Compared to the long history of earth, you and every other living thing around you have only been around for a moment of time. But the matter that makes up your body and that of other organisms has been around since the planet formed billions of years ago. How can that be? A biogeochemical cycle is a pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic (atmosphere) compartments of earth. In effect the element is recycled, although in some cycles there may be places where the element is accumulated or held for a long period of time. All materials that recycle through living organisms are important, but three substances are particularly important: carbon, nitrogen, and phosphorus. In almost all biogeochemical cycles, living organisms contain much less of the recycled substances than does the nonliving environment.
Of all the nonliving components of an ecosystem, water is one of those that most affects both living and nonliving components. Water is constantly moving in a cycle through organisms and the environment. Evaporation is the process by which water changes form a liquid to a gas. The energy to drive this process comes from the sun. This water returns to the surface of earth when precipitation – rain, sleet, snow – falls from the clouds. On land much of the water is heated by the sun, and it reenters the atmosphere through evaporation. After passing through a plant, water moves into the atmosphere through the process of transpiration, the evaporation of water from the leaves of plants.
If you could look at the organic molecules that make up living things, you’d find something similar in all of them – they all contain carbon. Carbon cycles through living organisms and the nonliving environment. Plants use carbon dioxide along with water to build organic molecules during photosynthesis. During cellular respiration, organisms use oxygen to release energy from carbon-containing organic molecules. Carbon dioxide is a product of this process, and it’s released into the air. Carbon also returns to the air during combustion – when materials burn. Large amounts of carbon may be tied up in the wood of trees for hundreds of years, and some remain locked away for million of years in the form of fossil fuels. Many marine organisms contain carbon in their calcium carbonate shells. During erosion, the limestone may become exposed and undergo chemical changes that return carbon to the atmosphere.
The atmosphere around you contains about 79% nitrogen gas, but only certain types of bacteria can use this form directly. The bacteria bind nitrogen atoms to hydrogen atoms to form ammonia, in a process called nitrogen fixation. Other bacteria in the soil convert ammonia into nitrates and nitrates during nitrification. Procedures can take these substances in and use them to make proteins. Bacteria may again change the ammonia into nitrates and nitrates, or other bacteria may convert it into nitrogen gas in a process called denitrification.
An ecosystem contains hundreds or thousands of interacting species. Although nutrient cycles enable the organisms in an ecosystem to reuse essential materials, the amounts of particular nutrients in an ecosystem can change. The results often have a ripple effect that changes not only the particular ecosystem, but other ecosystems too.


  1. Which is a process by which carbon is removed from the atmosphere?
  2. combustion
  3. photosynthesis
  4. respiration
  5. erosion

  1. Why is nitrogen fixation an important process in nitrogen cycle?
  2. most organisms can’t use nitrogen in the form it takes in the atmosphere
  3. all the nitrogen would become carbon dioxide
  4. plants product too much nitrogen
  5. bacteria release nitrogen atoms from hydrogen atoms to form ammonia

  1. If a puddle of water in the yard disappears, where did it go?

  1. How would an increase in nitrogen effect the bird population?