We designed two NGSS-aligned middle school classroom experiments to investigate the effects of biochar on plant growth and soil respiration. Biochar is a carbon-rich material, produced by heating organic matter under limited oxygen, that is added to soils to improve fertility, to promote plant growth, and as one possible strategy to help mitigate climate change. The experiments offer an ideal case study for students learning fundamentals of soil and plant interactions. Soils and biochar are accessible, are connected to global issues such as agriculture and climate change, and are the focus of ongoing research in soil science. These classroom experiments promote authentic science because students design replicated experiments, collect and analyze data, discuss variability in the data, and interpret their results in the context of recent research.

Introduction

Soils are the foundation of terrestrial ecosystems and produce food and fiber, filter freshwater, and regulate carbon cycling and therefore global climate (Wall et al., 2012). Demands for increased agricultural productivity, climate change, and impacts of drought (IPCC, 2014) put soils at the forefront of global environmental issues and provide unique opportunities for teaching real-world life science. Soils provide a great opportunity to study fundamental ecosystem processes in a local context because they are found everywhere! Most teachers and students have multiple ways to access soils – in their backyards, parks, local forests, or garden stores. Studying soils in classrooms is inexpensive, easy, and provides an ideal way to involve students in timely and authentic scientific inquiry.

We designed two classroom experiments focused on the use of biochar as a soil amendment to improve soil health. The activities are aligned with the Next Generation Science Standards at the middle school level but can be adapted to high school classrooms (NGSS Lead States, 2013). Biochar serves as an ideal case study for students learning fundamentals of soil and plant interactions because it is easily accessible, connected to global environmental issues, and is the focus of ongoing research. Studying biochar also supports students in engaging in authentic science by discussing data variability and interpreting contrasting research results.

Biochar is made by heating organic matter in the absence of oxygen (Figure 1). Indigenous people of the Amazon have long incorporated charcoal into soils to increase fertility and productivity in what would otherwise be infertile tropical soils (Figure 2). In South America, the treated soils are known as Terra Preta de Indio soils, or Amazonian Dark Earths (Glaser et al., 2001; Figure 2). Scientists have found that biochar can increase soil moisture (Basso et al., 2013; Yu et al., 2013) and stimulate the growth and activity of soil microorganisms (e.g., bacteria and fungi) to enhance nutrient availability to plants (Biederman & Harpole, 2013; Liu et al., 2016). Current investigations focus on how biochar might enhance soil fertility and improve crop productivity (Lehmann, 2007).

Figure 1.

Biochar is the carbon-rich product of biomass charring. It is highly porous (A), heterogeneous in structure (B), and added to agricultural soils to improve soil fertility (C). The biochar pictured here is derived from palm fronds (A) and pine wood (B, C). (Credits: image A by Nadav Ziv; images B and C by E. J. Foster)

Figure 1.

Biochar is the carbon-rich product of biomass charring. It is highly porous (A), heterogeneous in structure (B), and added to agricultural soils to improve soil fertility (C). The biochar pictured here is derived from palm fronds (A) and pine wood (B, C). (Credits: image A by Nadav Ziv; images B and C by E. J. Foster)

Figure 2.

A typical tropical soil (A) compared to the Terra Preta de Indio soil (B). (Credit: images by Bruno Glaser)

Figure 2.

A typical tropical soil (A) compared to the Terra Preta de Indio soil (B). (Credit: images by Bruno Glaser)

The impacts of biochar amendment depend on the type of biochar (i.e., from what and how it was produced) and the soil type (e.g., texture, organic matter content, nutrient availability, acidity; Lehmann & Joseph, 2015). These factors explain why many biochar experiments yield different results. So far, the greatest effects of biochar have been seen in tropical, arid, and acidic soils (Jeffery et al., 2011). Research to determine which types of biochar improve soil moisture and nutrient availability is still ongoing (Zhang et al., 2016).

Aside from improving soils for plant growth, research suggests that the production and application of biochar may store carbon in soils for decades to millennia, reduce the net amount of carbon released to the atmosphere, and help mitigate rising atmospheric CO2 concentrations responsible for climate change (Liu et al., 2016; Figure 3). The pyrolysis process to make biochar requires carbon and energy inputs. By using a local feedstock for the pyrolysis process or even using the solid waste product from biofuel production, biochar can sequester more carbon than is used in its production process (Woolf et al., 2010; Field et al., 2012). Biochar alone is not a sufficient strategy to address climate change, but it can serve as one part of a multipronged approach to mitigating climate change.

