Science of Water : Concepts and Applications

Water Ecology 161
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FIGURE 6.4 Carbon cycle.
process on Earth.” This process produces the food and oxygen that all organisms live on. Part of the
carbon produced remains in living matter; the other part is released as CO 2 in cellular respiration.
Miller (1988) points out that the carbon dioxide released by cellular respiration in all living organisms
is returned to the atmosphere.
Some carbon is contained in buried, dead animal and plant materials. Much of these buried
animal and plant materials were transformed into fossil fuels. Fossil fuels, coal, oil, and natural gas
contain large amounts of carbon. When fossil fuels are burned, stored carbon combines with oxygen
in the air to form carbon dioxide, which enters the atmosphere.
In the atmosphere, carbon dioxide acts as a benefi cial heat screen because it does not allow
the radiation of the Earth’s heat into space. This balance is important. The problem is that as more
carbon dioxide from burning is released into the atmosphere, the balance can be and is being
altered. Odum (1983) warns that the recent increase in consumption of fossil fuels “coupled with
the decrease in ‘removal capacity’ of the green belt is beginning to exceed the delicate balance.”
Massive increases of carbon dioxide into the atmosphere tend to increase the possibility of global
warming. The consequences of global warming “would be catastrophic … and the resulting climatic
change would be irreversible” (Abrahamson, 1988).
NITROGEN CYCLE
Respiration
Atmospheric CO 2
Decaying
and
burning
Organic C
plants & animals
Photosynthesis
Nitrogen is an essential element for all organisms. In animals, nitrogen is a component of crucial
organic molecules such as proteins and DNA and constitutes 1–3% of the dry weight of cells. Our
atmosphere contains 78% by volume of nitrogen, yet it is not a common element on the Earth.
Although nitrogen is an essential ingredient for plant growth, it is chemically very inactive, and
before the vast majority of the biomass can incorporate it, it must be “fi xed.” Special nitrogen-fi xing
bacteria found in soil and water fi x nitrogen. Thus, microorganisms play a major role in nitrogen
cycling in the environment. These microorganisms (bacteria) have the ability to take nitrogen gas
from the air and convert it to nitrate. This is called nitrogen fi xation. Some of these bacteria occur
as free-living organisms in the soil. Others live in a symbiotic relationship (a close relationship
between two organisms of different species, and one where both partners benefi t from the association)
with plants. An example of a symbiotic relationship, related to nitrogen, can be seen, for
example, in the roots of peas. These roots have small swellings along their length. These contain
millions of symbiotic bacteria, which have the ability to take nitrogen gas from the atmosphere
and convert it to nitrates that can be used by the plant. Then the plant is plowed into the soil after
the growing season to improve the nitrogen content. Price (1984) describes the nitrogen cycle as an
example “of a largely complete chemical cycle in ecosystems with little leaching out of the system.”
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