The four biogeochemical cycles side by side — build the rest of 56.1 on this
4 rows| Cycle | Main reservoir | Chemical forms | Key fluxes and who does them | Human disruption |
|---|---|---|---|---|
| Carbon | Atmosphere (CO₂, 0.03% of volume ≈ 750 billion metric tons) plus oceans, biomass, and fossil fuels | CO₂ gas, HCO₃⁻ in water, organic compounds, CH₄ | Photosynthesis fixes CO₂; respiration and combustion release it; methanogens make CH₄ anaerobically and it is oxidized abiotically back to CO₂ | Burning fossil fuels returns buried carbon in decades, raising atmospheric CO₂ and driving global warming |
| Water | Oceans, plus atmosphere, lakes and rivers, and groundwater in aquifers | Liquid, vapor, ice — a compound, so it can be made and destroyed | Evaporation and transpiration (about 90% of terrestrial input to the air) up; condensation and precipitation down; percolation recharges groundwater; respiration makes water and photosynthesis splits it | Deforestation converts transpiration to runoff; the Ogallala Aquifer is pumped faster than it recharges |
| Nitrogen | Atmosphere, 78% N₂ by volume — huge but unusable by plants and animals | N₂, NH₃/NH₄⁺, NO₃⁻, N₂O, organic N, urea | Fixation N₂→NH₃ (prokaryotes with nitrogenase); nitrification NH₃→NO₃⁻; ammonification urea→NH₃; denitrification NO₃⁻→N₂. Every step is prokaryotic | Industrial NH₄⁺ fertilizer has doubled the rate at which usable nitrogen enters soils and waters |
| Phosphorus | Rocks and deep-sea sediments — there is no significant atmospheric reservoir | A single oxidation state: PO₄³⁻ | Weathering of rock releases it; uptake by roots; phosphate remineralization by decay microbes returns it; rivers carry it one way to ocean sediments | Fertilizers from crushed phosphate rock and bone, plus phosphate detergents now restricted by law |
The nitrogen cycle by verb — who does what, and which way
5 rows| Process | Conversion | Who does it | Direction |
|---|---|---|---|
| Nitrogen fixation | N₂ → NH₃ | Prokaryotes with the nitrogenase (nif) complex — free-living, root-symbiotic in legumes/alders/myrtles, or aquatic cyanobacteria | Atmosphere into the ecosystem |
| Nitrification | NH₃ → NO₃⁻ | Other prokaryotic microbes (oxidation) | Within the soil or water |
| Ammonification | Organic N and urea → NH₃ | Decay microbes and urea-converting microbes | Dead matter back into the usable pool |
| Denitrification | NO₃⁻ → N₂ (or N₂O) | Prokaryotes — both bacteria and archaea | Ecosystem back to the atmosphere |
| Uptake | NH₃/NH₄⁺ or NO₃⁻ → organic N | Plants and algae (then animals by eating) | Soil or water into living tissue |
Hubbard Brook — the numbers the exam asks for
6 rows| Measurement | Undisturbed watershed | Deforested watershed | What it shows |
|---|---|---|---|
| Water leaving the valley | Baseline | +40% | Water once taken up and transpired by vegetation now runs off |
| Calcium loss | About 1% of the system per year | Ninefold increase | Undisturbed forest is highly efficient at retaining nutrients |
| Nitrate balance | Gaining ~5 kg/ha/yr | Losing ~53 kg/ha/yr | The most striking change — the valley flipped from nitrogen sink to nitrogen source |
| Stream nitrate concentration | About 0–3 mg/L | About 40–80 mg/L | Sharp rise right after the deforestation arrow on the graph |
| Phosphorus | Low | No increase | Phosphorus stayed locked in insoluble soil compounds — the exception to the pattern |
| Downstream effect | None notable | Massive algal blooms | Lost fertility upstream became eutrophication downstream |
Trophic levels, energy, and the three pyramids
7 rows| Item | Definition | Number to remember | Trap |
|---|---|---|---|
| Trophic levels | Producers → herbivores → primary carnivores → secondary carnivores, with detritivores feeding off every level | Level 1 is always the autotrophs | Omnivores and multi-level feeding make real systems food webs, not chains |
| GPP | Raw rate at which producers synthesize organic matter | About 1% of incoming sunlight is captured | GPP is not what herbivores get |
| NPP | GPP minus producer respiration | Rainforest and wetland ≈ 2000 g/m²/yr; desert ≈ 90 | NPP is what is available to herbivores |
| Ten percent rule | Energy available to a level is about 10% of the level below | 1000 → 100 → 10 → 1 | Can run as high as 30%; it is a generality, not a law |
| Pyramid of energy flow | Box widths proportional to productivity | Plankton 36,380 → herbivore 596 → carnivore 48 kcal/m²/yr | Always upright — required by the rules of energy flow |
