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Biogeochemical Cycles*

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Abstract

By the end of this section, you will be able to:

• Discuss the biogeochemical cycles of water, carbon, nitrogen, phosphorus, and sulfur• Explain how human activities have impacted these cycles and the potential consequences for Earth

Energy �ows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoau-totrophs) and leaving as heat during the many transfers between trophic levels. However, the matter thatmakes up living organisms is conserved and recycled. The six most common elements associated with organicmolecules�carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur�take a variety of chemical formsand may exist for long periods in the atmosphere, on land, in water, or beneath the Earth's surface. Ge-ologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates,all play a role in this recycling of materials. Because geology and chemistry have major roles in the studyof this process, the recycling of inorganic matter between living organisms and their environment is called abiogeochemical cycle.

Water contains hydrogen and oxygen, which is essential to all living processes. The hydrosphere is thearea of the Earth where water movement and storage occurs: as liquid water on the surface and beneaththe surface or frozen (rivers, lakes, oceans, groundwater, polar ice caps, and glaciers), and as water vaporin the atmosphere. Carbon is found in all organic macromolecules and is an important constituent of fossilfuels. Nitrogen is a major component of our nucleic acids and proteins and is critical to human agriculture.Phosphorus, a major component of nucleic acid (along with nitrogen), is one of the main ingredients inarti�cial fertilizers used in agriculture and their associated environmental impacts on our surface water.Sulfur, critical to the 3�D folding of proteins (as in disul�de binding), is released into the atmosphere by theburning of fossil fuels, such as coal.

The cycling of these elements is interconnected. For example, the movement of water is critical forthe leaching of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is amajor reservoir for carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entirebiosphere, from one living organism to another, and between the biotic and abiotic world.

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Head to this website1 to learn more about biogeochemical cycles.

1 The Water (Hydrologic) Cycle

Water is the basis of all living processes. The human body is more than 1/2 water and human cells aremore than 70 percent water. Thus, most land animals need a supply of fresh water to survive. However,when examining the stores of water on Earth, 97.5 percent of it is non-potable salt water (Figure 1). Ofthe remaining water, 99 percent is locked underground as water or as ice. Thus, less than 1 percent of freshwater is easily accessible from lakes and rivers. Many living things, such as plants, animals, and fungi, aredependent on the small amount of fresh surface water supply, a lack of which can have massive e�ects onecosystem dynamics. Humans, of course, have developed technologies to increase water availability, such asdigging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water

1http://openstaxcollege.org/l/biogeochemical



from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, thesupply of fresh water is still a major issue in modern times.

Figure 1: Only 2.5 percent of water on Earth is fresh water, and less than 1 percent of fresh water iseasily accessible to living things.

Water cycling is extremely important to ecosystem dynamics. Water has a major in�uence on climateand, thus, on the environments of ecosystems, some located on distant parts of the Earth. Most of the wateron Earth is stored for long periods in the oceans, underground, and as ice. Figure 2 illustrates the averagetime that an individual water molecule may spend in the Earth's major water reservoirs. Residence timeis a measure of the average time an individual water molecule stays in a particular reservoir. A large amountof the Earth's water is locked in place in these reservoirs as ice, beneath the ground, and in the ocean, and,thus, is unavailable for short-term cycling (only surface water can evaporate).



Figure 2: This graph shows the average residence time for water molecules in the Earth's water reser-voirs.

There are various processes that occur during the cycling of water, shown in Figure 3. These processesinclude the following:

• evaporation/sublimation• condensation/precipitation• subsurface water �ow• surface runo�/snowmelt• stream�ow



The water cycle is driven by the sun's energy as it warms the oceans and other surface waters. This leadsto the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor)of frozen water, which deposits large amounts of water vapor into the atmosphere. Over time, this watervapor condenses into clouds as liquid or frozen droplets and is eventually followed by precipitation (rain orsnow), which returns water to the Earth's surface. Rain eventually permeates into the ground, where it mayevaporate again if it is near the surface, �ow beneath the surface, or be stored for long periods. More easilyobserved is surface runo�: the �ow of fresh water either from rain or melting ice. Runo� can then make itsway through streams and lakes to the oceans or �ow directly to the oceans themselves.

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Head to this website2 to learn more about the world's fresh water supply.

