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TE Lesson: Dirty Decomposers
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Summary
Students design and conduct experiments to delennine what environmental factors favor
decomposition by soil microbes. They use chunks ofcarrors for the materials to be
decomposed, and their experiments are carried out in plastic bags filled with dirt. Every
few days students remove the carrots from the dirt and weigh them. Depending on the
experimental conditions, after a few weeks most of the carrots will have decomposed
completely.
Engineering Connection
Engineers must understand what causes vegetables to decompose in order to develop
methods for transporting and preserving them from the field to the groeery store.
Additionally, Engineers may design systems in the future that use microbes to break
down waste.
Contents
1. Learning Objeclives
2. Introductionlivlotivation
3.
4.
5.
G.
7.
Background
Associated Activities
Lcsson Closurc
Attachments
Assessment
8. E.xtensions
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Grade Level: 6 (6-8)
Lesson #: Not provided
Time Required: 5 hours
Lesson Dependency @:None
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The 5 hours is spread over several
weeks.
Keywords: soil, decomposition, nUlrient recyeling, baeteria, fungi
Edueational Standards
@:
Related Standards for California
•
California Science..
• ma. Develop a hypothesis. (Grade: 6) [1998]
• ill!. Communicale the steps and results from an investigation in written
reports and oral presentations. (Grade: 6) [1998)
• GJe. Recognize whether evidence is consistent with a proposed explanation.
(Grade: 6) [1998]
• ill!. Students know dirferenl kinds of organisms may play similar
eeological roles in similar biomes. (Grade: 6) [1998J
•
CIlc. Construct appropriate graphs from data and develop qualitative
statements about the relationships benveen variables. (Grade: 6) [1998]
• Wh.
Identify changes in natural phcnomcna ovcr lime without manipulating
the phenomena (e.g., a tree Iimh, a grove of trees, a stream, a hillslope).
(Grade: 6) [1998J
•
CDb. Students know maHer is transferred over time from one organism 10
others in the food web and between organisms and the physical
environment. (Grade: 6) [19981
•
G1. Students know populations of organisms can be categorized by the
functions they serve in an ecosystem. (Grade: 6) [1998]
•
GJa. Students know energy can be carried from one place to another by heat
flow or by waves, including water, light and sound \"'aves, or by moving
objects. (Grade: 6) [1998]
• illL Students know earthquakes, volcanic eruptions, landslides, and floods
change human and wildlire habitats. (Grade: 6) [1998J
•
We. Students know the number and types of organisms an ecosystem can
support depends on the resources available and on abiotic factors, such as
quantities of light and water, a range of temperatures, and soil composition.
(Grade: 6) [19981
•
LiJa. Students know evidence of plate tectonics is derived from the fit of the
continents; the location of earthquakes, volcanoes, and mid ocean ridges;
and the distribution of fossils, rock types, and ancient climatic zones.
(Grade: 6) [19981
• Wf.
Read a topographic map and a geologie map for evidence provided on
the maps and construct and interpret a simple scale map. (Grade: 6) [1998]
• rna. Students know energy entering eeosystems as sunlight is transferred by
producers into chemical energy through photosynthesis and then from
organism to organism through food webs. (Grade: 6) [1998]
• (i]3. Heal moves in a predictable flow from warmer objects to cooler
objects until all the objects are at the same temperature. As a basis for
understanding this concept: (Grade: 6) [1998]
•
California Math.
• CiJMeasurement and Geometry (Grades K - 7) [1997]
It
Gl2.0 Students analyze and use tables, graphs, and rules to solve problems
involving rates and proportions (Grade: 6) [1997]
• GJ1.2 Know common estimates oflt (3.14; 2217) and use these values to
estimate and calculate the circumference and the area of circles; compare
with actual measurements. (Grade: 6) [1997]
• (ill.3 Analyze data displays and explain why the way in which the question
was asked might have influenced the results obtained and why the way in
which the results were displayed might have influenced the conclusions
reached. (Grade: 6) [1997]
Learning Objectives (Relurn 10 C(}~le~IS)
• Students will be able to describe the role of decomposer organisms in nutrient
recycling and their importance in maintaining the flow of energy through an
ecosystem.