Figure 3.

The natural carbon cycle (left) compared with the biochar carbon cycle (right). In the natural carbon cycle, carbon withdrawn from the atmosphere through photosynthesis is balanced by carbon released to the atmosphere through respiration. In the biochar carbon cycle, a portion of the carbon withdrawn via photosynthesis is used for bioenergy production, which results in a biochar coproduct that can be sequestered in soils. (Credit: redrawn and adapted from Lehmann, 2007)

Figure 3.

The natural carbon cycle (left) compared with the biochar carbon cycle (right). In the natural carbon cycle, carbon withdrawn from the atmosphere through photosynthesis is balanced by carbon released to the atmosphere through respiration. In the biochar carbon cycle, a portion of the carbon withdrawn via photosynthesis is used for bioenergy production, which results in a biochar coproduct that can be sequestered in soils. (Credit: redrawn and adapted from Lehmann, 2007)

We present two classroom experiments that investigate the effect of biochar on soils and plants. We align these activities with NGSS by integrating disciplinary core ideas, science practices, and crosscutting concepts. The activities' objectives are to

  • gain an appreciation for soils as an important natural resource;

  • explore issues in soils and agriculture and examine biochar as a possible solution;

  • manipulate soil properties and biochar amendment to investigate impacts on soils and plants; and

  • conduct an experiment, collect and analyze data, and engage in scientific inquiry to interpret real-world data.

Properties of Biochar

Biochar is produced by breaking down organic matter under high heat (250–700°C) and limited oxygen (Lehmann & Joseph, 2015). Biochar is often a coproduct of bioenergy derived from naturally renewable organic matter sources, including agricultural products such as crop and animal wastes and nonagricultural products such as woody biomass and algae (Table 1). The physical and chemical properties of biochar vary depending on its source and how it is produced. Different sources and production temperatures yield biochars with different surface areas, pore sizes, pH values, and carbon contents. Biochar surface area is a function of pore size, with smaller pore size resulting in higher surface area. Pore size, and thus surface area, is controlled by the temperature at which biochars are produced (Lehmann & Joseph, 2015). As a consequence of this variability, not all biochars interact with the environment in the same way. Students can draw on their understanding of the variable properties of biochar to explain their own experimental results.

Table 1.
Properties of biochar from different feedstocks, including surface area, total carbon, and pH. Values are taken from research studies that incorporated different biochars into soils. Values vary depending on production conditions.
Biochar FeedstockProduction Temperature
(°C)
Surface Area
(m2 g−1)
Total Carbon
(%)
pH
Agricultural crop Corn stovera 550 12 74.3 9.89 
Wheat strawb 200 2.53 38.7 5.43 
Wheat strawb 350 3.48 59.8 8.69 
Wheat strawb 500 33.2 62.9 10.2 
Nonagricultural organic matter Pinec 400–700 232.72 72 9.2 
Grassb 500 3.33 62.1 10.2 
Bambood 300 1.3 66.2 7.9 
Algaee 305 1.15 28.9 8.0 
Animal waste Cow manureb 500 21.9 43.7 10.2 
Pig manureb 200 3.59 37.0 8.22 
Pig manureb 350 4.26 39.1 9.65 
Pig manureb 500 47.4 42.7 10.5 
Biochar FeedstockProduction Temperature
(°C)
Surface Area
(m2 g−1)
Total Carbon
(%)
pH
Agricultural crop Corn stovera 550 12 74.3 9.89 
Wheat strawb 200 2.53 38.7 5.43 
Wheat strawb 350 3.48 59.8 8.69 
Wheat strawb 500 33.2 62.9 10.2 
Nonagricultural organic matter Pinec 400–700 232.72 72 9.2 
Grassb 500 3.33 62.1 10.2 
Bambood 300 1.3 66.2 7.9 
Algaee 305 1.15 28.9 8.0 
Animal waste Cow manureb 500 21.9 43.7 10.2 
Pig manureb 200 3.59 37.0 8.22 
Pig manureb 350 4.26 39.1 9.65 
Pig manureb 500 47.4 42.7 10.5 

Biochar is easy to obtain from garden stores or online (https://www.biochar-international.org). Teachers should read the material safety data sheet provided by biochar producers and handle biochar appropriately. Dust inhalation is an important but manageable safety concern. We recommend purchasing pelletized biochar products or products with >3 mm particle size. To minimize dust, take care when opening a bag of biochar as dust may accumulate during transport. Wear facemasks and gloves when handling raw biochar. Spraying or adding small amounts of water to biochar can reduce dust. Students can observe large pellets of biochar under the microscope and handle the materials after the biochar has been added to soils.