Rain and surface runo� are major ways in which minerals, including carbon, nitrogen, phosphorus, andsulfur, are cycled from land to water. The environmental e�ects of runo� will be discussed later as thesecycles are described.

2http://openstaxcollege.org/l/freshwater



Figure 3: Water from the land and oceans enters the atmosphere by evaporation or sublimation, whereit condenses into clouds and falls as rain or snow. Precipitated water may enter freshwater bodiesor in�ltrate the soil. The cycle is complete when surface or groundwater reenters the ocean. (credit:modi�cation of work by John M. Evans and Howard Perlman, USGS)

2 The Carbon Cycle

Carbon is the second most abundant element in living organisms. Carbon is present in all organic molecules,and its role in the structure of macromolecules is of primary importance to living organisms. Carboncompounds contain especially high energy, particularly those derived from fossilized organisms, mainly plants,which humans use as fuel. Since the 1800s, the number of countries using massive amounts of fossil fuels hasincreased. Since the beginning of the Industrial Revolution, global demand for the Earth's limited fossil fuelsupplies has risen; therefore, the amount of carbon dioxide in our atmosphere has increased. This increasein carbon dioxide has been associated with climate change and other disturbances of the Earth's ecosystemsand is a major environmental concern worldwide. Thus, the �carbon footprint� is based on how much carbondioxide is produced and how much fossil fuel countries consume.

The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbonexchange among living organisms and the other dealing with the long-term cycling of carbon through geologicprocesses. The entire carbon cycle is shown in Figure 4.



Figure 4: Carbon dioxide gas exists in the atmosphere and is dissolved in water. Photosynthesis convertscarbon dioxide gas to organic carbon, and respiration cycles the organic carbon back into carbon dioxidegas. Long-term storage of organic carbon occurs when matter from living organisms is buried deepunderground and becomes fossilized. Volcanic activity and, more recently, human emissions, bring thisstored carbon back into the carbon cycle. (credit: modi�cation of work by John M. Evans and HowardPerlman, USGS)



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Click this link3 to read information about the United States Carbon Cycle Science Program.

2.1 The Biological Carbon Cycle

Living organisms are connected in many ways, even between ecosystems. A good example of this connectionis the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way ofatmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to buildmulti-carbon, high energy compounds, such as glucose. The energy harnessed from the sun is used bythese organisms to form the covalent bonds that link carbon atoms together. These chemical bonds therebystore this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbondioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid,H2CO3

−). However carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic

3http://openstaxcollege.org/l/carbon_cycle



organisms are responsible for depositing approximately 21 percent oxygen content of the atmosphere thatwe observe today.

Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary con-sumers, largely herbivores). Heterotrophs acquire the high-energy carbon compounds from the autotrophsby consuming them, and breaking them down by respiration to obtain cellular energy, such as ATP. The moste�cient type of respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolvedin water. Thus, there is a constant exchange of oxygen and carbon dioxide between the autotrophs (whichneed the carbon) and the heterotrophs (which need the oxygen). Gas exchange through the atmosphere andwater is one way that the carbon cycle connects all living organisms on Earth.

2.2 The Biogeochemical Carbon Cycle

The movement of carbon through the land, water, and air is complex, and in many cases, it occurs muchmore slowly geologically than as seen between living organisms. Carbon is stored for long periods in whatare known as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), oceansediment, soil, land sediments (including fossil fuels), and the Earth's interior.

As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide and is essentialto the process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly in�uenced by thereservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirsin�uences how much carbon is found in each location, and each one a�ects the other reciprocally. Carbondioxide (CO2) from the atmosphere dissolves in water and combines with water molecules to form carbonicacid, and then it ionizes to carbonate and bicarbonate ions (Figure 5)

Figure 5: Carbon dioxide reacts with water to form bicarbonate and carbonate ions.

The equilibrium coe�cients are such that more than 90 percent of the carbon in the ocean is found asbicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCO3), amajor component of marine organism shells. These organisms eventually form sediments on the ocean �oor.Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir onEarth.