• Students will be able to relate the presence of soil-dwelling organisms to soil
quality.
• Students will be able to describe some of the physical conditions that are
favorable or unfavorable to microbial decomposers.
IntroductionIM.otivalion
{Relurn 10 C"nwntsj
This lesson introduces students to the ideas of nutrient recycling by soil decomposers.
This is an important feature of the natural world, and it is not a difficult concept for
middle·sehool students to grasp. This lesson and its associated activity, however, take
advantage of this simple concept to allow students to independenlly design a controlled,
scientific experiment that will enhance their understanding of the processes involved in
decomposition. As students conduct the activity, they will go through the same steps any
research scientist does: asking a rclevant question, designing an experiment to answer the
question, executing the experiment, collecting data, and interpreting the data to find the
answer to the original question.
For many studenls, this may be the first time they have ever been challenged to ask their
own queslions and create their own experiments, as opposed to following someone else's
procedures to arrive at a usually predictable conclusion. When sludents are given the
opportunity to ask their own questions and lhen figure out a method to answer them. they
are not just learning about science, but they arc actually doing it Students arc generally
more motivated to execute experiments carefully, and more interested in the results, when
they can play key roles in deciding whatlhe experimenl will be about and how they will
go about conducting it.
A good way to introduce the lopic is to bring a few food items into class that can be left
out to begin rotting in full view of everyone. Do this alleasl a week before actually
staning the lesson. Items that work well inelude a large piece of pumpkin (a carved Jack
O'Lantern from Halloween works especially well); soft, ripe fruit, such as a peach, also
cUl open; and plain bread sprinkled lightly with waler to keep it moist. You can also pour
some canned, undiluted tomato soup into a bowl, or place a few small eubes of tofu or
cooked potato on a plate. For safety reasons, avoid anything containing eggs, meal, or
dairy products, since these can grow harmful bacteria.
You do not need to offer much explanation at the time you place these items in the
elassroom. Instead, simply tell the class you are curious to see what will happen, and ask
students to take a look at them eaeh day when they enler or leave the classroom, or
otherwise have a minute 10 spare. After a few days there should be notieeable ehanges,
sueb as discoloration and slimy or fuzzy materials growing on the ilems. By the time you
are ready to start the lesson, there should be enough "activity" on the food items to begin
a discussion by asking students what they think is happening to the items.
If there is a wooded area on the school grounds there is another way this lesson can be
introduced. Take the class to a place where there is a rotting log or tree stump. Ask
students to describe what they sec, and then ask what they think will happen as lime
eontinues. Let them poke at the log wilh sticks to get a hetter idea of what is happening
beneath the visible surfaces. If the log is well rotted, they should notice that it is
becoming more soil-like and less wood-like in compos ilion. They should also see several
insects or other invertebrates living within the rotting log, and many strands oflhread-like
fungus. Both the invenebrales and fungus are important decomposer organisms.
After the class has witnessed examples of decomposition, provide each student with a
copy of the Information for Students: An Introduction to the Decomposers handout
shown below. After students have read this material, let them know that they will work in
teams to design and conduct experiments to lest for the cffects of physical features of the
environmem on decomposition. Emphasize that they themselves wil1 be designing the
experiments - not you, thc tcacher.
After students have read this material, di vide the class
into working groups of four. Then provide each
student with a copy of the Designing an Experiment
handout shown below.
After students read through this handout. each group
should be able to come up with an idea for an
~,
experiment. The experiments are intended to identify
physical characteristics of either the sailor the environment that can affect the
decomposition rate of a carrot.
Examples of questions students have asked in the past include:
•
How does temperature affect decomposition?
•
Will wet soil cause faster decomposition than dry soLI?
•
Will the caITot decompose faster in soil from the woods than it will in sandy soil
from the playground?
• Does acid rain make decomposition occur faster?
