APES Unit 1.4–1.7 Key Cycles: Carbon, Nitrogen, Phosphorus, and Water

carbon sink

A reservoir that absorbs more carbon than it releases (e.g., ocean, plants, soil).

carbon source

A reservoir that releases more carbon than it absorbs (e.g., fossil fuel combustion, animal agriculture, deforestation).

photosynthesis

Absorbs CO₂ and makes glucose.

cellular respiration

Releases CO₂ and breaks down glucose.

ocean acidification

When increasing atmospheric CO₂ dissolves into the ocean, lowering pH and harming organisms with calcium carbonate shells.

fossil fuel combustion

Releases CO₂ very quickly, while fossil fuel formation takes millions of years.

N₂ gas

The form in which most nitrogen is stored in the atmosphere, which is not usable by plants.

nitrogen fixation

Conversion of N₂ into ammonia (NH₃) or nitrate (NO₃⁻) by bacteria or lightning.

nitrification

Soil bacteria converting ammonium (NH₄⁺) into nitrite (NO₂⁻) and then nitrate (NO₃⁻).

ammonification

Decomposers breaking down organic nitrogen (waste/dead matter) into ammonia (NH₃).

denitrification

Conversion of nitrate (NO₃⁻) into nitrous oxide (N₂O) or N₂ gas, releasing nitrogen back to the atmosphere.

human alteration of the nitrogen cycle

Synthetic fertilizer use, combustion, and agriculture → causes eutrophication, air pollution, and greenhouse gas emissions (N₂O).

main reservoir of phosphorus

Rocks and sediments.

limiting nutrient

Phosphorus is often a limiting nutrient because it cycles very slowly and has no atmospheric (gas) phase.

natural release of phosphorus

Weathering of rocks.

human addition of phosphorus

Mining phosphate for fertilizers and detergents.

excess phosphorus in aquatic systems

Eutrophication → algae blooms, oxygen depletion, fish kills.

water cycle driver

Energy from the sun.

transpiration

Plants releasing water vapor from leaves into the atmosphere.

evapotranspiration

Combined water movement from evaporation and transpiration into the atmosphere.

infiltration

Water trickling through soil to recharge groundwater.

runoff

Water flowing across land into surface waters, often carrying pollutants.

largest reservoir of water on Earth

The ocean.