Coordinated Science for the 21st Century597

Activity 9 The Nitrogen and Phosphorous Cycles

GOALSIn this activity you will:

• Investigate the chemicals thatpromote and inhibit thegrowth of plant material.

• Explain the importance ofnitrogen and phosphorous toorganisms.

• Describe how nitrogen cyclesin an ecosystem.

• Describe how phosphorouscycles in an ecosystem.

• Provide examples of howhuman activities can affectthe nitrogen cycle.

What Do You Think?

Nitrogen is essential to all forms of life. Yet, recent studieshave shown that excess nitrogen has been introduced into ourecosystems. It has had negative effects on the natural nitrogencycle.

• What are the sources of the excess nitrogen?

• What are some of the negative effects of too muchnitrogen?

Write your answer to these questions in your Active Biologylog. Be prepared to discuss your ideas with your small groupand other members of your class.

For You To Do

An excessive growth of algae (algal blooms) can make a lakevery unappealing. More importantly, it places other organismsin the ecosystem in peril through lack of oxygen. In thisactivity you will investigate some of the chemicals thatpromote the growth of algae.

Activity 9 The Nitrogen and Phosphorous Cycles

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1. Obtain three 1-L jars. Make sure thejars are rinsed thoroughly, so thatthere are no leftover traces of anychemicals, including soap. Fill eachjar about three-fourths full withdistilled water.

a) Why is it important that the jarsbe cleaned before beginning thisactivity?

2. To each jar add a 10-mL sample ofpond water. Stir the pond waterthoroughly before taking the sample.The pond water will contain algae.

3. Label the jars A through C.

4. To each jar add the following:• To Jar A, add 15 gm of detergent. • To Jar B, add 15 gm of lawn

fertilizer.• Do not add anything to Jar C.

a) Many detergents containphosphates. Fertilizers containnitrogen and phosphates. Write as a question what you areinvestigating in this activity.

b) What is the purpose of Jar C?

5. Cover each jar with plastic wrap sothat dirt will not settle into the jar,but allow for some air to enter thejar.

6. Use a glass marker to mark the water level in each jar.

7. Set all the jars in a well-lighted place, but not in direct sunlight.

a) Predict in which jar the algalgrowth will be the greatest? Theleast? Give reasons for yourpredictions.

8. Observe the jars each day for abouttwo weeks. As water evaporates

from the jars, add distilled water tobring the water back up to itsoriginal level. At the end of twoweeks, you will pass the water ineach jar through a separate filter.

a) Record your observations everytwo or three days.

9. Find the mass of each of threepieces of filter paper.

a) Record the mass of each in atable.

10. Fold the filter paper as shown andinsert it into a funnel. Place thefunnel in the mouth of another jarto collect the filtrate (the liquid thatpasses through the filter).

Filter the liquid in each of the threejars.

11. Allow the filter papers and the algaeresidue to dry thoroughly.

Find the mass of each piece of filterpaper and algae. Calculate the massof the algae.

a) Record your findings in a table.

b) Did your findings support yourpredictions? Explain anydifferences you found.

Place folded filter paper in funnel and soak with water.

Handle all of theliquids andchemicals verycarefully. Theyshould all beconsideredcontaminated andtoxic. Keep handsaway from eyesand mouth duringthe activity. Washyour hands wellafter the activity.Clean up anyspills immediately.

Be very carefulwith the liquidand the algaeresidue. Youshould assumethat diseaseorganisms havegrown in thewater during theactivity. Be verycareful to avoidingesting any ofthe water orresidue. Disposeof all materials asdirected by yourteacher whenfinished.

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Activity 9 The Nitrogen and Phosphorous Cycles

THE NITROGEN CYCLE

Nitrogen Fixation

Nitrogen is a basic building block of plant and animal proteins. It is anutrient essential to all forms of life. Nitrogen is also required to makedeoxyribonucleic acid or DNA. DNA is the hereditary material foundin all living things.The movement of nitrogen through ecosystems, thesoil, and the atmosphere is called the nitrogen cycle. Like carbon,nitrogen moves in a cycle through ecosystems. It passes through foodchains and from living things to their environment and back again. Lifedepends on the cycling of nitrogen.

The largest single source of nitrogen is the atmosphere. It is made upof 78 percent of this colorless, odorless, nontoxic gas.With this muchnitrogen available, you would think organisms would have no difficultygetting nitrogen. Unfortunately, this is not the case. Nitrogen gas is avery stable molecule. It reacts only under limited conditions. In orderto be useful to organisms, nitrogen must be supplied in another form,the nitrate ion (NO3�).

The nitrogen cycle is verycomplex. A simplified descriptionis shown in the diagram on thenext page.There are two ways inwhich atmospheric nitrogen canbe converted into nitrates.Thefirst method is lightning, and thesecond is bacteria in the soil.Theprocess of converting nitrogeninto nitrates is called nitrogenfixation.