| Pyramid of biomass | Box widths proportional to standing crop biomass | Usually upright (807 → 37 → 11 g/m²); inverted case 4 g/m² algae under 21 g/m² grazers | Can be inverted when producers turn over fast |
| Pyramid of numbers | Box widths proportional to counts of individuals | 4,000,000,000 plankton → 11 herbivores → 1 carnivore | Usually but not always upright |
Top-down versus bottom-up — how to tell them apart on an exam
6 rows| Feature | Top-down (trophic cascade) | Bottom-up |
|---|---|---|
| Where the change starts | An upper trophic level — a predator added or removed | The base — light, nutrients, or primary productivity changes |
| Direction of the effect | Flows down through two or more levels | Flows up from producers to higher levels |
| Classic experiment | Stream enclosures with or without trout; enclosures with or without large fish above damselfly nymphs | Eel River enclosures with clear versus tinted roofs controlling light |
| Signature result | Effect on the bottom level alternates with each level added — trout raise algae, big fish lower algae | Producer and carnivore biomass rise while herbivore biomass stays flat |
| Large-scale example | Sea otter → sea urchin → kelp; wolves and deer; overfished cod releasing shrimp and crabs | Rising productivity crossing thresholds that let herbivores, then carnivores, be supported |
| Human version | Removal of top carnivores — Barro Colorado Island losing 15 bird species after jaguars went | Fertilizer or nitrogen deposition raising productivity, as in eutrophication |
Exam traps
11 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
Matter cyclingvsEnergy flow
Atoms are recycled — Earth has a fixed number of them and they pass round the same reservoirs forever. Energy makes a one-way pass: sunlight in, heat out, never reused. If the question says "cycles," it is matter; if it says "flows," it is energy.
Carbon fixationvsNitrogen fixation
Same word, two cycles. Carbon fixation is photosynthesis, CO₂ → organic compounds, done by plants and algae. Nitrogen fixation is N₂ → NH₃, done only by certain prokaryotes with the nitrogenase complex. Both fit the general meaning: gaseous → nongaseous.
NitrificationvsDenitrification
Nitrification is oxidation within the soil: NH₃ → NO₃⁻, keeping nitrogen available to plants. Denitrification sends nitrogen out of the ecosystem: NO₃⁻ → N₂ gas. One builds the usable pool, the other drains it.
AmmonificationvsNitrogen fixation
Ammonification makes NH₃ from organic nitrogen already in the system — decaying tissue and urea. Fixation makes NH₃ from atmospheric N₂. Only fixation brings new nitrogen in from outside.
Phosphorus cyclevsNitrogen cycle
Phosphorus has no atmospheric reservoir, exists in one oxidation state (PO₄³⁻), and needs no microbial conversions to become usable. Nitrogen has a huge atmospheric reservoir, several oxidation states, and requires prokaryotes at every step. Phosphorus is the simple one — except it is also the one that leaks one way into the sea.
ReservoirvsFlux
A reservoir is a place where the element sits — the atmosphere, the ocean, fossil fuels, soil. A flux is a movement between reservoirs — photosynthesis, respiration, weathering, precipitation. Figure arrows are fluxes; figure boxes are reservoirs.
GPPvsNPP
GPP is the raw synthesis rate. NPP = GPP − producer respiration, and NPP is the only part herbivores can eat. If a question asks how much energy is available to the next level, subtract respiration first.
The 1% figurevsThe 10% figure
1% is how much of incoming solar energy the producers capture. 10% is how much of one trophic level's energy reaches the next. Do not swap them — 1% happens once, at the entry point; 10% happens at every hand-off after that.
Pyramid of energyvsPyramid of biomass
The energy/productivity pyramid is always upright because the laws of energy flow forbid a level from receiving more than the one below. The biomass pyramid is usually upright but can be inverted — fast-dividing phytoplankton hold little standing biomass while supporting more zooplankton biomass. Productivity high, biomass low.
Top-down effectvsBottom-up effect
Ask where the manipulation was applied. Predator added or removed at the top = top-down / trophic cascade. Light, nutrients, or productivity changed at the base = bottom-up. Both can run at once in the same system.
Area effectvsDistance effect
Both shift the equilibrium left to fewer species, but by moving different curves. Island size changes the extinction curve — small islands, small populations, higher extinction. Distance changes the colonization curve — far islands get fewer arrivals. Graph questions want to know which curve moved.