On land, carbon is stored in soil as a result of the decomposition of living organisms (by decomposers)or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirsby surface runo�. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed



remains of plants that take millions of years to form. Fossil fuels are considered a non-renewable resourcebecause their use far exceeds their rate of formation. A non-renewable resource, such as fossil fuel, iseither regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land(including land beneath the surface of the ocean) by the eruption of volcanoes and other geothermal systems.Carbon sediments from the ocean �oor are taken deep within the Earth by the process of subduction: themovement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano eruptsor from volcanic hydrothermal vents.

Carbon dioxide is also added to the atmosphere by the animal husbandry practices of humans. Thelarge numbers of land animals raised to feed the Earth's growing population results in increased carbondioxide levels in the atmosphere due to farming practices and the respiration and methane production.This is another example of how human activity indirectly a�ects biogeochemical cycles in a signi�cant way.Although much of the debate about the future e�ects of increasing atmospheric carbon on climate changefocuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account asthey model and predict the future impact of this increase.

3 The Nitrogen Cycle

Getting nitrogen into the living world is di�cult. Plants and phytoplankton are not equipped to incorporatenitrogen from the atmosphere (which exists as tightly bonded, triple covalent N2) even though this moleculecomprises approximately 78 percent of the atmosphere. Nitrogen enters the living world via free-livingand symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen �xation(conversion of N2). Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play akey role in nitrogen �xation. Cyanobacteria are able to use inorganic sources of nitrogen to ��x� nitrogen.Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts)and provide them with the organic nitrogen they need. Free-living bacteria, such as Azotobacter, are alsoimportant nitrogen �xers.

Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem pro-cesses, such as primary production and decomposition, are limited by the available supply of nitrogen. Asshown in Figure 6, the nitrogen that enters living systems by nitrogen �xation is successively convertedfrom organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrialsystems: ammoni�cation, nitri�cation, and denitri�cation. First, the ammoni�cation process converts ni-trogenous waste from living animals or from the remains of dead animals into ammonium (NH4

+) by certainbacteria and fungi. Second, the ammonium is converted to nitrites (NO2

−) by nitrifying bacteria, suchas Nitrosomonas, through nitri�cation. Subsequently, nitrites are converted to nitrates (NO3

−) by simi-lar organisms. Third, the process of denitri�cation occurs, whereby bacteria, such as Pseudomonas andClostridium, convert the nitrates into nitrogen gas, allowing it to re-enter the atmosphere.

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Figure 6: Nitrogen enters the living world from the atmosphere via nitrogen-�xing bacteria. Thisnitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria,which also supply terrestrial food webs with the organic nitrogen they need. (credit: modi�cation of workby John M. Evans and Howard Perlman, USGS)

Which of the following statements about the nitrogen cycle is false?

a.Ammoni�cation converts organic nitrogenous matter from living organisms into ammonium(NH4

+).b.Denitri�cation by bacteria converts nitrates (NO3

−) to nitrogen gas (N2).c.Nitri�cation by bacteria converts nitrates (NO3

−) to nitrites (NO2−).

d.Nitrogen �xing bacteria convert nitrogen gas (N2) into organic compounds.

Human activity can release nitrogen into the environment by two primary means: the combustion of fossilfuels, which releases di�erent nitrogen oxides, and by the use of arti�cial fertilizers in agriculture, which arethen washed into lakes, streams, and rivers by surface runo�. Atmospheric nitrogen is associated with severale�ects on Earth's ecosystems including the production of acid rain (as nitric acid, HNO3) and greenhousegas (as nitrous oxide, N2O) potentially causing climate change. A major e�ect from fertilizer runo� issaltwater and freshwater eutrophication, a process whereby nutrient runo� causes the excess growth ofmicroorganisms, depleting dissolved oxygen levels and killing ecosystem fauna.

A similar process occurs in the marine nitrogen cycle, where the ammoni�cation, nitri�cation, and den-itri�cation processes are performed by marine bacteria. Some of this nitrogen falls to the ocean �oor assediment, which can then be moved to land in geologic time by uplift of the Earth's surface and thereby



incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systemshas been traditionally seen as insigni�cant compared with nitrogen �xed from the atmosphere, a recent studyshowed that this process may indeed be signi�cant and should be included in any study of the global nitrogencycle.

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4 The Phosphorus Cycle

Phosphorus is an essential nutrient for living processes; it is a major component of nucleic acid and phospho-lipids, and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is oftenthe limiting nutrient (necessary for growth) in aquatic ecosystems (Figure 7).