A small amount of nitrogen isfixed into nitrates by lightning.The energy from lightning causesnitrogen gas to react with oxygenin the air, producing nitrates.Thenitrates dissolve in rain, falling toEarth and forming surface water.

Bio Words nitrogen cycle:the movement ofnitrogen throughecosystems, the soil,and the atmosphere

nitrogen fixation:the process bywhich certainorganisms producenitrogencompounds fromthe gaseousnitrogen in theatmosphere

Three processes are responsible for most of the nitrogen fixation in the biosphere:atmospheric fixation by lightning, biologicalfixation by certain microbes, and industrialfixation.The enormous energy of lightningbreaks nitrogen molecules apart. Only aboutfive percent of the nitrates produced bynitrogen fixation are produced by lightning.

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The nitrates enter the soil and then move into plants through theirroots. Plant cells can use nitrates to make DNA, and they can convertnitrates into amino acids, which they then string together to makeproteins.When a plant is consumed by an animal, the animal breaks

down the plantproteins intoamino acids.The animal canthen use theamino acids tomake theproteins itneeds.

Some bacteriaare capable offixing nitrogen.These bacteriaprovide thevast majority ofnitrates foundin ecosystems.They are foundmostly in soil,and in smalllumps callednodules on theroots oflegumes suchas clover,soybeans, peas,and alfalfa.Thebacteriaprovide theplant with abuilt-in supplyof usablenitrogen, whilethe plant

nga

nitrogen fixation(lightning)(lightning)

nitrogen fixationnitrogen fixation(bacteria)

artificialfertilizers

decomposers

feedingfeeding

wast an death

taken upby plants

nitrogen gasnitrogen gas(atmosphere)(atmosphere)

nitrogen innitrogen ininorganic formsinorganic forms

nitrogen innitrogen inprotein and protein and other organicother organiccompoundscompounds

THENITROGENCYCLE

The most familiar examples of biotic nitrogen fixing are the root nodulesof legumes, plants like peas, beans, and clover.

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Activity 9 The Nitrogen and Phosphorous Cycles

supplies the nitrogen-fixing bacteria with the sugar they need to makethe nitrates.This plant-bacteria combination usually makes much morenitrate than the plant or bacteria need.The excess moves into the soil,providing a source of nitrogen for other plants.The traditionalagricultural practices of rotating crops and mixed plantings of crops, oneof which is always a legume, capitalizes on bacterial nitrogen fixation.

All organisms produce wastes and eventually die.When they do,decomposers break down the nitrogen-containing chemicals in thewaste or body into simpler chemicals such as ammonia (NH3). Otherbacteria convert ammonia into nitrites, and still others convert thenitrites back to nitrates.These bacteria all require oxygen to function.The nitrates then continue the cycle when they are absorbed by plantroots and converted into cell proteins and DNA.

Farmers and gardeners who use manure and other decaying mattertake advantage of the nitrogen cycle. Soil bacteria convert thedecomposing protein in the manure into nitrates. Eventually, thenitrates are absorbed by plants.

Denitrification

At various stages in the decay process, denitrifying bacteria can breakdown nitrates to nitrites, and then nitrites to nitrogen gas. Eventually,the nitrogen gas is released back into the atmosphere.This process

A gardening magazine stated, "grass can actually poison itself as a result of the various chemicalprocesses that occur in the individual grass plants if the grass roots do not have enough air." To what "poison" is the magazine referring?

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Bio Words denitrification: theconversion of nitratesand nitrites to nitrogengas, which is releasedinto the atmosphere

phosphorous cycle: thecycling of environmentalphosphorous through along-term cycle involvingrocks on the Earth’s crust,and through a shortercycle involving livingorganisms

is called denitrification, and is carried out by bacteria that do notrequire oxygen. Denitrification ensures the balance between soilnitrates, nitrites, and atmospheric nitrogen, and completes thenitrogen cycle.

Older lawns often have many denitrifying bacteria.The fact thatdenitrifying bacteria grow best where there is no oxygen may helpexplain why people often aerate their lawns in early spring. Byexposing the denitrifying bacteria to oxygen, the breakdown ofnitrates to nitrogen gas is reduced. Nitrates will then remain in thesoil, and can be used by the grass to make proteins.

THE PHOSPHOROUS CYCLE

The phosphorous cycle is different from the water, carbon, andnitrogen cycles because phosphorous is not found in the atmosphere.

Phosphorous is anecessary elementin DNA, in manymolecules found inliving cells, and inthe bones ofvertebrate animals.Phosphorous tendsto cycle in twoways: a long-termcycle involving therocks of the Earth’scrust, and a short-term cycle involvingliving organisms.