These arc only examples, however. Be sure to allow students enough time (at least ten
minutes) to generate their own questions. If they need help you can ask leading questions
such as, "Do you think an apple core tossed on the ground in northern Alaska will
decompose at the same rate as one tossed on the ground in an Amazonian rainforest?"; or,
"Do you think one left in the mud in the Okeefenokee swamp will decompose faster or
slower than one in the sand of the Sahara desert?". Follow up these questions by asking
for the reasoning behind their answers. Students should be able to make connections such
as the faet that we keep perishable foods in the refrigerator to prevent rotting, and that
swampy places actually smell like they contain rotting vegetation. From the introductory
reading, smdents should also kno\\' that moisture is necessary for decomposition, and
there is little moisture in the Sahara desert. Once they have providcd good reasoning for
their ans\\!crs to the questions above, follow with the question, "How could you do an
cxperimcnt here to simulate those conditions and let you know if your prediction is likcly
to be right or not'?"
Studcnts can easily design a simple experiment to test the effects of different
environmental conditions on the dccomposition of an organic substance, such as a carrO!.
By giving every student two plastic hags to use as the decomposition chambcrs, they will
each havc one experimental chamber and one control chambcr. By working in a group of
four students, they will have four trials for their experiment. Of course, this means that
the group will first have to agree on one question to test.
If students want to test for thc effccts of temperature, including both warnl and cold
temperatures, you may need to combine two groups in order to obtain a large cnough
sample size for each condition. This way, with eight students participating, four could
keep their experimental carrots in a refrigerator, four could keep theirs in an incubator,
and all could compare their experimental carrots to their control can"Ots led at room
temperature. Likewise, if students want \0 test for different moisture levels, by combining
two groups students could compare nOl11lally-moist polling soil, wet potting soil, and
potting soil that has been dried by spreading it in a baking pan and placing it in a warm
oven (150° F) for an hour or two.
Students may ask to test substances other than a carrot, simply because they arc curious
and earrOlS may not be among their favorile foods. While comparing the decomposition
of different materials is a worthwhile undertaking, the curricular goals of lhis exercise are
related to environmental conditions sueh as temperature, water eontent, and types ofsoi!.
We have found carrots very suitable for this experiment, sinee they stay relatively intaet
throughoul the experiment. Carrots also have large enough starting masses and small
enough surfaee areas so any adhering soil partieles make up a relatively small proportion
of the total mass. In other words, errors in the mass measurements due to soil sticking to
the carrots are reasonably small. Fruits and vegetables that are softer or have higher water
contents than carrots do not share these desirable characteristics. (However, ifstudents
are still curious after conducting their experiments on carrot decomposition, they should
be encouraged to do another experiment comparing carrots to other fruits or vegetables-­
or any other testable questions they arc interested in.)
As indicated in the Designing an Experiment handout, each team should prepare a written
proposal that answers the seven questions found at the bottom of the handout. Go over
each team's responses, and if necessary, ask them to rethink their plans. They may need
help with the fIfth question, "How will you report your results quantitatively?" Point out
that in science, observations need to be quantified: it's not enough to say that the carrot
got smaller each day, but instead, they need to be able to say exactly how much smaller.
Thus, they can use balances to find the masses of the carrots each day, and report their
results in bolh a table of data and a graph that shows how much the carroL weighed each
time it was checked.
Also, make sure students understand the role of the control carrots. Ifa team wants to test
for the effects of wal11l temperature and only puts carrots in an incubator, how wou ld they
know lhat the same carrots wouldn't decompose in exactly the same way if they simply
left them at room temperature for the duration of the experiment?
Depending on what queslion a group decides on, you may also need to make sure they
don', introduce additional variables into their experiment. For example, if a group wants
to add a small amounl oflemonjuice to the soil oflheir experimental carrots fo lest for
the effects of acidity, they would also need to add an equal amount of water to the soil of
their control carrots. If they did not add water to the control carrots, the two soils would
contain different moisture levels. In that case, at the end of the cxperimcnt it would be
impossible to know if any diffcrences observed werc the result of aeidity, or if they were
simply the result of diffcrent moisture levels.