Phosphorus occurs in nature as the phosphate ion (PO43−). In addition to phosphate runo� as a re-

sult of human activity, natural surface runo� occurs when it is leached from phosphate-containing rock byweathering, thus sending phosphates into rivers, lakes, and the ocean. This rock has its origins in the ocean.Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from theirexcretions. However, in remote regions, volcanic ash, aerosols, and mineral dust may also be signi�cantphosphate sources. This sediment then is moved to land over geologic time by the uplifting of areas of theEarth's surface.

Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine ecosys-tems. The movement of phosphate from the ocean to the land and through the soil is extremely slow, withthe average phosphate ion having an oceanic residence time between 20,000 and 100,000 years.



Figure 7: In nature, phosphorus exists as the phosphate ion (PO43−). Weathering of rocks and volcanic

activity releases phosphate into the soil, water, and air, where it becomes available to terrestrial foodwebs. Phosphate enters the oceans via surface runo�, groundwater �ow, and river �ow. Phosphatedissolved in ocean water cycles into marine food webs. Some phosphate from the marine food webs fallsto the ocean �oor, where it forms sediment. (credit: modi�cation of work by John M. Evans and HowardPerlman, USGS)

Excess phosphorus and nitrogen that enters these ecosystems from fertilizer runo� and from sewagecauses excessive growth of microorganisms and depletes the dissolved oxygen, which leads to the death ofmany ecosystem fauna, such as shell�sh and �n�sh. This process is responsible for dead zones in lakes andat the mouths of many major rivers (Figure 7).



Figure 8: Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth ofmicroorganisms, which depletes oxygen and kills fauna. Worldwide, large dead zones are found in coastalareas of high population density. (credit: NASA Earth Observatory)

A dead zone is an area within a freshwater or marine ecosystem where large areas are depleted of theirnormal �ora and fauna; these zones can be caused by eutrophication, oil spills, dumping of toxic chemicals,and other human activities. The number of dead zones has been increasing for several years, and more than400 of these zones were present as of 2008. One of the worst dead zones is o� the coast of the United Statesin the Gulf of Mexico, where fertilizer runo� from the Mississippi River basin has created a dead zone ofover 8463 square miles. Phosphate and nitrate runo� from fertilizers also negatively a�ect several lake andbay ecosystems including the Chesapeake Bay in the eastern United States.

: Chesapeake Bay



Figure 9: This (a) satellite image shows the Chesapeake Bay, an ecosystem a�ected by phosphate andnitrate runo�. A (b) member of the Army Corps of Engineers holds a clump of oysters being used as apart of the oyster restoration e�ort in the bay. (credit a: modi�cation of work by NASA/MODIS; creditb: modi�cation of work by U.S. Army)

The Chesapeake Bay has long been valued as one of the most scenic areas on Earth; it is now indistress and is recognized as a declining ecosystem. In the 1970s, the Chesapeake Bay was one of the�rst ecosystems to have identi�ed dead zones, which continue to kill many �sh and bottom-dwellingspecies, such as clams, oysters, and worms. Several species have declined in the Chesapeake Bay dueto surface water runo� containing excess nutrients from arti�cial fertilizer used on land. The sourceof the fertilizers (with high nitrogen and phosphate content) is not limited to agricultural practices.There are many nearby urban areas and more than 150 rivers and streams empty into the bay thatare carrying fertilizer runo� from lawns and gardens. Thus, the decline of the Chesapeake Bay is acomplex issue and requires the cooperation of industry, agriculture, and everyday homeowners.

Of particular interest to conservationists is the oyster population; it is estimated that more than200,000 acres of oyster reefs existed in the bay in the 1700s, but that number has now declined toonly 36,000 acres. Oyster harvesting was once a major industry for Chesapeake Bay, but it declined88 percent between 1982 and 2007. This decline was due not only to fertilizer runo� and dead zonesbut also to overharvesting. Oysters require a certain minimum population density because theymust be in close proximity to reproduce. Human activity has altered the oyster population andlocations, greatly disrupting the ecosystem.

The restoration of the oyster population in the Chesapeake Bay has been ongoing for several yearswith mixed success. Not only do many people �nd oysters good to eat, but they also clean up thebay. Oysters are �lter feeders, and as they eat, they clean the water around them. In the 1700s, itwas estimated that it took only a few days for the oyster population to �lter the entire volume ofthe bay. Today, with changed water conditions, it is estimated that the present population wouldtake nearly a year to do the same job.



