Environmental Engineering Reference
In-Depth Information
(e.g., lead, cadmium, chromium), are highly toxic to organisms, but important in modern
industry, even in what we think of as “green” technologies (e.g., the use of cadmium in
some kinds of rechargeable batteries and solar panels). Still other elements are redistributed
in large quantities as a by-product of industrial activity (e.g., mercury from coal-burning or
waste incineration) and are problematic to organisms when in high concentration (see
Chapter 15).
Human use of materials for fertilizer and industry, the changes we have made in land
cover, and our combustion of large amounts of fossil fuels have led to profound changes
in many element cycles (
Vitousek 1994
). We have affected element cycles through activities
that lead to loss, retention, or redistribution of materials. For example, land-use changes
often result in soil erosion, increased retention time of water and sediment in reservoirs,
and movement of materials contained in crops from fields to cities. Globally, we have
doubled the amount of nitrogen fixed from the atmosphere into reactive forms
(see Chapter 7), and annually, humans mine more phosphorus for agricultural use than is
weathered from rocks (see Chapter 8). For both elements this has greatly increased the
transport from terrestrial to aquatic systems. Both the distribution and abundance of ele-
ments have taken on completely new geographical patterns as a result of human activity.
These changes ripple throughout ecosystems and across continents as we will see in the
next three chapters.
WHAT IS AN ELEMENT CYCLE?
Element cycles involve the movement and transformation of biotic and abiotic forms of
elements—particularly the elements that are essential to life—as they make their way
through ecosystems. There are numerous pathways that both organic and inorganic mate-
rials follow, and generally they can be described as transformation between abiotic and
biotic pools, within biotic pools, within abiotic pools, or physical movement while in any
pool (
Figure 5.1
). Nearly all elements may be taken up by organisms and incorporated
Production
Inorganic
Organic
Decomposition
Ecosystem boundary
FIGURE 5.1
Inorganic and organic materials follow many pathways as they move through ecosystems, but in
general they flow between abiotic and biotic pools as they are taken up by organisms and incorporated into living
biomass (production) and broken down (decomposition). They also move within inorganic pools (e.g., through
soil weathering) and within organic pools (e.g., in trophic transfer). Materials may be added or lost from these
pools as elements are transferred across ecosystem boundaries.
