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Sulfur Cycle
Sulfur (S), the tenth most abundant element in the universe, is a brittle, yellow, tasteless, and odorless nonmetallic element. It comprises many vitamins, proteins, and hormones that play critical roles in both climate
and in the health of various ecosystems. The majority of the Earth's sulfur is stored underground in rocks and
minerals, including as sulfate salts buried deep within ocean sediments.
The sulfur cycle contains both atmospheric and land processes. Within the land portion, the cycle begins with
the weathering of rocks, releasing the stored sulfur. The sulfur then comes into contact with air where it is
converted into sulfate (SO4). The sulfate is taken up by plants and microorganisms and is converted into
organic forms; animals then consume these organic forms through foods they eat, thereby moving the sulfur
through the food chain. As organisms die and decompose, some of the sulfur is again released as a sulfate
and some enters the tissues of microorganisms. There are also a variety of natural sources that emit sulfur
directly into the atmosphere, including volcanic eruptions, the breakdown of organic matter in swamps and
tidal flats, and the evaporation of water.
Phosphorus Cycle
Phosphorus is an important element for all forms of life. As phosphate (PO4), it makes up an important part of
the structural framework that holds DNA and RNA together. Phosphates are also a critical component of ATP –
the cellular energy carrier – as they serve as an energy ‘release' for organisms to use in building proteins or
contacting muscles. Like calcium, phosphorus is important to vertebrates; in the human body, 80% of
phosphorous is found in teeth and bones.
The phosphorus cycle differs from the other major biogeochemical cycles in that it does not include a gas
phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere,
contributing – in some cases – to acid rain. The water, carbon, nitrogen and sulfur cycles all include at least
one phase in which the element is in its gaseous state. Very little phosphorus circulates in the atmosphere
because at Earth's normal temperatures and pressures, phosphorus and its various compounds are not gases.
The largest reservoir of phosphorus is in sedimentary rock.
It is in these rocks where the phosphorus cycle begins. When it rains, phosphates are removed from the rocks
(via weathering) and are distributed throughout both soils and water. Plants take up the phosphate ions
from the soil. The phosphates then moves from plants to animals when herbivores eat plants and
carnivores eat plants or herbivores. The phosphates absorbed by animal tissue through consumption
eventually returns to the soil through the excretion of urine and feces, as well as from the final decomposition
of plants and animals after death.
Nitrogen Cycle
Nitrogen is both the most abundant element in the atmosphere and, as a building block of proteins and nucleic acids such
as DNA, a crucially important component of all biological life. The nitrogen cycle is a complex biogeochemical cycle in which
nitrogen is converted from atmospheric molecular form (N2) into a form that is useful in biological processes.
The nitrogen cycle contains several stages:
Nitrogen fixation
Atmospheric nitrogen occurs primarily as a form that few organisms can use; therefore it must be converted to an organic or fixed - form in a process called nitrogen fixation-this is done through biological processes. First, nitrogen is deposited
from the atmosphere into soils and surface waters, mainly through rain. Once in the soils and surface waters, nitrogen
undergoes a set of changes: its two nitrogen atoms separate and combine with hydrogen to form ammonia (NH4+). This is
done by bacteria.
A small amount of nitrogen is 'fixed' through a process as lighting strikes, converting atmospheric nitrogen into ammonia
(NH4+) and nitrates (NO3-). Nitrogen can also be fixed through man-made processes, primarily industry and nitrogen-rich
fertilizers.
Nitrification
While ammonia can be used by some plants, most of the nitrogen taken up by plants is converted by bacteria from
ammonia - which is highly toxic to many organisms - into nitrite (NO2-), and then into nitrate (NO3-). This process is called
nitrification.
Assimilation
Nitrogen compounds in various forms, such as nitrate, nitrite, ammonia, and ammonium are taken up from soils by plants
which are then used in the formation of plant and animal proteins.
Ammonification
When plants and animals die, or when animals emit wastes, the nitrogen in the organic matter reenters the soil where it is
broken down by other microorganisms, known as decomposers. This decomposition produces ammonia.
Denitrification
Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate
(NO3-) is converted back to nitrogen gas (N2). Denitrification occurs primarily in wet soils where the water
makes it difficult for microorganisms to get oxygen.
