Matter moves through the biosphere differently than the way in which energy moves.  Solar and chemical energy are captured by primary producers and then pass in a one-way fashion from one trophic level to the next—dissipating in the environment as heat along the way.  But while energy in the form of sunlight is constantly entering the biosphere, Earth doesn’t receive a significant, steady supply of new matter from space.

• Unlike the one-way flow of energy, matter is recycled within and between ecosystems.

Biogeochemical Cycles
Processes in which elements, chemical compounds, and other forms of matter are passed from one organism to another and one part of the biosphere to the other.

As that word suggests, cycles of matter involve biological processes, geological processes, and chemical processes.  Human activity can also play an important role.  As matter moves through these cycles, it is transformed.  It is never created or destroyed—just changed.

There are many ways in which the processes involved in biogeochemical cycles can be classified.  Here, we will use the following guidelines:

Biological Processes
Biological processes consist of any and all activities performed by living organisms.  These processes include eating, breathing, “burning” food, and eliminating waste products.

Geological Processes
Geological processes include volcanic eruptions, the formation and breakdown of rock, and major movements of matter within and below the surface of the earth.

Chemical and Physical Processes
Chemical and physical processes include the formation of clouds and precipitation, the flow of running water, and the action of lightning.

Human Activity
Human activities that affect cycles of matter on a global scale include the mining and burning of fossil fuels, the clearing of land for building and farming, the burning of forests, and the manufacture and use of fertilizers.

These processes, shown in Figure 3–14, pass the same atoms and molecules around again and again.  Imagine, for a moment, that you are a carbon atom in a molecule of carbon dioxide that has just been shot out of a volcano.  The leaf of a blueberry bush in a nearby mountain range absorbs you during photosynthesis.  You become part of a carbohydrate molecule in a blueberry.  A caribou eats the fruit, and within a few hours, you pass out of the animal’s body.  You are soon swallowed by a dung beetle, which gets eaten by a hungry shrew.  You are combined into the body tissues of the shrew, which is then eaten by an owl.  You are released back into the atmosphere when the owl exhales carbon dioxide, dissolve in a drop of rainwater, and flow through a river into the ocean.

This could just be part of the never-ending cycle of a carbon atom through the biosphere.  Carbon atoms in your body may once have been part of a rock on the ocean floor, the tail of a dinosaur, or even part of a historical figure such as Julius Caesar!

How does matter move through the biosphere?
 

REVIEW & DO NOW Answer the following questions:
How does matter move through the biosphere differently than energy? What are biogeochemical cycles?	What are four ways that biogeochemical cycles can be classified? How are they alike and how are they different?

The Water Cycle
How does water cycle through the biosphere?

Every time you see rain or snow, or watch a river flow, you are witnessing part of the water cycle.

• Water continuously moves between the oceans, the atmosphere, and land—sometimes outside living organisms and sometimes inside them.

Water vapor may be transported by winds over great distances.  If the air carrying it cools, water vapor condenses into tiny droplets that form clouds.  When the droplets become large enough, they fall to Earth’s surface as precipitation in the form of rain, snow, sleet, or hail.  On land, some precipitation flows along the surface in what scientists call runoff, until it enters a river or stream that carries it to an ocean or lake.  Precipitation can also be absorbed into the soil and is then called groundwater.  Groundwater can enter plants through their roots, or flow into rivers, streams, lakes, or oceans.  Some groundwater penetrates deeply enough into the ground to become part of underground reservoirs.  Water that re-enters the atmosphere through transpiration or evaporation begins the cycle anew.

How does water cycle through the biosphere?
 

REVIEW & DO NOW Answer the following questions:
What are two ways that water vapor enters the atmosphere?	What are three types of precipitation?

Nutrient Cycles
What is the importance of the main nutrient cycles?

The chemical substances that an organism needs to sustain life are called nutrients.

