Biogeochemical Cycles: Carbon, Phosphorus, Nitrogen, and Sulfur Cycles

BIOGEOCHEMICAL CYCLES
Question One
A. Carbon Cycle
Carbon in general terms occurs in two forms in nature. It can either be organic carbon or
inorganic carbon. Carbon enters the carbon cycle either by photosynthesis in plants in the form
of carbon dioxide or through the process of formation of rocks in the form of carbonates. Carbon
also leaves the carbon cycle through two main ways; Respiration and decomposition in the form
of carbon dioxide (Palmeri, Barausse, & Sven, 2013).
B. Phosphorus Cycle
Phosphorus occurs naturally in compounds in solid. The compounds that contain
phosphorus, called phosphates, are present in rock layers in the earth’s crust. The weathering of
these rocks forms the entry point of phosphorus into the phosphorus cycle. The decomposition of
organisms forms the point of exit of phosphorus from the phosphorus cycle (Palmeri, Barausse,
& Sven, 2013).
C. Nitrogen Cycle
Nitrogen occurs abundantly in the earth’s atmosphere in gaseous form, this form is
however not a readily utilizable form for organisms. The Nitrogen thus enters the nitrogen cycle
in the form of ammonia through the conversation process done by the nitrogen fixing bacteria.
Nitrogen then leaves the nitrogen cycle as nitrogen gas by the action of denitrifying bacteria on
nitrates and nitrites (Palmeri, Barausse, & Sven, 2013).
D. Sulfur Cycle
Sulfur is found in the form of compounds in the earth’s crust as well as in the pure
elemental form as sulfur. Therefore, there are two ways in which sulfur enters the sulfur cycle;
volcanic eruptions and weathering. In volcanic eruptions, sulfur enters the sulfur cycle in the
form of gaseous compounds such as sulfur dioxide. In the weathering process, sulfur enters the
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BIOGEOCHEMICAL CYCLES
sulfur cycle in its pure elemental form or as a compound. The exit of sulfur from the sulfur cycle
is through the actions of plants and fungi which convert sulfate compounds into organic sulfur in
the soil (Palmeri, Barausse, & Sven, 2013).
Question Two
A. Carbon Cycle
The autotrophs in the carbon cycle are the plants which extract carbon in its gaseous form
of carbon dioxide in the process of photosynthesis. The autotrophs decrease the amount of
carbon dioxide in the atmosphere thereby maintaining a balance in the atmospheric carbon
levels.
The heterotrophs in the carbon cycle obtain carbon from consuming other organisms who
are either autotrophs or heterotrophs. Therefore, in the carbon cycle the heterotrophs would be
the herbivores, carnivores and omnivores. The heterotrophs prevent the over extraction of carbon
by the autotrophs by acting as a control mechanism for the population of the autotrophs.
B. Phosphorus Cycle
The autotrophs in the phosphorus cycle extract phosphorus in a form that is soluble from
the soil while heterotrophs obtain nitrogen by consuming either autotrophs or heterotrophs.
Similar to the carbon cycle, the autotrophs are the plants while the heterotrophs are the
herbivores, carnivores and omnivores.
The roles of the autotrophs and the heterotrophs are also similar as in the case of the
carbon cycle. The autotrophs decrease the amount of phosphorus in the soil to maintain a balance
in the amounts while the heterotrophs act as a control mechanism for the autotrophs to prevent
over extraction of phosphorus that would cause imbalance.
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