2. Carbon cycle: Photosynthesis balances respiration in natural ecosystem; man has unbalanced the carbon cycle through the use of fossil fuels. Major environmental and economical impacts to consider in understanding the carbon cycle.
3. Nitrogen cycle: Atmospheric nitrogen cannot be used by plants -- must be converted to ammonium (ammonia ion); Nitrogen fixation via lightning or N-fixing bacteria; organic nitrogen (amino acids) converted to nitrate or nitrite. Nitrite is unusable to plants. Some bacteria convert nitrite to nitrate (useable form); others may convert nitrites and nitrates to atmospheric nitrogen.
4. Phosphorus cycle: Major pool of phosphorus is sedimentary. Inorganic phosphate cannot be used by animals. Inorganic phosphate converted by plants to a form useful to animals. This phosphate can be put back into the ecosystem by animals as waste and by decomposers as the organic material is broken down.
5. Hydrologic cycle: evaporation and precipitation cycle water at the earth's surface. Less than 3% of the global water supply is fresh, the rest is saline. Water availability is a critical factor in distribution of human population. Environmental and economic impacts of hydrologic cycle are extremely important.