Natural Biogeochemical Cycles
Carbon Cycle
Carbon is the basic building block of life and the fundamental element found in the carbohydrates, fats, proteins, and
nucleic acids (DNA and RNA). Carbon is changed among the biosphere, geosphere, hydrosphere, and atmosphere.
Although carbon is found in rocks, it is a minor component when compared with the mass of either oxygen or
silicon atoms in rock. Carbon is found in carbon dioxide (CO2)m which makes up less than 1% of the atmosphere.
A portion of the atmospheric carbon –15%- is removed through photosynthesis, where the carbon is incorporated
into the plant structure and compounds; and dissolving into cooler ocean waters. The oceans are gaining
approximately 2 gigatons (4 X 10^12 kg) of carbon each year.
Carbon is released back into the atmosphere through:
1.
2.
3.
4.
5.
Cellular respiration of plants and animals that break down glucose into carbon dioxide and water
Decay of organic material by the action of decomposers; if oxygen is present , the carbon is released in the form
of carbon dioxide; if oxygen is absent, it is released in the form of methane (CH4)
Weatherization of rocks and especially the erosion of limestone, marble, and chalk, which break down carbon to
carbon dioxide and carbonic acid (H2CO3)
Volcanic eruptions
Release of carbon dioxide by warmer ocean waters
Anthropogenic Effects:
6.
7.
Burning fossil fuels, wood, coal and so on
Clearing trees and other plants (deforestation)
The oceans contain the largest amount of dissolved carbon dioxide. However, most of it is not involved with rapid
exchange with the atmosphere. Removing carbon dioxide from water raises the pH making the water more basic.
The oceans are gaining a net 2 gigaton of carbon each year (1 GT = 2 X 10^12 Kg), the lithosphere 7 Gt, and the
atmosphere 5 Gt.
Carbon Sink
Carbon Sink
Amount
(Billions of metric tons)
Marine sediments and sedimentary rocks (Limestone and Marble)
Ocean Actual water itself
Fossil fuel deposits
Soil organic matter
Atmosphere
~75,000,000
~40,000
~4000
~1500
578 (1700 c.e.) to 766
(in 200 c.e.)
Terrestrial plants
~580
Nitrogen Cycle
Nitrogen is an essential element needed to make amino acids, proteins, and nucleic acids. Nitrogen makes up 78% of
the atmosphere, however plants and animals CAN NOT use that nitrogen gas directly. Other nitrogen stores
included organic matter in soil and the ocean (1 million times more nitrogen is found in the atmosphere than is
contained in either land or ocean waters)
The nitrogen cycle involves:
 Nitrogen Fixation
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



Nitrification
Assimilation
Ammonification
Denitrification
Nitrogen Fixation
Nitrogen fixation is the conversion of atmospheric nitrogen (N2) to ammonia ( NH3) or nitrate (NO3-) ions. Nitrate
is the product of high-energy fixation by lightning, cosmic radiation, or meteorite trails. In high-energy fixation,
atmospheric nitrogen and oxygen combine to form nitrates, which are carried to Earth’s surface in rainfall as nitric
acid (HNO3). High-energy fixation accounts for about 10% of the nitrate entering the nitrogen cycle.
In contrast, Biological fixation accounts for 90% of the fixed nitrogen in the cycle. In biological fixation,
molecular nitrogen (N2) is split into two free nitrogen atoms (N2  N+N). The nitrogen atoms combine with
hydrogen to yield ammonia (NH3) Reaction: N2 + 6 H+ + 6 e− → 2 NH3
The fixation process is accomplished by a series of different microorganisms. The symbiotic bacteria
Rhizobium which does a great deal of this nitrogen fixation lives in the roots of a family of plants called legumes.