In the long cycleliving things divertphosphates fromthe normal rockcycle. Phosphorousis found in bedrockin the form of

phosphates phosphates

detrivoresi

pin rock

rockrock

ts als

upliftingof rockof rock

weatheringweatheringof rockof rock

THEPHOSPHOROUSCYCLE

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phosphate ions combined with a variety of elements. Phosphates aresoluble in water and so can be drawn out of rock as part of the watercycle. Dissolved, phosphates can be absorbed by photosyntheticorganisms and so pass into food chains. Phosphates eroded from rockare also carried by water from the land to rivers, and then to theoceans. In the ocean phosphates are absorbed by algae and otherplants, where they can enter food chains.Animals use phosphates tomake bones and shells.When they die, these hard remains formdeposits on the ocean floor. Covered with sediment, the depositseventually become rock, ready to be brought to the surface again.Thecycle can take millions of years to complete.In the short cycle, wastesfrom living things are recycled by decomposers, which break downwastes and dead tissue and release the phosphates.The short cycle ismuch more rapid.

AGRICULTURE AND THE NITROGEN AND PHOSPHOROUS CYCLES

The seeds, leaves, flowers, and fruits of plants all contain valuablenutrients, which is why we eat them. However, as crops are harvested,the valuable nitrogen and phosphorous in these plant body parts areremoved and do not return to the field or orchard they came from.This diversion of nitrates and phosphate from their cycles would soondeplete the soil unless the farmer replaced the missing nutrients.Fertilizers are materials used to restore nutrients and increaseproduction from land. In this activity you investigated the effectfertilizer had on the growth of algae. Some estimates suggest thatfertilizers containing nitrogen and phosphates can as much as doubleyields of cereal crops such as wheat and barley. However, fertilizersmust be used responsibly. More is not necessarily better.

The accumulation of nitrogen and phosphate fertilizers produces anenvironmental problem.As spring runoff carries decaying plant matterand fertilizer-rich soil to streams and then lakes, the nutrients allowaquatic plants to grow more rapidly in what is called an algal bloom.When the plants die, bacteria use oxygen from the water todecompose them. Because decomposers flourish in an environment

Bio Words fertilizer: amaterial used toprovide or replacesoil nutrients

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with such anabundant foodsource, oxygenlevels in lakesdrop quickly, sofish and otheranimals maybegin to die.Dying animalscan only makethe problemworse, asdecomposersbegin to recyclethe matter fromthe dead fish,allowing thepopulations ofbacteria to grow

even larger, anduse still moreoxygen.

Reflecting on the Activity and the Challenge

You have now investigated how severaldifferent types of matter cycle throughecosystems. You have also had anopportunity to learn about howhumans can influence any one of thesecycles. Consider how you will describethe importance of each of these cycles

to the public. Also, consider whether or not the environmental issue that youhave chosen deals specifically with oneof these cycles. You will need toexamine if any solution you providewill create a problem in any one ofthese cycles.

Algae are generally thought of as simple, aquatic plants that donot have roots, stems, or leaves. A recurring problem in manybodies of water is algal bloom. An algal bloom is an abnormalincrease of algae in a body of water.The most serious algalblooms are associated with human activities. Algal blooms depletethe water of oxygen and nutrients. In turn, this can kill otherspecies in the water.

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1. The “new-tank syndrome”

Research to find out what is meant bythe “new-tank” syndrome. How is itrelated to the nitrogen cycle?

2. Too much of a good thing

Which human activities impact on thenitrogen cycle? Choose one and explainhow the impact of this activity on theenvironment could be reduced.

Inquiring Further

1. Why is nitrogen important to organisms?

2. If plants cannot use the nitrogen in the atmosphere, how do they obtain thenitrogen they need?

3. How do animals obtain their usable nitrogen?

4. Explain why it is a good practice to aerate lawns.

5. Why is phosphorous important to living things?

6. With each harvest, nitrogen is removed from the soil. Farmers have traditionallyrotated crops. Wheat, planted one year, is often followed by legumes planted thefollowing year. Because the legumes contain nitrogen-fixing bacteria, nitrogenlevels are replenished. The use of nitrogen-rich fertilizers has allowed farmers tonot use crop rotation.

a) What advantages are gained from planting wheat year after year?

b) New strains of crops have been especially bred to take up high levels ofnitrogen and harvests have increased dramatically. Speculate about somepossible long-term disadvantages that these crops might present forecosystems.

7. Before municipal sewers, the backyard outhouse was standard behind homes.They can still be found in some areas. To make an outhouse, a hole was dug in the ground to collect human wastes. Explain why the outhouse poses a risk to neighboring lakes, using information that you have gained about thenitrogen cycle.

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