Cycles Information
Water cycle
The water cycle describes how water is exchanged through Earth's land, ocean, and
atmosphere. Water always exists in all three places, and in many forms—as lakes
and rivers, glaciers and ice sheets, oceans and seas, underground aquifers, and
vapor in the air and clouds.
The water cycle consists of three major processes: evaporation, condensation, and
precipitation.
Evaporation​: ​process of a liquid's surface changing to a gas. In the water cycle,
liquid water (in the ocean, lakes, or rivers) evaporates and becomes water vapor.
The water cycle evaporation process is driven by the sun. As the sun interacts with
liquid water on the surface of the ocean, the water becomes an invisible gas (water
vapor).
Condensation: ​process of a gas changing to a liquid. In the water cycle, water
vapor in the atmosphere condenses and becomes liquid.
Condensation can happen high in the atmosphere or at ground level. Clouds form as
water vapor condenses, or becomes more concentrated (dense). Water vapor
condenses around tiny particles called cloud condensation nuclei (CCN). CCN can
be specks of dust, salt, or pollutants. Clouds at ground level are called fog or mist.
Precipitation: ​Unlike evaporation and condensation, precipitation is not a process.
Precipitation describes any liquid or solid water that falls to Earth as a result of
condensation in the atmosphere. Precipitation includes rain, snow, and hail.
Fog is not precipitation. The water in fog does not actually precipitate, or liquify and
fall to Earth. Fog and mist are a part of the water cycle called suspensions: They are
liquid water suspended in the atmosphere.
Precipitation is one of many ways water is cycled from the atmosphere to the Earth
or ocean.
Other Processes: Evaporation, condensation, and precipitation are important parts of
the water cycle. However, they are not the only ones.
1
Runoff, for instance, describes a variety of ways liquid water moves across land.
Snowmelt, for example, is an important type of runoff produced as snow or glaciers
melt and form streams or pools.
Transpiration is another important part of the water cycle. Transpiration is the
process of water vapor being released from plants and soil. Plants release water
vapor through microscopic pores called stomata. The opening of stomata is strongly
influenced by light, and so is often associated with the sun and the process of
evaporation.
Infiltration is the process by which water on the ground surface enters the ​soil​.
Water can be absorbed by the soil and may stay in the soil for a long time until it
gradually gets evaporated. If there is a lot of vegetative cover (green plants) the
infiltrated water can also be absorbed by plant roots and later transpired.
States of Water
Through the water cycle, water continuously circulates through three states: solid,
liquid, and vapor.
Ice is ​solid water. Most of Earth's freshwater is ice, locked in massive glaciers, ice
sheets, and ice caps.
As ice melts, it turns to ​liquid. The ocean, lakes, rivers, and underground aquifers all
hold liquid water.
Water vapor is an invisible ​gas. Water vapor is not evenly distributed across the
atmosphere. Above the ocean, water vapor is much more abundant, making up as
much as 4% of the air. Above isolated deserts, it can be less than 1%.
Water cycle video:​ https://www.youtube.com/watch?v=9pqh6tlEOhs
Nitrogen cycle
Nitrogen, a component of ​proteins and ​nucleic acids​, is essential to ​life on ​Earth​.
Although 78 percent by volume of the ​atmosphere is nitrogen ​gas​, this abundant
reservoir exists in a form unusable by most organisms. Through a series of microbial
transformations, nitrogen is made available to ​plants​, which in turn ultimately sustain
all ​animal life. The steps, which are not altogether sequential, fall into the following
classifications: ​nitrogen fixation​, nitrogen assimilation, ammonification, nitrification,
and ​denitrification​.
2
The nitrogen cycle describes how ​nitrogen moves between plants, animals, bacteria,
the atmosphere (the air), and soil in the ground. Nitrogen is an important element to
all life on Earth.
Different Nitrogen States
For Nitrogen to be used by different life forms on Earth, it must change into different
states. Nitrogen in the atmosphere, or air, is N​2​. Other important states of nitrogen
include Nitrates (N0​3​), Nitrites (NO​2​), and Ammonium (NH​4​).
This picture shows the flow of the nitrogen cycle. The most important part of the
cycle is ​bacteria​. Bacteria help the nitrogen change between states so it can be
used. When nitrogen is absorbed by the soil, different bacteria help it to change
states so it can be absorbed by plants. Animals then get their nitrogen from the
plants.
Processes in the Nitrogen Cycle:
● Fixation - Fixation is the first step in the process of making nitrogen usable by
plants. Here bacteria change nitrogen into ammonium.
● Nitrification - This is the process by which ammonium gets changed into
nitrates by bacteria. Nitrates are what the plants can then absorb.
● Assimilation - ​This is how plants get nitrogen. They absorb nitrates from the
soil into their roots. Then the nitrogen gets used in amino acids, nucleic acids,
and chlorophyll.
● Ammonification - This is part of the decaying process. When a plant or
animal dies, decomposers like fungi and bacteria turn the nitrogen back into
ammonium so it can reenter the nitrogen cycle.
● Denitrification - ​Extra nitrogen in the soil gets put back out into the air. There
are special bacteria that perform this task as well.
Why is nitrogen important to life?
Plants and animals could not live without nitrogen. It is an important part of many
cells and processes such as amino acids, ​proteins​, and even our ​DNA​. It is also
needed to make chlorophyll in plants, which plants use in photosynthesis to make
their food and energy.
How have humans altered the nitrogen cycle?
