Biogeochemical cycles describe the flow of essential elements from the environment to living things and back to the environment. The following list outlines the major storage locations (reservoirs) for essential elements, the processes through which each element incorporates into terrestrial plants and animals (assimilation), and the processes through which each element returns to the environment (release).

1. Hydrologic cycle (water cycle).

• Reservoirs: oceans, air (as water vapor), groundwater, glaciers. (Evaporation, wind, and precipitation move water from oceans to land.)

• Assimilation: plants absorb water from the soil; animals drink water or eat other organisms (which are mostly water).

• Release: plants transpire; animals and plants decompose.

2. Carbon cycle. Carbon is required for the building of all organic compounds.

• Reservoirs: atmosphere (as CO2), fossil fuels (coal, oil), peat, durable organic material (cellose, for example).

• Assimilation: plants use CO2 in photosynthesis; animals consume plants or other animals.

• Release: plants and animals release CO2 through respiration and decomposition; CO2 is released when organic material (such as wood and fossil fuels) is burned.

3. Nitrogen cycle. Nitrogen is required for the manufacture of all amino acids and nucleic acids.

• Reservoirs: atmosphere (N2); soil (NH4 + or ammonium, NH3 or ammonia, NO2 – or nitrite, NO3 – or nitrate).

• Assimilation: plants absorb nitrogen either as NO3 – or as NH4 +; animals obtain nitrogen by eating plants or other animals. The stages in the assimilation of nitrogen are as follows:

Nitrogen fixation: N2 to NH4 + by prokaryotes (in soil and root nodules); N2 to NO3 – by lightning and UV radiation.

Nitrification: NH4 + to NO2 – and NO2 – to NO3 – by various nitrifying bacteria. NH4 + or NO3 – to organic compounds by plant metabolism.

• Release: denitrifying bacteria convert NO3 – back to N2 (denitrification); detrivorous bacteria convert organic compounds back to NH4 + (ammonification); animals excrete NH4 + (or NH3), urea, or uric acid.

4. Phosphorus cycle. Phosphorus is required for the manufacture of ATP and all nucleic acids. Biogeochemical cycles of other minerals, such as calcium and magnesium, are similar to the phosphorus cycle.

• Reservoirs: rocks. (Erosion transfers phosphorus to water and soil; sediments and rocks that accumulate on ocean floors return to the surface as a result of uplifting by geological processes.)

• Assimilation: plants absorb inorganic PO4 3– (phosphate) from soils; animals obtain organic phosphorus when they eat plants or other animals.

• Release: plants and animals release phosphorus when they decompose; animals excrete phosphorus in their waste products. Part II: Subject Area Reviews with Sample Questions and Answers