Water Cycle
Earth is the water planet with more than two-thirds of its surface covered by water. Most of life on Earth is also primarily
composed of water; our cells, and those of plants and animals are composed of approximately 70 percent water. Vast
quantities of water also cycle through the Earth's atmosphere, oceans, land, and biosphere over both short and long time
scales. This grand cycling of water is called the hydrologic cycle. The cycling of water shapes our weather and climate,
supports plant growth, and makes life itself possible. The water cycle is dominated by the oceans, where 96 percent of the
water on Earth is found and where 86 percent of global evaporation occurs.
When rain and other precipitation falls on land, some of it runs off into surface waters such as lakes and streams. Much of
it, however, seeps into the ground. This process – the movement of water into and through the soil and rocks – is called
infiltration. This is where the water is purified. The extent to which the water is “cleaned” depends on the state of the
environment and the amount of pollution in the water. Passing through layers of rock helps to filter pollutants out, allowing
the pure water to pass through. Generally, the deeper groundwater is found, the cleaner it will be.
Water that is not absorbed into the soil flows across the landscape to rivers, lakes, streams, and eventually to the oceans,
as runoff. While some runoff waters originate from precipitation, others stem from melting snow or ice, and are called melt
water runoff.
Rather than seep into the soil or run off into surface waters, some water returns to the air in gas form (water vapor)
through evaporation. However, of all water that returns to the atmosphere through evaporation, ocean evaporation is the
most prevalent, consisting of about 80 percent of total global evaporation. For land-based evaporation, roughly half occurs
on the surface area of plants and is called transpiration.
The process in which water vapor is converted back into liquid is called condensation. A familiar type of condensation is
the formation of dew drops on blades of grass or on the outside of a cold glass. A more important type of condensation
within the hydrologic cycle takes place in the atmosphere. As water vapor moves upward in the atmosphere it cools. The
droplets formed from atmospheric condensation gather together in the clouds.
Water in the atmosphere, after condensing and forming into clouds, returns to Earth through precipitation, which can take
many forms.
Water is stored for periods of time in various types of reservoirs. The primary reservoirs are the oceans, polar ice and
glaciers, the atmosphere, groundwater, lakes, soils, atmosphere, rivers and streams.
Carbon Cycle
Carbon is a fundamental building block of life; life on Earth is comprised of carbon-based life forms.
Photosynthesis
The first step in the biological carbon cycle is the conversion of atmospheric carbon into a biological form. This 'fixing' of
carbon in biological form takes place within plants and other organisms - known as producers - in a process called
photosynthesis, by which energy from sunlight is converted into chemical form.
Cycling and Storage
Within the oceans, a large amount of organic carbon sinks to the ocean floor to be buried into the crust of the earth. In
plants and animals - known as consumers - carbon dioxide reenters the air through respiration, as food molecules are
broken down for energy and CO2 gas is emitted.
The carbon that is absorbed from the atmosphere by plants and animals can take several paths before reentering the air as
carbon dioxide. When a plant dies, it is broken down by microorganisms - called decomposers - that feed on the dead
organic matter. As the microorganisms consume the plant matter, they release some of the plant's carbon into the
atmosphere in the form of CO2, and some is used in the body. These animals then return more of the carbon to the
atmosphere as CO2 through respiration, although some will be stored within their bodies until they die and decompose in
the soil.
The Two Carbon Cycles and Human Alteration
Carbon buried under the ocean floor might take tens of millions of years to return to the atmosphere, if it does at all.
Throughout the Earth's history, this emission of CO2 (and many other gases) from deep below the planet's surface happens
as geological events, such as volcanic eruptions.
Human beings tap into the carbon cycle by extracting oil and coal for use in automobiles and power plants. A byproduct of
this combustion is CO2 gas. Since the Industrial Revolution began, carbon dioxide levels in the atmosphere have
increased measurably, mostly as a result of human use of fossil fuels.
Humans have also altered the carbon cycle by increasing atmospheric CO2 levels through forest clearing and land use.
Trees store large amounts of carbon; when they die and decompose, much of this stored carbon is released as CO2.
However, when humans clear large areas of forest, primarily through the use of fire, the levels of atmospheric carbon are
increased in two ways. First, during combustion, stored carbon is released directly into the air as CO2, and second, the
clearing of land takes away a key mechanism for removing carbon dioxide from the atmosphere (via photosynthesis).
Since carbon dioxide is a primary greenhouse gas, many scientists argue that the increase in atmospheric CO2 from
human activities has resulted in an enhanced greenhouse effect and could result in corresponding changes in our global
climate, including higher global temperatures.