• Every organism needs nutrients to build tissues and carry out life functions.  Like water, nutrients pass through organisms and the environment through biogeochemical cycles.  The three pathways, or cycles that move carbon, nitrogen, and phosphorus through the biosphere are especially critical for life.

The Carbon Cycle

Carbon is a major component of all organic compounds, including carbohydrates, lipids, proteins, and nucleic acids.  In fact, carbon is such a key ingredient of living tissue and ecosystems that life on Earth is often described as “carbon-based life.” Carbon in the form of calcium carbonate (CaCO3) is an important component of many different kinds of animal skeletons and is also found in several kinds of rocks.  Carbon and oxygen form carbon dioxide gas (CO2), which is an important component of the atmosphere and is dissolved in oceans.

Some carbon-containing compounds that were once part of ancient forests have been buried and transformed by geological processes into coal.  The bodies of marine organisms containing carbon have been transformed into oil or natural gas.  Coal, oil, and natural gas are often referred to as fossil fuels because they are essentially “fossilized” carbon.  Major reservoirs of carbon in the biosphere include the atmosphere, oceans, rocks, fossil fuels, and forests.

Figure 3–17 shows how carbon moves through the biosphere.  Carbon dioxide is continuously exchanged between the atmosphere and oceans through chemical and physical processes.  Plants take in carbon dioxide during photosynthesis and use the carbon to build carbohydrates.  Carbohydrates then pass through food webs to consumers.  Many animals—both on land and in the sea—combine carbon with calcium and oxygen as the animals build skeletons of calcium carbonate.  Organisms release carbon in the form of carbon dioxide gas by respiration.  Also, when organisms die, decomposers break down the bodies, releasing carbon to the environment.  Geologic forces can turn accumulated carbon into carbon-containing rocks or fossil fuels.  Carbon dioxide is released into the atmosphere by volcanic activity or by human activities, such as the burning of fossil fuels and the clearing and burning of forests.

Scientists know a great deal about the biological, geological, chemical, and human processes that are involved in the carbon cycle, but important questions remain.  How much carbon moves through each pathway? How do ecosystems respond to changes in atmospheric carbon dioxide concentration? How much carbon dioxide can the ocean absorb? Later in this unit, you will learn why answers to these questions are so important.

The Nitrogen Cycle

All organisms require nitrogen to make amino acids, which are used to build nucleic acids, which combine to form DNA, RNA, and proteins.  Many different forms of nitrogen occur naturally in the biosphere.  Nitrogen gas (N2) makes up 78 percent of Earth’s atmosphere.  Nitrogen-containing substances such as ammonia (NH3), nitrate ions (NO3?), and nitrite ions (NO2?) are found in soil, in the wastes produced by many organisms, and in dead and decaying organic matter.  Dissolved nitrogen also exists in several forms in the ocean and other large water bodies.  Figure 3–18 shows how different forms of nitrogen cycle through the biosphere.

Although nitrogen gas is the most abundant form of nitrogen on Earth, only certain types of bacteria can use this form directly.  These bacteria live in the soil and on the roots of certain plants, such as peanuts and peas, called legumes.  The bacteria convert nitrogen gas into ammonia, in a process known as nitrogen fixation.  Other soil bacteria convert that fixed nitrogen into nitrates and nitrites.  Once these forms of nitrogen are available, primary producers can use them to make proteins and nucleic acids.  Consumers eat the producers and reuse nitrogen to make their own nitrogen-containing compounds.  Decomposers release nitrogen from waste and dead organisms as ammonia, nitrates, and nitrites that producers may take up again.  Other soil bacteria obtain energy by converting nitrates into nitrogen gas, which is released into the atmosphere in a process called denitrification.  A relatively small amount of nitrogen gas is converted to usable forms by lightning in a process called atmospheric nitrogen fixation.  Humans add nitrogen to the biosphere through the manufacture and use of fertilizers.  Excess fertilizer is often carried into surface water or groundwater by precipitation.

