestack disrupts two cycles at once — a great systems-thinking FRQ answer.

Example 4 (AP-style): Reservoir reasoning

The ocean is the largest active reservoir exchanging CO₂ with the atmosphere. Predict what happens to ocean chemistry as atmospheric CO₂ rises.

Solution: More atmospheric CO₂ dissolves into the ocean, forming carbonic acid and lowering ocean pH — ocean acidification (Unit 9). This threatens shell- and coral-building organisms.

Interpretation: Reservoirs aren't passive — pushing one (atmosphere) pushes the connected one (ocean).

Common Mistakes

• Giving phosphorus an atmospheric step. The P cycle has no significant gas phase. This is the single most tested distinction between the N and P cycles.
• Confusing nitrogen fixation with nitrification. Fixation = N₂ → ammonia. Nitrification = ammonia → nitrate. Both are bacterial; they are different steps.
• Saying plants take nitrogen straight from the air. With rare exceptions, plants cannot use N₂ gas; they absorb nitrate/ammonium from soil. Bacteria do the converting.
• Forgetting transpiration. Plants release huge amounts of water vapor; deforestation measurably reduces regional rainfall.
• Treating combustion as natural-only. Emphasize that fossil-fuel combustion moves carbon far faster than natural fluxes — that speed is the climate problem.

Practice Problems

11. (FRQ-style) Describe how excess fertilizer from a farm can move through the nitrogen and phosphorus cycles to cause eutrophication in a nearby lake. Name the specific ions involved.

12. (Data) A watershed receives 40 kg/ha of nitrogen from fertilizer; monitoring shows only 25 kg/ha taken up by crops. (a) How much is "excess"? (b) Give two possible fates of that excess nitrogen.

FRQ Practice — Analyze an Environmental Problem and Propose a Solution (10 pts)

Farmers in a river basin apply large amounts of synthetic nitrogen and phosphorus fertilizer. Downstream, a lake experiences frequent algal blooms and fish kills.

(a) Identify the human activity that has disrupted the nitrogen and phosphorus cycles here. (1 pt) (b) Describe the process by which nutrient runoff leads to fish kills, in correct sequence. (3 pts) (c) Explain why the phosphorus that reaches the lake is unlikely to leave quickly through an atmospheric pathway. (2 pts) (d) Propose two specific practices farmers could adopt to reduce nutrient runoff, and explain how each works. (4 pts)