Lesson Dirty Decomposers

Learning Objectives

• 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.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Introduction/Motivation

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-school students to grasp. This lesson and its associated activity, however, take advantage of this simple concept to allow students to independently design a controlled, scientific experiment that will enhance their understanding of the processes involved in decomposition. As students conduct the How Fast Can a Carrot Rot? activity, they will go through the same steps any research scientist does: asking a relevant 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 students, this may be the first time they have ever been challenged to ask their own questions and create their own experiments, as opposed to following someone else's procedures to arrive at a usually predictable conclusion. When students are given the opportunity to ask their own questions and then figure out a method to answer them, they are not just learning about science, but they are actually doing it. Students are generally more motivated to execute experiments carefully, and more interested in the results, when they can play key roles in deciding what the experiment will be about and how they will go about conducting it.

A good way to introduce the topic is to bring a few food items into class that can be left out to begin rotting in full view of everyone. Do this at least a week before actually starting the lesson. Items that work well include a large piece of pumpkin (a carved Jack O'Lantern from Halloween works especially well); soft, ripe fruit, such as a peach, also cut open; and plain bread sprinkled lightly with water to keep it moist. You can also pour some canned, undiluted tomato soup into a bowl, or place a few small cubes of tofu or cooked potato on a plate. For safety reasons, avoid anything containing eggs, meat, 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 classroom. Instead, simply tell the class you are curious to see what will happen, and ask students to take a look at them each day when they enter or leave the classroom, or otherwise have a minute to spare. After a few days there should be noticeable changes, such as discoloration and slimy or fuzzy materials growing on the items. 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 see, and then ask what they think will happen as time continues. Let them poke at the log with sticks to get a better 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 composition. They should also see several insects or other invertebrates living within the rotting log, and many strands of thread-like fungus. Both the invertebrates 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 test for the effects of physical features of the environment on decomposition. Emphasize that they themselves will be designing the experiments – not you, the teacher.

After students have read this material, divide 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 soil or 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 soil?
• Will the carrot decompose faster in soil from the woods than it will in sandy soil from the playground?
• Does acid rain make decomposition occur faster?

These are 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 fact 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, students should also know that moisture is necessary for decomposition, and there is little moisture in the Sahara desert. Once they have provided good reasoning for their answers to the questions above, follow with the question, "How could you do an experiment here to simulate those conditions and let you know if your prediction is likely to be right or not?"

Students can easily design a simple experiment to test the effects of different environmental conditions on the decomposition of an organic substance, such as a carrot. By giving every student two plastic bags to use as the decomposition chambers, they will each have one experimental chamber and one control chamber. 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 the effects of temperature, including both warm and cold temperatures, you may need to combine two groups in order to obtain a large enough 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 carrots left at room temperature. Likewise, if students want to test for different moisture levels, by combining two groups students could compare normally-moist potting 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 are curious and carrots may not be among their favorite foods. While comparing the decomposition of different materials is a worthwhile undertaking, the curricular goals of this exercise are related to environmental conditions such as temperature, water content, and types of soil. We have found carrots very suitable for this experiment, since they stay relatively intact throughout the experiment. Carrots also have large enough starting masses and small enough surface areas so any adhering soil particles 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, if students 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 are 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 both a table of data and a graph that shows how much the carrot weighed each time it was checked.

Also, make sure students understand the role of the control carrots. If a team wants to test for the effects of warm temperature and only puts carrots in an incubator, how would they know that 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 question a group decides on, you may also need to make sure they don't introduce additional variables into their experiment. For example, if a group wants to add a small amount of lemon juice to the soil of their experimental carrots to test 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 experiment it would be impossible to know if any differences observed were the result of acidity, or if they were simply the result of different moisture levels.

Lesson Background and Concepts for Teachers

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, bacteria and fungi are the microbial decomposers that outnumber all the other decomposer organisms combined, with billions of individuals existing in a single handful of soil. 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 suburban gardeners use composted grass clippings to enrich their flowerbeds and vegetable patches. By breaking down proteins, starches, and other complex organic molecules that were once part of a living organism, decomposers, as products of their 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 chemical reactions, increased temperatures cause more rapid decomposition reactions, unless the temperature is so high that the microbes are adversely affected. Moisture content of the soil also affects decomposition, with most decomposers benefiting from moist conditions. However, waterlogged soils can become anaerobic, thereby killing some decomposers (those that require oxygen for respiration) but allowing others (those that require little or no oxygen) to thrive. Since many of the soil microbes do their work underground, bright light may also adversely affect some decomposers.