A legume is a plant that produces a simple dry fruit that develops from a simple carpel and usually dehisces (opens
along a seam) on two sides. A common name for this type of fruit is a pod, although "pod" is also applied to a few
other fruit types. Well-known legumes include alfalfa, clover, peas, beans, lentils, lupins, mesquite, carob, soy, and
peanuts. Another bacterial form (not in legume roots) that does N-fixation would be blue-green algae
(Cyanobacteria) can also fix nitrogen both in the soil and in water, yielding ammonia as the stable end product.
Nitrification
Nitrification is the process in which ammonia is oxidized to nitrite (NO2-) and nitrate (NO3-). Two groups of
microorganisms are involved in the two steps of nitrification. 1) Nitrosomonas oxidizes ammonia to nitrite (nitrites
are toxic to plants, particularly in high concentrations) and water. 2) Nitrobacter oxidizes the nitrite to nitrate (the
most usable form by plants.)
Assimilation
Nitrates and ammonia (NH3 or NH4+ any time you add h20 to NH3 you’ll get NH4) are the forms of nitrogen most
commonly assimilated by plants through their root hairs. Nitrogen is used by plants to synthesize amino acids, oils,
and nucleic acids. Rains and extensive irrigation can leach soluble nitrates and nitrites into groundwater, which, in
high amounts, can interfere with blood-oxygen levels in human infants. Soluble nitrates that run off the land and
enter aquatic and wet land habitats result in cultural eutrophication and the destruction of these habitats. Animals
assimilate nitrogen-based compounds by consuming plants and other organisms that consume plants.
Ammonification
Ammonification is a one-way reaction. In it, organisms break down amino acids and produce ammonia (NH3).
Denitrification
Denitrification is the process in which nitrates are reduced to gaseous nitrogen. This process is used by facultative
anaerobes. These organisms flourish in an aerobic environment but are also capable of breaking down oxygencontaining compounds (N03-) to obtain oxygen in anaerobic environments. Examples fungi and the bacteria
Pseudomonas.
Anthropogenic Effects
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Burning any fossil fuel adds large amount of nitric oxide into the atmosphere. Nitric oxide combines with oxygen to
form nitrogen dioxide gas. This then reacts with water to form nitric acid (HNO3). Nitric acid is a component of
acid deposition which (1) damages and weakens trees (2) upsets aquatic ecosystems (3) corrodes metals (4) damages
marble, stone, and other building materials.
Anaerobic bacteria decomposing livestock waste and commercial inorganic fertilizers release nitrous oxide (N2O)
which is a greenhouse gas. Therefore, this contributes to global warming.
Agricultural runoff (fertilizer/livestock waste) and municipal sewage discharge add excess nutrients to aquatic
ecosystems stimulating rapid growth of algae (know as an algae bloom) and other aquatic plants. This rapid growth
will eventually deplete the nutrient supply. The decomposition of that plant matter by aerobic bacteria then
consumes all of the available dissolved oxygen. The results in a fish kill which the catastrophic collapse of all
oxygen consuming organisms in the ecosystem.
The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen and hydrogen,
over an iron catalyst, to produce ammonia. The Haber process is important because ammonia is difficult to produce
on an industrial scale, and the fertilizer generated from the ammonia is responsible for sustaining one-third of the
Earth's population. Even though 78.1% of the air we breathe is nitrogen, the gas is relatively unreactive because
nitrogen molecules are held together by strong triple bonds. It was not until the early 20th century that this method
was developed to harness the atmospheric abundance of nitrogen to create ammonia, which can then be oxidized to
make the nitrates and nitrites essential for the production of nitrate fertilizer and munitions.
PHOSPHORUS CYCLE
Phosphorus is essential for the production of the nucleotides, production of ATP, fats in cell membranes, bones,
teeth, and shells. Phosphorus is not found in the atmosphere but, rather, in sedimentary rocks and does not depend
on the action of bacteria. Generally, phosphorus is found in the form of the phosphate ion (PO4 3-) or the hydrogen
phosphate ion (HPO4 2-) . Phosphorus is slowly released from terrestrial rocks by weathering and the action of acid
rain. It then dissolves into the soil and is taken up by plants. It is often a limiting factor for soils due to its low
concentration and solubility. Phosphorus is a Key element in fertilizer. A fertilizer labeled 6-24-26 contains 6%
nitrogen, 24% phosphorus, and 26 % potassium.
