Biology 12

Chapter 13 Ecological Principles • MHR 453

The Carbon Cycle

Another important biogeochemical cycle is the
carbon cycle (shown in Figure 13.27). Other than
oxygen and hydrogen, which are mostly locked up
in water molecules, carbon is the most abundant
element in living things. Plants and other
autotrophs take in carbon dioxide as a raw material
for photosynthesis, and almost all organisms
(including autotrophs) give it off as a waste product
of cellular respiration. The carbon that accumulates
in long-lived, durable plant material (such as the
wood of trees) does not cycle as rapidly, unless the
trees are burned.
In some environments, living things die faster
than decomposers can break them down. This was
particularly evident in the warm, moist conditions
of the Mesozoic era, roughly 150 to 250 million
years ago. During that time, giant ferns and a
multitude of other plants grew thickly in vast
forests. Under these conditions, so much detritus
built up that it became compressed, resulting in the
formation of coal or oil deposits. Today, we refer to
these substances as fossil fuels. Since these
deposits were formed from what were once living
things, they contain substantial amounts of carbon.

As described above, these carbon supplies are not

accessible to living things unless the carbon is

released by burning — either naturally or in a coal

stove or oil furnace.

In one sense, the burning of fossil fuels simply

returns to the atmosphere the carbon that was

removed by photosynthetic activity in Mesozoic

forests. However, during the time these carbon

supplies were locked up underground, a new

balance developed in the global carbon cycle. With

the addition of substantial amounts of so-called

new carbon to the cycle, the environment is

changing. What effects do you think this might

have on the biosphere? This is an issue you will

consider further in Chapter 15.

The Nitrogen Cycle

Nitrogen is another element that is relatively

abundant in ecosystems, making up almost

80 percent of Earth’s atmosphere. But since

nitrogen gas (N 2 ) cannot be taken up by

photosynthetic organisms, it is not available to

heterotrophs. In fact, plants can only use nitrogen

when it is a part of ammonium (NH 4 +) or nitrate

(NO 3 −) molecules. Nitrogen gas is occasionally

decomposers

(soil microbes and others)

cellular respiration

photosynthesis

plants, algae,

cyanobacteria

primary consumers

higher-level

consumers

detritus

burning

wood and

fossil fuels

carbon in atmosphere ( )CO 2

Figure 13.27On a global scale, the amount of carbon
entering the atmosphere as a result of decomposition and
respiration balances the carbon removed by photosynthesis.

The burning of wood and fossil fuels unbalances the

equation. What might result from this practice?