Chapter 13 Ecological Principles • MHR 45113.4 Nutrient Cycling in Ecosystems
In addition to energy, organisms must also obtain
17 chemical elements (termed essential nutrients)
for proper growth and repair of body tissue. Several
of these elements, including carbon, hydrogen,
oxygen, and nitrogen, are required in large
amounts, since they make up 95 percent of the
mass of all living organisms. Calcium, iron, and
other elements are needed in smaller amounts,
while only trace (very small) amounts are required
of the remaining nutrients.
Energy and nutrients are vital for the survival of
organisms. Although both are carried as part of the
same complex molecules from one trophic level to
another, their overall paths through ecosystems are
different. Energy is continuously supplied to Earth
by the Sun, and while some is captured and flows
through the ecosystem, much is lost to the
environment along the way.
In contrast, the supply of nutrients is not
constantly replenished. The existence of life in the
biosphere depends on the recycling of chemical
elements. Recycling begins when organisms die,
parts are lost (such as dead skin cells or broken
branches), or wastes are eliminated.
Decomposers then go to work, releasing the
nutrients that were contained in these bodies into
the atmosphere or soil. From these “pools,”
nutrients are picked up by various types of
organisms and re-used to build new bodies —
which can later be eaten by other organisms.
Since the routes these chemicals travel involve
both biotic and abiotic components of the
environment (including rocks and soil that are
geological features), they are referred to as
biogeochemical cycles. Read on to learn about the
general features of biogeochemical cycles.
Biogeochemical Cycles
The route a specific element takes in its
biogeochemical cycle depends on the element
and on the trophic structure of the ecosystem in
which it is travelling. However, there are two
general types of cycles. The first type includes the
cycles of carbon, oxygen, nitrogen, and sulfur —
elements that can occur as gases in the atmosphere.
These nutrients have global cycles because
individual nutrient atoms may travel long distances.
For example, a plant living in one location may
take up carbon (in a carbon dioxide molecule) that
was released by an animal living far away.
The second type of cycle involves nutrients
that are more static and tend not to move around
(including phosphorus, potassium, calcium, and
trace elements). These elements are typically found
in the soil rather than the atmosphere. They are
absorbed by plant roots and return to the same soil
“pool” when the plant dies.
In general, all biogeochemical cycles involve the
movement of elements between four reservoirs —
which can be thought of as nutrient “banks” (see
Figure 13.25 on the following page). Nutrients can
be easily withdrawn by organisms from two of
these banks. The first of these consists of living
organisms and the bodies of recently deceased
organisms; therefore it is a biotic bank. The
second storehouse consists of abiotic parts of the
environment from which nutrients can be easily
accessed — atmosphere, soil, and water.
In the other two reservoirs, nutrients are held
more tightly and cannot be accessed by living
organisms. Again, one is biotic and the other is
abiotic. The biotic reservoir is formed from the
compressed (fossilized) remains of organisms that
died long ago. Over time, the nutrients contained
in the bodies of these organisms became incorporatedEXPECTATIONS
Distinguish between how energy and nutrients move in ecosystems.
Describe the general features shared by all biogeochemical cycles.
Differentiate between biogeochemical cycles of materials that exist in the
environment in gaseous form and those that do not.
Illustrate some of the processes that move nutrients through ecosystems,
using one nutrient cycle as an example.
Recognize the impact that humans and plants have on biogeochemical cycles.