Humans have impacted the phosphorus cycle in several ways: First humans have mined large quantities of
rock containing phosphorus for inorganic fertilizers and mined large quantities of rocks containing phosphorus for
inorganic fertilizers and detergents. Second, clear-cutting tropical habitats for agriculture decreases the amount of
available phosphorus as it is contained in the vegetation. Third, humans allow runoff from feedlots, from fertilizers,
and from the discharge of municipal sewage plants. The runoff collects in lakes, streams, and ponds. It causes an
increase in cyanobacteria ( blue-green bacteria), green algae, and aquatic plants. In turn, this growth results in
decreased oxygen content in the water, which then kills other aquatic organism in the food web. Fourth, humans
apply phosphorus-rich guano (an excrement from sea birds, bats, or seals that is high in phosphates and used to
produce fertilizer) and other phosphate-containing fertilizers to fields.
Sulfur Cycle
Most sulfur is found in underground rocks and deep in oceanic deposits. The Natural release of sulfur into the
atmosphere comes from the weathering of rock and gases released from seafloor vents and volcanic eruptions.
Sulfur is released mostly in the form of hydrogen sulfide (H2S) and sulfur dioxide (SO2) from these volcanic
eruptions. This sulfur dioxide is converted to sulfur trioxide (SO3) and eventually to tiny droplets of sulfuric acid
(H2SO4). Sulfuric acid mixes with rain to fall back to Earth and is known as acid rain, acid precipitation, or acid
deposition. Sulfate (SO4 2- ) salt particles enter the atmosphere from sea sprays, dust storms, and forest fires. These
sulfate ions are eventually absorbed by plants, which then incorporate them into proteins. Certain marine algae
produce gaseous dimethyl sulfide (DMS), which in the atmosphere also forms tiny condensation nuclei for rain.
Humans contribute to adding sulfur to the cycle through refining oil, burning fossil fuels, and by converting
(smelting) sulfur-containing metallic mineral ore into free metals such as copper, lead, and zinc. Some industrial
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processes like tire and paper plants can also release sulfur into the atmosphere. Forest fires can be anthropogenic as
well. These manmade causes also add to acid deposition.
Water Cycle
The water cycle is powered by energy from the sun. Solar energy evaporates water from oceans, lakes, streams, soil,
and vegetation. The oceans hold 97% of all water on the planet and are the source of 78% of all the global
precipitation. Oceans are also the source of 86% of all global evaporation, with evaporation from the sea surface
keeping Earth from overheating. If there were no oceans, surface temperature on land would rise to and average of
153 F (67 C).
The water cycle is in a state of dynamic equilibrium according to the law of conservation of mass. The rate
of evaporation equals the rate of precipitation. Warm air holds more water vapor than cold air. Processes involved in
the water cycle included; evaporation, transpiration (the evaporation of water from plants. It occurs chiefly at the
leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis), condensation, infiltration
(the process by which water on the ground surface enters the soil), percolation (the movement of water or air into
deeper layers of soil), runoff, and precipitation.
Human Impact on water Cycle
Human Activity
Impact on water cycle
Withdrawing water from lakes,
Aquifers, and rivers
Groundwater depletion and saltwater intrusion
Clearing of land for agriculture and urbanization
Agriculture
Increased runoff decreased infiltration increased
Flood risks accelerated soil erosion
Runoff contains nitrates, phosphates, ammonia, etc.
Destruction of wetlands
Disturbing natural processes that purify water
Pollution of water sources
increased occurrence of infectious agents such as
Cholera, dysentery, etc
Sewage runoff, feedlot runoff
Cultural eutrophication
Building power plants
increased thermal pollution
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