Lesson Background & Concepts for Teachers lR~tlIm w C""t~n\,~
An important feature of the biosphere is the cycling of materials such as carbon, water,
and other nutrients between the biotic (living) and abiotic (non-living) components of the
environment. This cycling of materials is dependent on soil-dwelling decomposer
organisms, including earthworms, snails, millipedes, and insects. Although we can't see
them, baeteria and fungi are the microbial decomposers that outnumber all the other
decomposer organisms combined, with billions of individuals existing in a single handful
ofsoi!. These microbes are vital to the breakdown of dead and discarded organic
materials, thereby supplying the plants growing in the soil with a continuous source of
nutrients.
Humans have capitalized on the work and abundance of decomposers for centuries, if not
millennia. Farmers spread manure on their fields to fertilize them, and suhurban
gardeners use composted grass clippings to enrich their flowerbeds and vegetable
patches. By breaking down proteins, starches, and olber complex organic molecules that
were once part ofa living organism, decomposcrs, as products ofthcir own metabolism.
convert elements such as nitrogen, phosphorus, calcium, and sulfur into forms that can be
utilized by plants.
What factors affect the ability of soil microbes to do this work? Like all chcmical
reactions, increased temperalUrcs cause mOre rapid decomposition reactions, unless the
temperature is so high that the microbes are adversely affected. Moisture contcnt ofthc
soil also affects decomposition, with most decomposers beneflling from moist conditions.
However, waterloggcd soils can bccomc anacrobic, thcreby killing somc dccomposcrs
(those that require oxygen for respiration) but allowing others (those that rcquirc Iitllc or
no oxygen) to thrive. Sincc many ofthc soil microbes do their work underground, bright
light may also adversely affect some decomposers.
Soil microbcs havc evolvcd along with the once-alive-but-now-dead organisms they
usually encounter, so dccomposition of "organic" materials such as animal flesh, fecal
material, fallen leaves, and acorn shells will occur fairly rapidly. Human-made materials
such as paper. cardboard, and corton fabric, which consist largcly ofccllulosc, are also
dccomposed, but not as quickly as unaltered plant material. The chemical changcs in the
cellulose fibers produced by the high temperatures involvcd in manufacturing proccsses,
and those brought about by additivcs such as sulfur and dyes, can also hinder the work of
decomposer microbes.
Weathcring processcs cventually can break down metals and plastics, bUL ordinarily soil
microbes have little or nothing to do with their decomposition. However, mierobiologists,
scientists and engineers involved in the plastics industry are currently at work developing
products that can be broken down by soil microbes. The cmphasis is on making plastics
that are based on vcgctablc starches rather than petroleum products. The backhone of
starch molecules consisls of carbon.oxygen bonds, which can bc more casily broken by
microbial enzymes than the strongcr carbon-carbon bonds of petroleum-based plastics.
Although it is unlikely that microbes alone will be the answer to our waste disposal
problems, lheir traditional role in ecosystems should not go unrecognized. The aetivity
associatcd with this lcsson will allow students to see for themselves what dccomposer
mierobes ean accomplish, even if they can't actually see the microbes. Howcver, it is
important that students not be misled in their thinking of how this work is actually done.
A soil microbe does not eat the malerial being decomposed, like a miniature Pac·Man,
but rather, it releases digestive enzymes that make their way into the small gap betwccn
the cell membrane and the cell wall. Through holes in the cell wall, the enzymes make
contact with the material to be decomposed. The enzymes thcn brcak thc matcrial's large
organic molecules into smaller molecules that can be used by the microbe. For example,
cellulose can be split into glucose and phosphate moleeules. The mierobe will use the
glucose for its own cellular needs, and release the phosphate into the soil, where it can be
taken up by as a nutrient by living plant root cells.
Associated Activities (ReIIJlTlI"ConlcnI5)
•
How Fast Can a CarroL Rot? - Students design and conduct controlled
experiments to detennine what environmental conditions favor the decomposition
of carrots by soil microbes.
