Biology of Soil - Lesson 9 - Bacteria in Soil

Biology of Soil - Lesson 9 - Bacteria in Soil

Target Grade Level / Age Range:

Grades 9-12

Estimated Time:

90 minutes

Purpose:

Students will identify the role bacteria plays in the soil and how bacteria interact with other organisms in the soil.  

Materials:

  • Soil, at least 500 ml (200 mL per treatment / 100 mL per container)
    • A good topsoil with loamy characteristics (not very sandy or clayey) will be best. Clean of rocks, leaves, roots, etc.
    • Should be somewhat moist, but not saturated/muddy
  • 2 or more CO2 probes (Vernier, or equivalent--probe-computer interface, software, etc) (Note: in a pinch you could use only one probe and do several runs in series.)
  • 2 or more 250 ml Nalgene bottles with openings to fit probes
  • 250 ml graduated cylinder
  • 10 ml graduated cylinder or teaspoon
  • 4 or more 9 oz (~250 ml) cups
  • 2 or more spoons, for mixing solutions into soils
  • Water (tap water or distilled)

Essential Files (maps, charts, pictures, or documents)

Vocabulary (with definitions)

  • Necromass: dead cells of bacteria and fungi adsorbed to particle surfaces
  • Soil aggregate: a soil structure formed from a mass of fragments or particles held together by biological components including polysaccharides, bacterial filaments, and fungal hyphae.
  • Rhizosphere: the region of soil in the vicinity of plant roots in which the chemistry and microbiology is influenced by their growth, respiration, and nutrient exchange.
  • Exudate: suite of substances in the rhizosphere that are secreted by the roots of living plants and microbially modified products of these substances. Could include sugars, amino acids, organic acids, enzymes, etc. Provide carbon and energy for bacteria.
  • Cyst: membrane that surrounds one or more bacteria to allow them to wait for more favorable environmental conditions
  • Autotrophs: organism that can form nutritional organic substances from simple inorganic substances such as carbon dioxide.

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

Soil bacteria are the most numerous (though not the largest by mass) component of the soil microbe community. They are typically 3 µm in size. Bacteria are often found adhering to soil particles via chemical bonds, and many are aquatic in that they survive in thin films of water adhering to soil particles. Some are capable of movement using one or more flagella that extend from their cell membrane. They reproduce asexually by budding or binary fission. Some bacteria only survive in aerobic conditions, some prefer aerobic conditions but can tolerate anaerobic ones, and some can only survive in anerobic conditions.

There is a wide variety of bacteria in the soil. When compared to bacteria grown in a lab culture, soil bacteria in their natural habitat are enveloped by a thick mucilaginous shell that is thought to protect them from changes in moisture, pH, and other conditions. Bacteria must be able to adapt to periods of severe lack of food or water by shifts in metabolic activity and structures such as mucilaginous shell. Some species form spores under extremely dry conditions and will return to their normal shape when moisture returns.

The type of bacteria in the soil depends on the conditions of the soil. Oxygen-deprived soils are likely to be dominated by members of the genus Clostridium. Bacteria are also more common in smaller soil pore spaces where they are protected from predation by protozoa and nematodes.

Besides their important role in decomposition, bacteria are also key to nitrogen cycling, as they are the most important soil microbe involved in fixing atmospheric nitrogen into an organic form and also, in some groups such as those in the genus Nitrosomonas, mineralizing it to an inorganic form.

Actinomycetes are a type of bacteria that are often discussed separately because of their unique shape. Rather than being rod or sphere shaped as other bacteria are, actinomycetes form long stringy, hair-like networks called hypha that reproduce asexually via spores. Their growth is similar to that of fungi but their hypha are considerably smaller. Actinomycetes decompose a wide variety of substances, many of which are difficult to break down and include chitin and cellulose. They are far more common in high pH soils and tend to be replaced by bacteria and fungi in lower pH soils. They are also more common in soils located in drier and hotter regions.

These microbes also have life history traits of wide interest to people. They are responsible for secreting many antibiotics now used in medicinal applications – such as streptomycin – which were discovered in the 1950s to be able to combat bacterial infections in humans such as strep throat. In their natural habitat, it is thought that these compounds play a role in protecting actinomycetes from predation by other soil microbes. A second interesting fact is that actinomycetes secrete an organic compound, geosmins, that contribute to the earthy smell of soil most easily noticed after a rainfall.

Interest Approach – Engagement (what will you do to engage students at the beginning of the lesson)

Hold up a teaspoon for the entire class to see and then fill it with soil. Ask the students to guess how many bacteria are present in that teaspoon of soil. Students may throw out a lot of different responses but will likely lowball it. Explain that there are over one million bacteria present in that single teaspoon of soil.

