A biogeochemical cycle describes the transformations that occur in a substance that is fundamental to the environment as it cycles through Earth's lithosphere (upper mantle), biosphere (life-supporting areas), hydro-sphere (water and water vapor), and atmosphere (layer of gases). The substances most often studied in biogeo-chemical cycling include carbon, nitrogen, phosphorous, hydrogen, oxygen, and sulfur.
Historical Background and Scientific Foundations
Biogeochemical cycles are characterized by reservoirs, also called sinks, and transformations. A substance can remain trapped or fixed in a reservoir for some period of time. Examples of reservoirs are rocks for phosphorous, oceans for hydrogen and oxygen, and forests for carbon. Eventually, something will happen to the reservoir that will cause a transformation of the substance. In some cases, these transformations are state changes, as when water in the ocean evaporates and becomes vapor in the atmosphere. In other cases, the transformation may involve breaking and forming chemical bonds, as when photosynthesis by plants converts carbon dioxide in the atmosphere into carbohydrates.
In the biogeochemical cycle for carbon, for example, the atmosphere serves as a reservoir of carbon dioxide. Atmospheric carbon dioxide is removed by photosynthesis of plants on land and algae in water, both of which represent significant carbon sinks. When plants and algae are buried, they can become fossilized and the carbon becomes transformed into fossil fuel. This reservoir is another significant carbon sink. When fossil fuels or forests are burned, carbon is transformed back into carbon dioxide and returns to the atmosphere.
The differences in biogeochemical cycles result from differences in the chemical properties of the different substances. These differences affect the types of reservoirs, the residence times in the different reservoirs, as well as the rates of transformation.
Impacts and Issues
As the global climate undergoes changes, both the reservoirs and transformations between reservoirs of all the biogeochemical cycles are affected. In particular, reliance on petroleum products is causing widespread changes to the carbon biogeochemical cycle. The enormous reservoir of carbon bound in fossilized plant material is being used at rapid rates through the burning of petroleum products. This combustion adds carbon in the form of carbon dioxide to the atmosphere. The size of the atmospheric carbon reservoir has increased significantly since the Industrial Revolution began in the late 1700s. In addition, many forests, which compose another large carbon reservoir, have been harvested without being replanted, transferring a portion of the carbon that was bound in trees to the atmosphere. The additional carbon in the atmosphere from the depletion of carbon in fossil fuels and forests is the major suspected cause of global climate change.
The carbon cycle is not the only one impacted by human practices since the Industrial Revolution. Oxygen and hydrogen biogeochemical cycles are affected by climate change because of their role in the hydrologic cycle. Global warming has caused melting of the ice sheets at the poles at a rapid rate. The cycling of nitrogen and sulfur are linked to acid rain, which results from burning fossil fuels. Both the phosphorus and nitrogen cycles are also significantly affected by the spread of fertilizer on agricultural fields, contributing to high nutrient runoff and eutrophication (over-enrichment with minerals and nutrients) of lakes and rivers.
WORDS TO KNOW
: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.
: The sum total of all life-forms on Earth and the interaction among those life-forms.
: Carbon reservoirs such as forests or oceans that take in and store more carbon (carbon sequestration) than they release. Carbon sinks can serve to partially offset greenhouse-gas emissions.
: The process whereby a body of water becomes rich in dissolved nutrients through natural or human-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.
: The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30% of the mid-latitudes' heat transport from the equatorial to polar regions.
: The totality of water encompassing Earth, comprising all the bodies of water, ice, and water vapor in the atmosphere.
: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.
: The rigid, uppermost section of Earth's mantle, especially the outer crust.
: A natural or artificial receptacle that stores a particular substance for a period of time
: The processes involved in the transfer of a substance from one reservoir to another.
See Also Acid Rain; Atmospheric Circulation; Carbon Cycle; Carbon Sequestration Issues; Carbon Sinks; Forests and Deforestation; Greenhouse Gases; Hydrologic Cycle; Melting; Ocean Circulation and Currents; Rainfall; Sea Level Rise; Sequestration; Sink; Water Vapor.
Raven, Peter H., and Linda R. Berg. Environment. Hoboken, NJ: John Wiley and Sons, 2006.
“Biogeochemical Cycles.” Environmental Literacy Council, October 30, 2006. < http://www.enviroliteracy.org/subcategory.php/198.html> (accessed October 17, 2007).
“Global Biogeochemical Cycles & the Physical Climate System.” University Corporation for Atmospheric Research. < http://www.ucar.edu/communications/gcip/m4bgchem/m4overview.html> (accessed October 17, 2007).
Juli M. Berwald