Connections to Next Generation Science Standards

We developed two classroom experiments that mirror recent experiments conducted by scientists: effects of biochar on (1) plant growth and (2) soil respiration. Both experiments align with NGSS through addressing disciplinary core ideas, crosscutting concepts, and science practices (Table 2).

Table 2.
NGSS Performance Expectations linked to plant and soil science classroom experiments.
NGSS Performance ExpectationAssociated Learning Goals for Plant Growth ExperimentAssociated Learning Goals for Soil Respiration Experiment
MS-LS1-5. Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms. Explain how and why environmental factors including soil properties (e.g., texture, organic matter content, moisture, nutrients, pH, biochar) may impact plant growth metrics including height, aboveground biomass, and belowground biomass. Explain how and why environmental factors including soil properties (e.g., texture, organic matter content, moisture, nutrients, pH, biochar) may impact soil biota respiration as measured by CO2 flux. 
MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem. Collect and analyze data, identify patterns, and provide evidence concerning relationships among resource availability, biochar amendment, soil type, and plant growth metrics. Collect and analyze data, identify patterns, and provide evidence concerning relationships among resource availability, biochar amendment, soil type, and soil biota respiration. 
MS-ETSI-1. Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions. Use knowledge and practice developed through the plant growth investigation to propose and evaluate whether/how biochar amendment would be appropriate to increase crop yield in different conditions – considering issues such as soil and crop type. Use knowledge and practice developed through the soil respiration investigation to propose and evaluate whether/how biochar amendment would be an appropriate strategy for increasing soil carbon storage in different conditions – considering issues such as soil type. 
NGSS Performance ExpectationAssociated Learning Goals for Plant Growth ExperimentAssociated Learning Goals for Soil Respiration Experiment
MS-LS1-5. Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms. Explain how and why environmental factors including soil properties (e.g., texture, organic matter content, moisture, nutrients, pH, biochar) may impact plant growth metrics including height, aboveground biomass, and belowground biomass. Explain how and why environmental factors including soil properties (e.g., texture, organic matter content, moisture, nutrients, pH, biochar) may impact soil biota respiration as measured by CO2 flux. 
MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem. Collect and analyze data, identify patterns, and provide evidence concerning relationships among resource availability, biochar amendment, soil type, and plant growth metrics. Collect and analyze data, identify patterns, and provide evidence concerning relationships among resource availability, biochar amendment, soil type, and soil biota respiration. 
MS-ETSI-1. Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions. Use knowledge and practice developed through the plant growth investigation to propose and evaluate whether/how biochar amendment would be appropriate to increase crop yield in different conditions – considering issues such as soil and crop type. Use knowledge and practice developed through the soil respiration investigation to propose and evaluate whether/how biochar amendment would be an appropriate strategy for increasing soil carbon storage in different conditions – considering issues such as soil type. 

The experiments follow the claim, evidence, reasoning (CER) instructional framework (McNeill & Krajcik, 2008), in which students construct a scientific argument based on experimental evidence from the students' experiments and from published studies. This integrated approach encourages students to use multiple lines of evidence, including their own observations, to support a claim with reasoning.

Soil & Plant Science Classroom Investigations

We describe two classroom experiments that were codeveloped by K–12 educators and university researchers and conducted in classroom settings. The data shown here were collected by students. Prior to conducting the experiments, provide students with an introduction to biochar and its application in soils and agriculture. Ask students to observe biochar and different soil types under a microscope and discuss differences in properties such as porosity (Figure 1) and pH ( Appendix 1). Discuss how different properties of the biochar and soil they observed might influence plant growth and soil respiration.

Experiment 1: Plant Growth

Biochar is typically added to agricultural soils, and scientists are interested in how biochar will affect crop productivity. Here, students conduct an analogous experiment in which they ask the question “How does biochar affect plant growth in different soil types?” Prompt students to make a prediction about whether biochar will increase, decrease, or have no effect on plant growth and to explain their ideas about why ( Appendix 2). Ask them to list the dependent variables they would measure in order to answer this question ( Appendix 2). Variables may include plant height, plant biomass, stem diameter, and number of leaves, among others. Scientists typically assess crop productivity by measuring plant height and aboveground (shoot) and belowground (root) biomass, and compare the amount of biomass plants have allocated to roots in relation to total plant biomass (root mass ratio). Students could measure plant height and aboveground and belowground biomass, and other variables that emerge from your classroom discussions.

