Biogeochemical Cycles in Ecosystems (APES Unit 1)

Carbon cycle

The movement of carbon through biotic components (living organisms) and abiotic components (atmosphere, oceans, rocks, and soils).

Reservoir

A storage location for a chemical in a biogeochemical cycle (e.g., atmosphere, oceans, biomass, fossil fuels).

Flux

A process that moves a chemical from one reservoir to another (e.g., photosynthesis, respiration, combustion).

Photosynthesis

A process in which producers take in CO2 (from air or water) and convert it into organic molecules; the main biological entry point of carbon into food webs.

Cellular respiration

A process where organisms break down organic molecules for energy and release CO2 back to the atmosphere or water.

Decomposition

Breakdown of dead organic matter by decomposers, returning carbon mainly as CO2 (and sometimes CH4 in low-oxygen conditions).

Combustion

Burning biomass or fossil fuels, rapidly converting stored carbon into CO2.

Ocean acidification

A shift in seawater chemistry caused by increased CO2 absorption by oceans, which can make it harder for corals and shell-formers to build calcium carbonate structures.

Nitrogen cycle

The cycling of nitrogen through ecosystems, involving both movement and chemical transformations between forms such as N2, NH4+, and NO3-, largely driven by bacteria.

Nitrogen fixation

Conversion of atmospheric N2 into ammonia-related forms (often NH4+ in soil) that can enter food webs; occurs via certain bacteria, lightning, or industrial processes.

Nitrification

A bacterial process that converts NH4+ to NO2− (nitrite) and then to NO3− (nitrate).

Assimilation (nitrogen)

Uptake of NH4+ or NO3− by plants and incorporation into organic molecules (proteins, DNA); animals get nitrogen mainly by eating organic matter.

Ammonification (mineralization)

Conversion of organic nitrogen from dead organisms and wastes into NH4+ by decomposers.

Denitrification

A bacterial process (usually in anaerobic, low-oxygen conditions) that converts NO3− back into N2 gas (and sometimes nitrous oxide), returning nitrogen to the atmosphere.

Nitrate (NO3−)

A highly water-soluble form of nitrogen commonly found in soils; important for plant uptake but easily transported by water (prone to being washed out of soils).

Eutrophication

Nutrient enrichment of a water body (often from fertilizer runoff) that causes rapid algae growth followed by decomposition that reduces dissolved oxygen.

Hypoxia (dead zone)

Low dissolved oxygen conditions in aquatic systems, often caused by eutrophication, where many organisms cannot survive.

Phosphorus cycle

The cycling of phosphorus mainly through rocks, soils, water, sediments, and organisms; it typically lacks a major atmospheric (gaseous) phase.

Phosphate (PO4^3−)

The main biologically usable form of phosphorus taken up by plants from soil and water; often binds to particles and can move with eroded soil.

Limiting nutrient

The nutrient in shortest supply relative to demand that constrains growth and primary productivity (often phosphorus in freshwater and many soils).

Weathering and erosion (phosphorus)

Processes that release phosphate from rocks into soils and waters over time; can be accelerated by land disturbance and soil loss.

Hydrologic (water) cycle

The movement of water through Earth’s systems (atmosphere, land, oceans, surface water, and groundwater); a major transport pathway for nutrients and pollutants.

Infiltration

Water soaking into the soil surface.

Runoff

Water flowing over land into streams and rivers when precipitation exceeds infiltration capacity.

Watershed (drainage basin)

The land area where all water drains to a common outlet (such as a stream, river, lake, or ocean), linking land use to downstream water quality.