The Living World - Environmental Science - page 49

Key components of each cycle include where the nutrients are stored, how long they remain in the storage
areas, and the process of movement between the living and nonliving parts of each cycle. Nutrient storage is
commonly referred to as reservoirs or sinks. These reservoirs may be different for each nutrient or they may
be common, depending on the nutrient. How long they remain depends on whether the nutrients are in living
organisms or nonliving components. In living organisms, a reservoir may exist for only a few hours (in some
bacteria) or as long as several thousand years (in a redwood or bristlecone pine). In the nonliving components,
nutrients could be locked up for millions or even billions of years, either in the atmosphere or Earth's crust.
The movement of each nutrient between the living and nonliving is described in each nutrient cycle.
Carbon Cycle
As a major part of life, carbon exists on Earth in living organisms, decomposing components of ecosystems,
and in abiotic factors of the environment. Also, it is found in gaseous and solid states. Carbon enters living or-
ganisms through plants when CO 2 is converted to carbohydrates during photosynthesis. Carbon moves through
animals when they consume plants and other animals. Carbon is released into the atmosphere in the form of
CO 2 during cellular respiration and when decomposers break down the remains of dead plants and animals.
The major reservoirs of the carbon cycle include plants, oceans, and sedimentary deposits. Plant matter, while a
small sink, is one that humans must consider as we grow more plants and animals for human food consump-
tion. Some of this carbon is stored for a short time, such as in the annual plants we grow for food, while other
carbon is stored for thousands of years in giant sequoias, for example. Because plants are constantly releasing
and capturing CO 2 , they are considered to cause zero change in the atmosphere.
Carbon dioxide is soluble in water, so the world's oceans are a major sink for carbon. It is dissolved in the
world's oceans just as oxygen is dissolved. This dissolved CO 2 is important for aquatic plant photosynthesis.
Carbon is also found in the shells and skeletons of marine organisms. An exchange of CO 2 occurs between the
atmosphere and marine waters. Because of the net increase in CO 2 in the atmosphere from the combustion of
fossil fuels, it is believed that the world's oceans are becoming more acidic.
Earth's rocks contain some carbon, although silicon and oxygen are their primary components. Rocks that con-
tain carbon are called carbonate rocks, and the form of carbon is calcium carbonate.
A final sink and the form that is the primary focus of human interventions in the carbon cycle is the carbon in
coal, gas, and crude oil, what we refer to as fossil fuels. This carbon—originally from living organisms that
have undergone a chemical change over time, with pressure and heat, to be converted into coal and oil—has
been sequestered for millions of years. Thus, the combustion of fossil fuels has contributed to a shift from car-
bon stored in the lithosphere to the atmosphere, creating a net gain of CO 2 in the atmosphere.