Lesson Closure
(Remm IQ(ontenIS)
When most of the experimental carrots (from the Associated Activity) arc nearly or
completely decomposed, have each group share its finding with the rest of the class. A
good way to do this is to have each group prepare a poster. Scientists frequently use
poslers as an efficient and timely means of communicating with each other when they get
together at meetings devoted to a particular topic area. Their posters contain the same
type of infonnation a formal papcr published in a scientific journal would:
• a descriptive tiile
• a description of the methods used to conduct the experiment, including diagrams
if appropriaie
• the results of (he experiment, shown in tables and graphs, and summarized in
words
• the conclusions drawn from the data
Allow students a day or two of class time to prepare a "semi-fomw}" poster to display in
the classroom. The poster should be fonnal in the sense that it must give a succinct and
objective reporting of the experiment, be neat, and use good grammar and correct
spelling. However, students can still be allowed to exercise their creativity in the way
they layout and embellish their posters with color, illustrations, etc.
Because there are several components of the poster's preparation, all sluuents will have
opportunities to contribute in ways that highlight their own particular strenglhs. When
their posters are done, each group should present its poster to the rest of the class with a
brief summary of the results and conclusions. Other students should be encouraged Lo ask
questions and gi ve feedback to the presenting group.
AnachInenls IRClLim 10 COnlcnt»
• Infonnation tor Students: An Introduction to the Decomposers
• Designing an Experiment
Assessment
(Ilelum 10 Contents)
• Ask students to write a paragraph describing the role of decomposer organisms in
nutrient recycling and thcir importancc in maintaining Ihe flow of energy through
an ecosystem.
•
Ask students to list somc of the physical conditions thai are favorable to microbial
decomposers.
•
Ask studcnts 10 complete a writtcn assignment in which thcy list the steps they
would use to design a controlled experiment that would address the question,
"Will fertilizer added 10 the soil increase the rate of decomposition ofa carrot by
soil microbes?"
•
If students are already knowledgeable aboul different biomes, provide them with a
list of biomes and ask them to put them in order from the ones in which they think
decomposition by soil microbes will occur most quickly to the ones in which they
think it will occur most slowly. Then ask them to write a paragraph justifying
their ran kings.
Lesson Extension Aetivities i!l.etumto C,,,,tl'IIlS)
An example of a human-made materiul that generated a great deal of controversy is the
insecticide known as DDT-clichloro-diphcnyl-triehloroethane. One of its important
characteristies is that it is non-biodegradable, so bacteria and fungi eannot break it down.
This makes it very effective as an insecticide, because ir doesn't have to be reapplied vcry
often. Unfortunately, ill addition to killing pests such as mosquitoes, DDT proved to be
quite harmful to many differem kinds of other, more beneficial organisms. Rachel
Carson, in the I960's, published rhe book Silent Spring, which documented some of these
harmful affects. DDT killed some bird speeies outright. Others laid eggs with thin shells
that cracked and broke before the chicks were ready to hatch. Furthermore, benefieial
insects were killed just as readily as the harmful ones. And DDT has even been
implicated in certain kinds of cancer in humans. Though it has been banned in the United
States for many years, it continues to be used in some other parts of the world.
Students can conduct library and/or Internel research to find out for themselves about
DDT and its history in the United Statcs. While the story of DDT is very interesting and
'\\'orthwhile on its own, what is especially significant about Rachel Carson's best-selling
book is thal it essentially launehed an environmental awareness movement. Never before
had the public been made aware of the real and potential long-term effects ofpesticides,
and because of this new awareness, many people began to ask questions aboul other
common pollutants that were then being sent into the atmosphere and waterways.
Other Related Infonnation
(Rctum \0 Conlenls)
This lesson and its associated activity were originally published, in slightly modified
foml, by Duke University'S Ccnter for Inquiry Based Learning (CIBL). Please visit the
wehsite http://tasc.Dralt.dukc.edu/indcx.php for infonnation about CIBL and other
resources for K-12 seience and math teachers.
Owner lRclum IQ C<m!£!!l_)
Engineering K-Ph.D. Program, Pratt School of Engineering, Dukc University
Contributors
Mary R. Hebrank, Projeet Writer and Consultanl, Duke University
Copyright
© 2004 by Engineering K-Ph.D. Program, Prall School of Engineering, Duke University
including copyrighted works from other educational inslitutions and/or U.S. government
ageneies; all rights reserved.
Lasl Modilicd: J3nuary 21. 2009