Supplies

  • Pots with holes at the bottom (3 in2 [7.6 cm2] recommended)

  • Window screen mesh

  • Commercial potting soil (fill to 80% volume of the pot)

  • Topsoil (collected locally; fill to 80% volume of the pot)

  • Sand (fill to 80% volume of the pot)

  • Biochar (10% by volume)

  • Balance

  • Forceps

  • Foil

  • Mung bean seeds

  • pH paper or probes

  • Deionized water

  • Metric ruler

  • Camera to document growth

Experimental Design & Protocols

The experiment is a comparative manipulative study with three soil types (potting soil, topsoil, and sand) and two biochar amendments (no biochar [control] and 10% biochar). Prior to the experiment, germinate the mung bean seeds by placing them between two damp paper towels for 24 hours. Prepare half the soil for biochar amendment by mixing in 10% biochar by volume; this is a concentration that is typically used in laboratory experiments and agricultural fields (Biederman & Harpole, 2013). Leave the other half of the soil unamended as the control. Clearly label the pots with soils with the appropriate six experimental treatments (potting soil control; potting soil biochar; topsoil control; topsoil biochar; sand control; sand biochar). Place germinated seeds on the soil surface and cover with 3–5 mm of soil. Place the potted plants under fluorescent grow lights (13–50 watt bulbs) and maintain them at room temperature. Each experimental treatment should be replicated at least three times to make it possible to investigate variability by calculating means and standard deviations. If necessary, use pipe cleaners or other wires to help keep plants upright during the experiment. Add more water immediately after planting and as needed.

Teacher Considerations

We grew fast-growing and easy-to-maintain mung beans in natural topsoil, commercial potting soil, and sand, but other plant species and soil types could be used. Have students work in groups to maintain one replicate of a soil type and biochar treatment combination (e.g., potting soil control and potting soil biochar).

Student Activities

Plant growth. Prepare and label each pot with the appropriate experimental treatment for your group as described above. Record the date of planting and the date that the plants break through the soil surface. After each watering, record visual observations about the plant. On day 7, select the tallest plant and measure the height (cm) with a ruler. Place a toothpick near your selected plant to mark which plant you will measure each week and record the height of this plant after each watering ( Appendix 3). Take a photo of your plants each week. Plants are delicate and must be handled carefully each time you measure them to prevent them from breaking.

Plant biomass harvest. After at least three weeks, take a final height measurement and then harvest plant biomass (Figure 4). Gently remove the plants and soil from the pot. Using forceps and a small paintbrush, carefully remove soil from the roots. You can rinse the roots with water to remove remaining soil. Separate the roots (belowground biomass) from the stems and leaves (aboveground biomass). Place the plant material into foil labeled with the experimental treatment and aboveground or belowground biomass. Fold the aluminum foil to minimize losses during transport. Dry the plant material in an oven at 60°C for at least 24 hours. You can leave the plant material in the oven over the weekend for up to 72 hours. Weigh dried biomass and record ( Appendix 3).

Figure 4.

Example of aboveground and belowground biomass harvest from sand (control) and sand + biochar treatments for the classroom plant growth experiment. Aboveground biomass (shoots) and belowground biomass (roots) are separated, dried, and weighed. (Credit: photo by M. Hunter-Laszlo)

Figure 4.

Example of aboveground and belowground biomass harvest from sand (control) and sand + biochar treatments for the classroom plant growth experiment. Aboveground biomass (shoots) and belowground biomass (roots) are separated, dried, and weighed. (Credit: photo by M. Hunter-Laszlo)

Soil pH. Students can also investigate the impact of biochar on soil pH as an indicator of the chemical and biological environment of the soil ( Appendix 1). Soil pH is considered the “master variable” in determining which soil organisms are present in the soil and how they function (Fierer & Jackson, 2006; Wu et al., 2011). Protocols for measuring soil pH are detailed in  Appendix 1.

Data analysis. Students can graph plant growth over time using plant height data from each treatment over the experimental period (Figure 5). Plant biomass data can be plotted in a bar graph as shown in Figure 6. Students can then clearly see the differences between aboveground and belowground biomass production. Notice that belowground responses do not necessarily mimic aboveground responses to the same treatment. Plant and soil scientists commonly represent these data as ratios. Students can calculate and graph root mass ratios from their biomass data by dividing root biomass by the total plant biomass ( Appendix 3 and Figure 7). Students can also calculate means and standard deviations and add these to the graphs (Figures 5, 6, and 7).