Restoration e�orts have been ongoing for several years by non-pro�t organizations, such as theChesapeake Bay Foundation. The restoration goal is to �nd a way to increase population densityso the oysters can reproduce more e�ciently. Many disease-resistant varieties (developed at theVirginia Institute of Marine Science for the College of William and Mary) are now available andhave been used in the construction of experimental oyster reefs. E�orts to clean and restore thebay by Virginia and Delaware have been hampered because much of the pollution entering thebay comes from other states, which stresses the need for inter-state cooperation to gain successfulrestoration.

The new, hearty oyster strains have also spawned a new and economically viable industry�oysteraquaculture�which not only supplies oysters for food and pro�t, but also has the added bene�t ofcleaning the bay.

5 The Sulfur Cycle

Sulfur is an essential element for the macromolecules of living things. As a part of the amino acid cysteine,it is involved in the formation of disul�de bonds within proteins, which help to determine their 3-D foldingpatterns, and hence their functions. As shown in Figure 9, sulfur cycles between the oceans, land, andatmosphere. Atmospheric sulfur is found in the form of sulfur dioxide (SO2) and enters the atmosphere inthree ways: from the decomposition of organic molecules, from volcanic activity and geothermal vents, andfrom the burning of fossil fuels by humans.



Figure 10: Sulfur dioxide from the atmosphere becomes available to terrestrial and marine ecosystemswhen it is dissolved in precipitation as weak sulfuric acid or when it falls directly to the Earth as fallout.Weathering of rocks also makes sulfates available to terrestrial ecosystems. Decomposition of livingorganisms returns sulfates to the ocean, soil and atmosphere. (credit: modi�cation of work by John M.Evans and Howard Perlman, USGS)

On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rockweathering, and geothermal vents (Figure 10). Atmospheric sulfur is found in the form of sulfur dioxide(SO2), and as rain falls through the atmosphere, sulfur is dissolved in the form of weak sulfuric acid (H2SO4).Sulfur can also fall directly from the atmosphere in a process called fallout. Also, the weathering of sulfur-containing rocks releases sulfur into the soil. These rocks originate from ocean sediments that are movedto land by the geologic uplifting of ocean sediments. Terrestrial ecosystems can then make use of these soilsulfates (SO4−), and upon the death and decomposition of these organisms, release the sulfur back into theatmosphere as hydrogen sul�de (H2S) gas.



Figure 11: At this sulfur vent in Lassen Volcanic National Park in northeastern California, the yellowishsulfur deposits are visible near the mouth of the vent.

Sulfur enters the ocean via runo� from land, from atmospheric fallout, and from underwater geothermalvents. Some ecosystems () rely on chemoautotrophs using sulfur as a biological energy source. This sulfurthen supports marine ecosystems in the form of sulfates.

Human activities have played a major role in altering the balance of the global sulfur cycle. The burningof large quantities of fossil fuels, especially from coal, releases larger amounts of hydrogen sul�de gas intothe atmosphere. As rain falls through this gas, it creates the phenomenon known as acid rain. Acid rainis corrosive rain caused by rainwater falling to the ground through sulfur dioxide gas, turning it into weaksulfuric acid, which causes damage to aquatic ecosystems. Acid rain damages the natural environment bylowering the pH of lakes, which kills many of the resident fauna; it also a�ects the man-made environmentthrough the chemical degradation of buildings. For example, many marble monuments, such as the LincolnMemorial in Washington, DC, have su�ered signi�cant damage from acid rain over the years. These examplesshow the wide-ranging e�ects of human activities on our environment and the challenges that remain for ourfuture.



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Click this link5 to learn more about global climate change.

6 Section Summary

Mineral nutrients are cycled through ecosystems and their environment. Of particular importance are water,carbon, nitrogen, phosphorus, and sulfur. All of these cycles have major impacts on ecosystem structure andfunction. As human activities have caused major disturbances to these cycles, their study and modeling isespecially important. A variety of human activities, such as pollution, oil spills, and events) have damagedecosystems, potentially causing global climate change. The health of Earth depends on understanding thesecycles and how to protect the environment from irreversible damage.

