Notes: Biogeochemical Cycles (Cycles of Matter)

Unlike the one-way flow of energy, matter is recycled within and between ecosystems. Elements, chemical compounds, and other forms of matter are passed from one organism to another and from one part of the biosphere to another through cycles that connect living things to the earth.


Biogeochemical cycles connect living things to the earth.

The four chemicals that make up 95% of living things are:

These elements are constantly being cycled through living and non-living organic matter.



1. Energy flows through an ecosystem and nutrients cycle within an ecosystem. Nutrients are substances such as water, carbon and nitrogen which are necessary for the survival of living things.

Scientists create models to represent natural interactions in the environment which make it easier to study and understand the 'big picture' without having to physically measure an entire ecosystem. Diagrams of the nutrient cycles could be considered models of the actual cycles as they occur in nature. The use of models also allows scientists to make predictions about how a particular event might effect an ecosystem without having to expose the real ecosystem to that event. For example, how might the loss of trees, and therefore transpiration, affect rainfall in a particular ecosystem?


2. In an Ecosystem the Three Primary Nutrient Cycles:

A. The Water Cycle

B. The Carbon Cycle

C. The Nitrogen Cycle


THE WATER (Hydrologic) CYCLE: The movement of water between different reservoirs on the earth, under ground and in the atmosphere is known as the water cycle.


1. To a large degree, availability of water determines the diversity of organisms in an ecosystem. Water is crucial to Life. Cells contain 70 - 90 % water, and water provides the aqueous environment in which most of life's reactions occur.

2. The availability of WATER is one of the key factors that regulates the productivity of terrestrial (land) ecosystems.

3. Bodies of water such as lakes, rivers, streams, and the oceans contain a substantial percentage of the Earths water. The atmosphere also contains water in the form of:


Processes of the Water Cycle:

A. EVAPORATION: from lakes, rivers, and oceans.

B. TRANSPIRATION: from plants and trees.

C. CONDENSATION: Cloud Formation

D. PRECIPITATION: Rain, Snow, Sleet, Hail.

E. RUN OFF, or RETURNED back into the Cycle.


The Hydrologic (Water) Cycle

For more information about the Hydrologic Cycle, use the interactive resource below from Teacher's Domain. You can stop the animation at any point and read the information. Click on each process at the top of the page and read details on the steps in the water cycle:

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1. Together, photosynthesis and cellular respiration form the basis of the carbon cycle. Carbon is found in all of the major macromolecules (carbohydrates, nucleic acids, proteins and lipids) which are necessary for all living systems. 

2. The Earth's atmosphere contains carbon in the form of carbon dioxide (CO2). There are five major reservoirs of carbon:

3. Processes of the Carbon Cycle: 

Photosynthesis: During photosynthesis, plants and other autotrophs use CO2 along with water and solar energy, to build organic molecules (carbohydrates), thus storing the carbon for themselves and other organisms.

Cellular Respiration: Both autotrophs and heterotrophs use oxygen to break down carbohydrates during cellular respiration. Consumers obtain energy-rich molecules that contain carbon by eating plants and animals.

Volcanic Eruptions and geothermal vents: carbon from deep within the earth's interior is brought back to the surface during eruptions of steam, gasses and lava

Decomposition: Carbon is returned to the environment through decomposers and cellular respiration (breathing releases CO2 back to the atmosphere).

Combustion: When wood or fossil fuels are burned, the chemical reaction releases carbon dioxide back into the atmosphere

Deposition: Coal, petroleum, and calcium carbonate rock are deposited in sediment and underground. Calcium carbonate deposits are eroded by water to form carbon dioxide. Large amounts of carbon are tied up in wood, only returning to the atmosphere when wood is burned. 

Image of the Carbon Cycle

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The black numbers in the image above indicate how much carbon is stored in various reservoirs, in billions of tons ("GtC" stands for GigaTons of Carbon. The figures are circa 2004. The dark blue numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and kerogen.


For more information about the Carbon Cycle, consider playing the Carbon Cycle Game. Click on the image below and follow the onscreen instructions.

Screenshot of the Carbon Cycle Game

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  1. ALL organisms need nitrogen, an important nutrient, to make proteins and nucleic acids.
  2. Most nitrogen is found in the atmosphere (80%) as N2, and most living things cannot use it. ALL organisms rely on the actions of bacteria that are able to transform nitrogen gas into a usable form.
  3. Nitrogen-fixing bacteria (Cyanobacteria and Rhizobium) play a key role in the nitrogen cycle. They live in the soil and in the roots of some kinds of plants, such as beans, peas, clover, and alfalfa. These bacteria have enzymes that can break the atmospheric N2 bonds. Nitrogen atoms are then free to bond with hydrogen atoms to form Ammonia (NH3).


Processes of the Nitrogen Cycle:

NITROGEN FIXATION is the conversion of nitrogen gas to ammonia; Ammonia can be absorbed by plants from the soil, and used to make proteins, and enter the food web for consumers.


ASSIMILATION: Consumers obtain nitrogen from the plants and animals they eat by digesting the food's proteins and using it to make their own proteins



Decomposers return the nitrogen from the remains of dead plants and animals back to the soil.

Nitrogen is also returned from animal and plant waste by decomposers (dung, urine, leaves and bark).

Through ammonification, nitrogen that would be lost, is recycled back into the ecosystem.



Denitrification occurs when anaerobic bacteria (chemoautotrophs) break down nitrates and release nitrogen gas back into the atmosphere.



Bacteria convert ammonia into nitrogen compounds that plants can utilize more easily

Autotrophs (plants) are therefore DEPENDENT on nitrogen-fixing bacteria, and all other organisms are DEPENDENT on autotrophs!

Visit the interactive Nitrogen Cycle resource below. Roll the mouse over the terms to see detailed explanations for reach process.

Image of the Nitrogen Cycle

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For more information about the Nitrogen Cycle, use the interactive resource below from Teacher's Domain. You can stop the animation at any point and read the information. It is a great review and presentation of the cycle.

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