Ask students to consider one acre of soil (approximately the size of a football field). Ask them how many pounds of biomass they think is below ground (in the form of worms, fungi, protozoa, and bacteria). Again, answers will vary but will likely be low. Explain that one acre of soil may hold 10-30,000 pounds of biomass. How many cows would it take to equal 10-30,000 pounds? Approximately 20-30 full grown cows.

Procedures

  1. Present the content in the PowerPoint slide deck titled Bacteria in Soil.pptx, slides 1-6.
    1. Bacteria
      1. Widely variable to air, moisture, temperature, and acidity tolerance in habitat
      2. Consume chitin and cellulose
      3. Autotrophs – organism that is able to form nutritional organic substances from simple inorganic substances such as carbon dioxide.
      4. Nitrogen fixation and release
      5. Some bacteria will release nutrients to the plant
      6. Some bacteria will protect the plant from diseases
      7. Some are parasites and/or cause plant disease
      8. Microorganisms (specifically bacteria) need right condition – water, pH, oxygen, temperature, food (carbon organic matter).
      9. Up to 90% of microorganisms can be in soil in a ‘resting’ state until conditions become just right.
    2. Bacteria in Soil
      1. Much of soil organic matter is the remains of microbial cells rather than plant cells.
      2. Tilling provides oxygen to the bacteria speeding up their metabolism and respiration.
      3. They eat more of the available organic matter and release the carbon dioxide into the atmosphere.
      4. No-till slows down respiration.
    3. Symbiosis
      1. Some bacteria will protect the plant from drought by covering the root in a sticky biofilm that minimizes water loss.
      2. Plant produces root exudates (sugars) that feed the microbiome.
      3. The plant is incentivizing different organisms to a symbiotic relationship.
      4. Plant sends hormones into the soil attracting soil bacteria live on the root or enter and live in the plant root.
    4. Bacteria – Building Soil Structure
      1. Bacteria produce a sticky polysaccharide (carbohydrate like starch) that helps holds soil particles together to form aggregates.
      2. Actinobacteria have filaments that bridge two minerals together that help build soil aggregates.
  2. Ensure that everyone has a solid understanding of the material by asking the students questions to test their comprehension. Encourage notetaking throughout of the key information. Using the key facts that the students captured in their notebooks, instruct them to work with a partner or team to rewrite the lyrics of a song using those key facts and information. Allow 10 minutes for this activity and after that time, ask each pair or group to share the new version of their song. Simple songs like Row, Row, Row Your Boat work well. Popular songs like Wild Thing, Imagine, or more current pop culture songs also work well.
  3. Option 1: If time and resources allow, have students complete the lab on measuring microbial activity in soil. Distribute the lab report sheets and supplies to student groups and have them follow the prescribed procedure.
  4. Option 2: If time or resources are limited, have students watch the video detailing measuring microbial activity in soil: Measuring Soil Respiration https://youtu.be/1EkkB8JaIzQ
  5. Have students turn in or report on their lab findings. Review the learnings by finishing with the content in the PowerPoint slide deck, slides 7-8.

Did You Know? (Ag facts)

  • Up to one million bacteria can be found living in just one teaspoon of soil.
  • Up to 400,000 different species of bacteria can be found living in just one teaspoon of soil.
  • One pound of roots is worth 1.5 pounds of above ground plant material when trying to build soil organic matter.
  • One acre of soil may hold 10,000 to 30,000 pounds of biomass below ground. That is equivalent to 20-30 full grown cows.

Extension Activities (how students can carry this beyond the classroom)

  • Measuring soil microbial activity: This activity examines how soil microbes, such as bacteria and fungi, are involved in carbon cycling. Students design experiments to explore the relationship between microbial respiration rates and soil variables such as temperature, habitat, soil type, and agricultural management choices. Four methods for measuring CO2 released from soil are provided, one in the field (CO2 probe), and three in the lab (CO2 probe, bromothymol blue (BTB) and acid-base titration). The full teacher guide and student guide is available here: https://www.glbrc.org/outreach/educational-materials/measuring-soil-microbial-activity.

Suggested Companion Resources (books and websites)

Sources/Credits

  • This material is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under number NR196114XXXXC003. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
  • The grant by which this project is funded is administered by the Conservation Districts of Iowa.
  • Soil Biology https://youtu.be/su29HS9q61c
  • The Living Soil: How Unseen Microbes Affect the Food We Eat (360 Video) https://youtu.be/-dhdUoK7s2s
  • Dr. Kristine Nichols - Soil Biology Builds Resilience in Organic Systems https://youtu.be/hC9mGS_gIRk

Author(s)

Will Fett

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

T1.9-12.b. Describe resource and conservation management practices used in agricultural systems (e.g., riparian management, rotational grazing, no-till farming, crop and variety selection, wildlife management, timber harvesting techniques).

Iowa Core Standards

  • HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy. 
  • HS-LS2-1. Use mathematical and/or computational representations to support explanations of factors that affect carrying capacity of ecosystems at different scales.
  • HS-LS2-3. Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
  • HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.