Figure 5.

Plant height data collected by students during plant growth experiment. Points represent means. Error bars represent one standard deviation from the mean. Numbers in legend are number of replicates within each treatment.

Figure 5.

Plant height data collected by students during plant growth experiment. Points represent means. Error bars represent one standard deviation from the mean. Numbers in legend are number of replicates within each treatment.

Figure 6.

Aboveground (A) and belowground (B) biomass data collected by students during plant growth experiment. Bars represent means. Error bars represent one standard deviation from the mean. Sample sizes shown above bars are number of plant replicates in each treatment.

Figure 6.

Aboveground (A) and belowground (B) biomass data collected by students during plant growth experiment. Bars represent means. Error bars represent one standard deviation from the mean. Sample sizes shown above bars are number of plant replicates in each treatment.

Figure 7.

Ratios of root mass to total plant biomass from data collected by students during plant growth experiment. Bars represent means. Error bars represent one standard deviation from the mean. Sample sizes shown above bars are number of plant replicates in each treatment.

Figure 7.

Ratios of root mass to total plant biomass from data collected by students during plant growth experiment. Bars represent means. Error bars represent one standard deviation from the mean. Sample sizes shown above bars are number of plant replicates in each treatment.

Experiment 2: Soil Respiration

Students conduct an experiment in which they add biochar to soil and compost and measure the response of soil respiration. Students ask, “How does adding biochar to soil and compost affect soil respiration?” Based on their observations of biochar, prompt your students to predict whether biochar will increase, decrease, or not affect soil respiration and to explain their ideas about why before they begin the experiment. Soil microbes metabolize organic and inorganic compounds via extracellular enzymes, respire carbon dioxide (CO2), and excrete nitrogen as byproducts of growth and reproduction (Schimel & Bennett, 2004; Paul, 2014). Scientists measure the flux of CO2, or “soil respiration,” to assess microbial activity and rates of CO2 entering the atmosphere. Scientists are also interested in the nitrogen released by microbial activity because nitrogen is necessary for plant growth.

Supplies

  • Garden soil

  • Compost

  • Biochar

  • Buckets for soil collection

  • 50 mL graduated cylinders

  • CO2 probe, sensor, or gas detecion tube

  • Data collection device

  • Sample container (200 mL minimum)

  • Deionized water

Experimental Design & Protocols

We collected garden soil and compost, but other soil types may be used. Compost supports high microbial activity and respiration and serves as a useful comparison with natural soils. Use half of each soil type as the unamended control and prepare the other half with biochar amendment by adding 10% biochar by volume. Label the soil and compost with the four experimental treatments (compost control; compost biochar; garden soil control; garden soil biochar) and leave them open to air for a day before taking measurements to allow soils and microbial communities to re-equilibrate after the disturbance of setup. Prepare the soil or compost sample by filling the sample container to the 150 mL mark with your assigned soil. Label the bottle with your group name, soil type (compost or garden soil), and experimental treatment (control or biochar). Using a graduated cylinder, moisten the soil with 20 mL of water. Set up the experiment away from windows to maintain stable temperature conditions (ideally around 25–30°C). Replicate each treatment at least three times to calculate means and standard deviations. Prepare the CO2 measurement instrument for data collection. For probes or sensors, allow the instrument to warm up until the readings begin to stabilize. Calibrate the sensor before setting up the experiment. Set up the data collection device to take a measurement every 1 minute for 24 hours (1440 measurements). Place the CO2 sensor in the sample container (Figure 8). For gas detection tubes, measure CO2 concentration at least three times: at the start, at 30 minutes, and at 24 hours. You can adjust intervals to fit your classroom and scheduling needs.

Figure 8.

Example setup for one experimental treatment replicate for soil respiration classroom investigation using a CO2 sensor and data collection device. (Credit: photo by S. Bucko)

Figure 8.