The Phosphorus Cycle

Phosphorus is essential to living organisms because it forms a part of vital molecules such as DNA and RNA.  Although phosphorus is of great biological importance, it is not abundant in the bio sphere.  Unlike carbon, oxygen, and nitrogen, phosphorus does not enter the atmosphere in significant amounts.  Instead, phosphorus in the form of inorganic phosphate remains mostly on land, in the form of phosphate rock and soil minerals, and in the ocean, as dissolved phosphate and phosphate sediments, as seen in Figure 3–19.

As rocks and sediments gradually wear down, phosphate is released.  Some phosphate stays on land and cycles between organisms and soil. Plants bind phosphate into organic compounds when they absorb it from soil or water.  Organic phosphate moves through the food web, from producers to consumers, and to the rest of the ecosystem.  Other phosphate washes into rivers and streams, where it dissolves.  This phosphate may eventually makes its way to the ocean, where marine organisms process and incorporate it into biological compounds.

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REVIEW & DO NOW Answer the following questions:
What are nutrients? What are the three pathways that move nutrients through the biosphere? What role does oxygen play in the nutrient cycles? Why is carbon important to life? What are two important carbon compounds, and why are they important? Why is nitrogen important to life?	How much of Earth's atmosphere is composed of nitrogen gas? What are three major nitrogen compounds, and where are they found? What is nitrogen fixation and why is it important? What is denitrification? Why is phosphorus important to living things? Does phosphorus enter the atmosphere?

Nutrient Limitation
How does nutrient availability relate to the primary productivity of an ecosystem?

Ecologists are often interested in an ecosystem’s primary productivity—the rate at which primary producers create organic material.

• If ample sunlight and water are available, the primary productivity of an ecosystem may be limited by the availability of nutrients.

Nutrient Limitation in Soil

In all but the richest soil, the growth of crop plants is typically limited by one or more nutrients that must be taken up by plants through their roots.  That’s why farmers use fertilizers! Most fertilizers contain large amounts of nitrogen, phosphorus, and potassium, which help plants grow better in poor soil.  Micronutrients such as calcium, magnesium, sulfur, iron, and manganese are necessary in relatively small amounts, and these elements are sometimes included in specialty fertilizers.  (Carbon is not included in chemical fertilizers because plants acquire carbon dioxide from the atmosphere during photosynthesis.) All nutrient cycles work together like the gears in Figure 3–20.  If any nutrient is in short supply—if any wheel “sticks”—the whole system slows down or stops altogether.

Nutrient Limitation in Aquatic Ecosystems

The open oceans of the world are nutrient-poor compared to many land areas.  Seawater typically contains only 0.00005 percent nitrogen, or 1/10,000 of the amount often found in soil.  In the ocean and other saltwater environments, nitrogen is often the limiting nutrient.  In streams, lakes, and freshwater environments, phosphorus is typically the limiting nutrient.

Sometimes, such as after heavy rains, an aquatic ecosystem receives a large input of a limiting nutrient—for example, runoff from heavily fertilized fields.  When this happens, the result can be an algal bloom.

Algal Bloom
A dramatic increase in the amount of algae and other primary producers that results from a large input of limiting nutrients, discoloring the water, producing foul odors and bad taste, or sometimes producing toxins or using up the free oxygen in the water, leading to fish die-off.

Why can runoff from fertilized fields produce algal blooms? More nutrients are available, so producers can grow and reproduce more quickly.  If there are not enough consumers to eat the algae, an algal bloom can occur, in which case algae can cover the water’s surface and disrupt the functioning of an ecosystem.

How does nutrient availability relate to the primary productivity of an ecosystem?
 

REVIEW & DO NOW Answer the following questions:
What is a limiting nutrient? What are five important micronutrients? What is the major limiting nutrient in seawater?	What is the major limiting nutrient in freshwater? What is an algal bloom? In what ways can an algal bloom be harmful?