BIOGEOCHEMICAL CYCLES

All of the substances that occur in organisms cycle through ecosystems. Although there are local exceptions to this generality, on a global scale the bulk of these substances is not contained within the bodies of organisms, but rather exists in the atmosphere, the water, or in rocks. Carbon (in the form of carbon dioxide), nitrogen, and oxygen primarily enter the bodies of organisms from the atmosphere, whereas phosphorus, potassium, sulfur, magnesium, calcium, sodium, iron, and cobalt, all of which are required for plant growth, come from rocks. All organisms require carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur in relatively large quantities; the other elements are required in smaller amounts.
We speak of the cycling of materials in ecosystems because they first are incorporated from the atmosphere or from weathered rock into the bodies of organisms; they then sometimes pass from these organisms into the bodies of other organisms that feed on the primary ones, and ultimately through decomposition are returned to thi:
nonliving world. When this occurs, the nutrients may possibly be incorporated again into the bodies of other organisms. Some examples will help to clarify the ways in which different cycles function.