Soil microbes have evolved along with the once-alive-but-now-dead organisms they usually encounter, so decomposition 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 cotton fabric, which consist largely of cellulose, are also decomposed, but not as quickly as unaltered plant material. The chemical changes in the cellulose fibers produced by the high temperatures involved in manufacturing processes, and those brought about by additives such as sulfur and dyes, can also hinder the work of decomposer microbes.

Weathering processes eventually can break down metals and plastics, but ordinarily soil microbes have little or nothing to do with their decomposition. However, microbiologists, scientists and engineers involved in the plastics industry are currently at work developing products that can be broken down by soil microbes. The emphasis is on making plastics that are based on vegetable starches rather than petroleum products. The backbone of starch molecules consists of carbon-oxygen bonds, which can be more easily broken by microbial enzymes than the stronger carbon-carbon bonds of petroleum-based plastics.

Although it is unlikely that microbes alone will be the answer to our waste disposal problems, their traditional role in ecosystems should not go unrecognized. The activity associated with this lesson will allow students to see for themselves what decomposer microbes can accomplish, even if they can't actually see the microbes. However, 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 material being decomposed, like a miniature Pac-Man, but rather, it releases digestive enzymes that make their way into the small gap between 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 then break the material's large organic molecules into smaller molecules that can be used by the microbe. For example, cellulose can be split into glucose and phosphate molecules. The microbe 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

• How Fast Can a Carrot Rot? - Students design and conduct controlled experiments to determine what environmental conditions favor the decomposition of carrots by soil microbes.

Lesson Closure

When most of the experimental carrots (from the Associated Activity) are 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 posters 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 information a formal paper published in a scientific journal would:

• a descriptive title
• a description of the methods used to conduct the experiment, including diagrams if appropriate
• the results of the 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-formal" poster to display in the classroom. The poster should be formal 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 lay out and embellish their posters with color, illustrations, etc.

Because there are several components of the poster's preparation, all students will have opportunities to contribute in ways that highlight their own particular strengths. 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 to ask questions and give feedback to the presenting group.

Assessment

• Ask students to write a paragraph describing the role of decomposer organisms in nutrient recycling and their importance in maintaining the flow of energy through an ecosystem.
• Ask students to list some of the physical conditions that are favorable to microbial decomposers.
• Ask students to complete a written assignment in which they list the steps they would use to design a controlled experiment that would address the question, "Will fertilizer added to the soil increase the rate of decomposition of a carrot by soil microbes?"
• If students are already knowledgeable about 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 rankings.

Lesson Extension Activities

An example of a human-made material that generated a great deal of controversy is the insecticide known as DDT—dichloro-diphenyl-trichloroethane. One of its important characteristics is that it is non-biodegradable, so bacteria and fungi cannot break it down. This makes it very effective as an insecticide, because it doesn't have to be reapplied very often. Unfortunately, in addition to killing pests such as mosquitoes, DDT proved to be quite harmful to many different kinds of other, more beneficial organisms. Rachel Carson, in the 1960's, published the book Silent Spring, which documented some of these harmful affects. DDT killed some bird species outright. Others laid eggs with thin shells that cracked and broke before the chicks were ready to hatch. Furthermore, beneficial 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 U.S. for many years, it continues to be used in some other parts of the world.

Students can conduct library and/or Internet research to find out for themselves about DDT and its history in the U.S. While the story of DDT is very interesting and worthwhile on its own, what is especially significant about Rachel Carson's best-selling book is that it essentially launched an environmental awareness movement. Never before had the public been made aware of the real and potential long-term effects of pesticides, and because of this new awareness, many people began to ask questions about other common pollutants that were then being sent into the atmosphere and waterways.

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Contributors

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Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

This lesson and its associated activity were originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Please visit http://ciblearning.org/ for information about CIBL and other resources for K-12 science and math teachers.