5http://openstaxcollege.org/l/climate_change



7 Art Connections

Exercise 1 (Solution on p. 22.)

Figure 6 Which of the following statements about the nitrogen cycle is false?

a. Ammoni�cation converts organic nitrogenous matter from living organisms into ammonium(NH4

+).b. Denitri�cation by bacteria converts nitrates (NO3

−) to nitrogen gas (N2).c. Nitri�cation by bacteria converts nitrates (NO3


d. Nitrogen �xing bacteria convert nitrogen gas (N2) into organic compounds.

8 Review Questions


The movement of mineral nutrients through organisms and their environment is called a ________cycle.

a. biologicalb. bioaccumulationc. biogeochemicald. biochemical


Carbon is present in the atmosphere as ________.

a. carbon dioxideb. carbonate ionc. carbon dustd. carbon monoxide


The majority of water found on Earth is:

a. iceb. water vaporc. fresh waterd. salt water


The average time a molecule spends in its reservoir is known as ________.

a. residence timeb. restriction timec. resilience timed. storage time


The process whereby oxygen is depleted by the growth of microorganisms due to excess nutrientsin aquatic systems is called ________.

a. dead zoning



b. eutrophicationc. retro�cationd. depletion


The process whereby nitrogen is brought into organic molecules is called ________.

a. nitri�cationb. denitri�cationc. nitrogen �xationd. nitrogen cycling

9 Free Response


Describe nitrogen �xation and why it is important to agriculture.


What are the factors that cause dead zones? Describe eutrophication, in particular, as a cause.


Why are drinking water supplies still a major concern for many countries?



Solutions to Exercises in this Module

to Exercise (p. 20)Figure 6 C: Nitri�cation by bacteria converts nitrates (NO3


to Exercise (p. 20)Cto Exercise (p. 20)Ato Exercise (p. 20)Dto Exercise (p. 20)Ato Exercise (p. 20)Bto Exercise (p. 21)Cto Exercise (p. 21)Nitrogen �xation is the process of bringing nitrogen gas from the atmosphere and incorporating it intoorganic molecules. Most plants do not have this capability and must rely on free-living or symbiotic bacteriato do this. As nitrogen is often the limiting nutrient in the growth of crops, farmers make use of arti�cialfertilizers to provide a nitrogen source to the plants as they grow.to Exercise (p. 21)Many factors can kill life in a lake or ocean, such as eutrophication by nutrient-rich surface runo�, oil spills,toxic waste spills, changes in climate, and the dumping of garbage into the ocean. Eutrophication is a resultof nutrient-rich runo� from land using arti�cial fertilizers high in nitrogen and phosphorus. These nutrientscause the rapid and excessive growth of microorganisms, which deplete local dissolved oxygen and kill many�sh and other aquatic organisms.to Exercise (p. 21)Most of the water on Earth is salt water, which humans cannot drink unless the salt is removed. Some freshwater is locked in glaciers and polar ice caps, or is present in the atmosphere. The Earth's water suppliesare threatened by pollution and exhaustion. The e�ort to supply fresh drinking water to the planet's ever-expanding human population is seen as a major challenge in this century.

Glossary

De�nition 11: acid raincorrosive rain caused by rainwater falling to the ground through sulfur dioxide gas, turning it intoweak sulfuric acid; can damage structures and ecosystems

De�nition 11: biogeochemical cyclecycling of mineral nutrients through ecosystems and through the non-living world

De�nition 11: dead zonearea within an ecosystem in lakes and near the mouths of rivers where large areas of ecosystemsare depleted of their normal �ora and fauna; these zones can be caused by eutrophication, oil spills,dumping of toxic chemicals, and other human activities

De�nition 11: eutrophicationprocess whereby nutrient runo� causes the excess growth of microorganisms, depleting dissolvedoxygen levels and killing ecosystem fauna

De�nition 11: falloutdirect deposit of solid minerals on land or in the ocean from the atmosphere



De�nition 11: hydrospherearea of the Earth where water movement and storage occurs

De�nition 11: non-renewable resourceresource, such as fossil fuel, that is either regenerated very slowly or not at all

De�nition 11: residence timemeasure of the average time an individual water molecule stays in a particular reservoir

De�nition 11: subductionmovement of one tectonic plate beneath another


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