Biogeochemical Cycles
Spheres of B. Cycles
  Nitrogen Properties
  Simple Nitrogen Cycle
  Human Influences
  Spheres of the N Cycle
  Choose a Sphere


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Chemical elements can be found in four different spheres: the atmosphere, biosphere, geosphere, and hydrosphere. The atmosphere consists predominantly of gases and aerosols and extends from the earth's surface to the edge of space. The geosphere predominantly includes the soil, sediments, and rocks that make up the solid part of the earth. The hydrosphere predominantly includes all liquid and solid water standing and flowing over and through the geosphere. The biosphere is made up of all organisms. Clearly, reality is more complex than this, as the four spheres are not mutually exclusive, but overlap and intersect. For example, surface soils contain air and exchange gases with the atmosphere, thus causing the geosphere and atmosphere to overlap. Sediments, soils, and aquifers contain water, so the geosphere and hydrosphere overlap. Dust from the geosphere and water from the hydrosphere occur in the atmosphere. Organisms are present in water bodies, soils, aquifers, and the atmosphere, so the biosphere overlaps with the other three spheres.

The majority of most elements in the geosphere will be in the solid form, in the atmosphere the gaseous form, and in the hydrosphere the liquid or dissolved form. All three phases of matter are found in the biosphere in significant amounts. The chemical reactions that make up the cycles of all elements occur in all four spheres.

The four most abundant elements that make up organisms - carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) - mainly come from the atmosphere (C, O, N) and the hydrosphere (H, O). The other essential mineral nutrients mainly come from the geosphere. Because substances from the other spheres are processed by the biosphere, the biosphere significantly influences the nature and chemical composition of the atmosphere, hydrosphere, and geosphere.

The most abundant gases in the atmosphere are nitrogen (N2), oxygen (O2), and argon (Ar). Dry air contains 75.5% N2, 23.1% O2, and 1.3% Ar by mass (78.3%, 21.0%, and 0.9% by volume). Air also contains water vapor (0 to 4% by volume), and carbon dioxide (CO2, ~365 parts per million by volume).

All elements can be found in the geosphere in some amounts. The thin layer of the earth that is called the soil is heavily populated by all kinds of microbes, flora, and fauna.

Lakes, rivers, and oceans are the major and obvious components of the hydrosphere. However, large amounts of liquid water also exist within the geosphere as ground water. The hydrosphere serves as a solvent, a physical weathering agent, and transport mechanism for many dissolved minerals and gaseous elements, as well as particulate materials essential for life - materials from the other three spheres. It also serves as the medium for the cellular biochemical reactions essential for the functioning of life.

Chemical elements are transferred within and among the four spheres. The total mass of the elements in all of the spheres is conserved. However, chemical transformations can change the chemical form.

Nitrogen is one specific example of these chemical elements and is the focus of this project.     

Suggested Reading:

Furley, P.A., and W.W. Newey. 1983. Geography of the Biosphere. Butterworths, London.

Mason, B. 1966. Principles of Geochemistry. Third Edition. John Wiley & Sons, Inc., New York.

Odum, E.P. 1963. Ecology. Holt, Rinehart and Winston, New York.

Stevenson, F.J. 1982. Origin and distribution of nitrogen in soil. Nitrogen in Agricultural Soils. F.J. Stevenson (ed.). Agronomy Monograph No. 22. American Society of Agronomy, Madison, WI., pp. 1-42.

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