Unfortunately, human activity has altered the cycle. We do this by adding nitrogen
into the soil with ​fertilizer as well as other activities that put more nitrous oxide gas
into the atmosphere. This adds in more nitrogen than is needed by normal cycle and
upsets the cycle's balance.
3
Fun Facts
● Around 78% of the atmosphere is nitrogen. However, this is mostly not usable
by animals and plants.
● Nitrogen is used in fertilizer to help plants grow faster.
● Nitrogen has no color, odor, or taste.
● It is used in many explosives.
● About 3% of your body weight is nitrogen.
Video link:​ ​https://www.youtube.com/watch?v=HOpRT8BRGtk
Phosphorus cycle
The phosphorus cycle is a slow process, which involves five key steps, as shown in
the diagram below and described as follows:
Weathering: ​Since the main source of phosphorus is found in rocks, the first step of
the phosphorus cycle involves the extraction of phosphorus from the rocks by
weathering. Weather events, such as rain and other sources of erosion, result in
phosphorus being washed into the soil.
Absorption by Plants and Animals: ​Once in the soil, plants, fungi, and
microorganisms are able to absorb phosphorus and grow. In addition, phosphorus
4
can also be washed into the local water systems. Plants can also directly absorb
phosphorus from the water and grow. In addition to plants, animals also obtain
phosphorus from drinking water and eating plants.
Return to the environment by decomposition: ​When plants and animals die,
decomposition results in the return of phosphorus back to the environment via the
water or soil. Plants and animals in these environments can then use this
phosphorus, and step 2 of the cycle is repeated.
Human Impact on the Phosphorus Cycle
Humans have had a significant impact on the phosphorus cycle due to a variety of
human activities, such as the use of fertilizer, the distribution of food products, and
artificial eutrophication. Fertilizers containing phosphorus add to the phosphorus
levels in the soil and are particularly detrimental when such products are washed into
local aquatic ecosystems. When levels of phosphorus are too high, the
overabundance of plant nutrients serves to drive the excessive growth of algae.
However, these algae die or form algae blooms, which are toxic to the plants and
animals in the ecosystem.
The phosphorus cycle is the process by which phosphorus moves through the
lithosphere, hydrosphere, and biosphere​. Phosphorus is essential for ​plant and
animal growth, as well as the health of microbes inhabiting the soil, but is gradually
depleted from the soil over time. The phosphorus cycle is an extremely slow
process, as various weather conditions (e.g., rain and erosion) help to wash the
phosphorus found in rocks into the soil. In the soil, the organic matter (e.g., plants
and ​fungi​) absorb the phosphorus to be used for various biological processes.
Video: ​https://www.youtube.com/watch?v=_IBx0zpNoEM
Carbon cycle
Carbon is an element. It is part of oceans, air, rocks, soil and all living things. Carbon
doesn’t stay in one place. It is always on the move!
Carbon moves from the atmosphere to plants.
In the atmosphere, carbon is attached to oxygen in a gas called carbon dioxide
(CO2). With the help of the Sun, through the process of photosynthesis, carbon
dioxide is pulled from the air to make plant food from carbon.
Carbon moves from plants to animals.
5
Through food chains, the carbon that is in plants moves to the animals that eat them.
Animals that eat other animals get the carbon from their food too.
Carbon moves from plants and animals to the ground.
When plants and animals die, their bodies, wood and leaves decay bringing the
carbon into the ground. Some becomes buried miles underground and will become
fossil fuels in millions and millions of years.
Carbon moves from living things to the atmosphere.
Each time you exhale, you are releasing carbon dioxide gas (CO2) into the
atmosphere. Animals and plants get rid of carbon dioxide gas through a process
called respiration.
Carbon moves from fossil fuels to the atmosphere when fuels are burned.
When humans burn fossil fuels to power factories, power plants, cars and trucks,
most of the carbon quickly enters the atmosphere as carbon dioxide gas. Each year,
five and a half billion tons of carbon is released by burning fossil fuels. That’s the
weight of 100 million adult African elephants! Of the huge amount of carbon that is
released from fuels, 3.3 billion tons enters the atmosphere and most of the rest
becomes dissolved in seawater.
Carbon moves from the atmosphere to the oceans.
The oceans, and other bodies of water, soak up some carbon from the atmosphere.
Animals that live in the ocean use the carbon to build their skeletons and shells.
Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and
other greenhouse gases, Earth would be a frozen world. But humans have burned so
much fuel that there is about 30% more carbon dioxide in the air today than there
was about 150 years ago. The atmosphere has not held this much carbon for at least
420,000 years according to data from ice cores. More greenhouse gasses such as
carbon dioxide in our atmosphere are causing our planet to become warmer.
Carbon moves through our planet over longer time scales as well. For example, over
millions of years weathering of rocks on land may add carbon to surface water which
eventually runs off to the ocean. Chemical weathering of silicate minerals, in
particular, can have an effect on the amount of carbon dioxide in the atmosphere.
Additionally, over long time scales, carbon is removed from seawater when the shells
6
and bones of marine animals and plankton collect on the sea floor. These shells and
bones are made of limestone, which contains carbon. When they are deposited on
the sea floor, carbon is stored from the rest of the carbon cycle for some amount of
time. The amount of limestone deposited in the ocean depends somewhat on the
amount of warm, tropical, shallow oceans on the planet because this is where prolific
limestone-producing organisms such as corals live. The carbon can be released back
to the atmosphere if the limestone melts or is metamorphosed in a subduction zone.
7