Example setup for one experimental treatment replicate for soil respiration classroom investigation using a CO2 sensor and data collection device. (Credit: photo by S. Bucko)

Teacher Considerations

Organize students into groups of two to four to conduct the experiment on one of the four treatments. Treatment combinations and replicates can be spread across class periods and students can analyze the combined data from all classes. Test the CO2 probes or sensors to ensure they are working properly before beginning the experiment with students. For CO2 gas detection tubes, ensure that the CO2 concentration of your soil and compost is within the detectable range of the instrument. Prior to setting up the experiment, determine the number of measurements you will make over the course of the experiment and the time between measurements. Consider using a larger container to avoid inhibiting respiration via oxygen depletion if you conduct the experiment over a longer period.

Student Activities

Prepare and label each pot with the appropriate experimental treatment for your group and familiarize yourself with the CO2 probes or sensors as described above. Start collecting data. Download the data after 24 hours. If you are using CO2 gas detection tubes, keep the sample container closed until you are ready to make a measurement. Place the tube into the sample container and read the CO2 concentration from the side of the tube. Repeat this at the time intervals you have chosen.

Data analysis. Students can graph the cumulative CO2 concentration for each treatment over the 24-hour period (Figure 9). In our experiments, students observed two patterns: (1) CO2 concentration increased over time and eventually leveled off in the compost treatments. This pattern could be due to a decline in oxygen concentration inhibiting respiration in the small chamber, a decrease in organic matter availability, or, in the case of the compost treatment, CO2 concentration exceeding the detection limit of the sensor. (2) CO2 concentration differed between compost and garden soil. The cumulative CO2 concentration tends to be higher in compost with biochar than without it, but lower in garden soil with biochar than without it (Figure 9). Biochar may have impacted soil pH or moisture that positively affected microorganisms and thus cumulative CO2 concentration in compost, but not in garden soil. However, cumulative CO2 concentration was not statistically different between the control and biochar treatments for either soil type. Discuss these results with your students and ask them whether their results confirmed or conflicted with their predictions and why.

Figure 9.

CO2 concentration data collected by students during soil respiration classroom experiment. Points represent means. Error bars represent one standard deviation from the mean. Sample sizes shown in the legend are number of replicates in each treatment.

Figure 9.

CO2 concentration data collected by students during soil respiration classroom experiment. Points represent means. Error bars represent one standard deviation from the mean. Sample sizes shown in the legend are number of replicates in each treatment.

Data Interpretation

Students can compare their findings with other experimental results from working scientists (Figure 10). As is often the case, results from other experiments may be similar to or quite different from classroom findings. Given the variability in biochar and soil properties, it is likely that classroom findings will differ from findings of other experiments. Results in Figure 10 were taken from published field, greenhouse, and laboratory experiments to show variability in responses of plant growth (A and B) and soil respiration (C and D) to biochar addition. These previous experiments have shown positive, negative, and neutral results of biochar soil amendment, depending on the soil type and the biochar properties. One reason for this variability is soil pH ( Appendix 1). Differences in pH between biochar and control treatments and soil types may help explain differences in plant growth and soil respiration. For example, biochar has a large effect on corn yield in an acidic soil (Figure 10A), whereas biochar has no measurable effect on corn yield in a basic soil (Figure 10B). Students can measure soil pH and visualize differences between treatments by graphing dot charts ( Appendix 1).

Figure 10.

Previous experiments show the variation in responses of plant growth (A, B) and soil respiration (C, D) to biochar addition. Details of the soil type, soil pH, type of biochar, amount of biochar, and experimental designs are given in the table below the graphs.

Figure 10.

Previous experiments show the variation in responses of plant growth (A, B) and soil respiration (C, D) to biochar addition. Details of the soil type, soil pH, type of biochar, amount of biochar, and experimental designs are given in the table below the graphs.

Comparing classroom experimental designs and results with previous experiments gives context to the students' work and allows for meaningful discussion about the nature of scientific investigation. For example, our classroom soil respiration experiment was conducted over a 24-hour period, while the laboratory (Figure 10C) and greenhouse (Figure 10D) soil respiration experiments were conducted over 50 days and 10 weeks, respectively. Ask your students why the soil respiration results over 10 weeks differ from those of their 24-hour experiment. In this case, the classroom experimental design resulted in a CO2 concentration that reached the maximum detectable limit in the compost treatments (Figure 9), whereas the garden-soil CO2 concentrations continued to increase, similar to the results in Figure 10. Use these results as an opportunity for students to objectively critique the experimental design and suggest ways to improve it, rather than criticizing themselves for “doing it wrong.” Teachers and students can refer to Figure 10 to discuss variability in experimental results and show that scientific experiments rarely come to one “right” answer or conclusion. Using the CER framework, task students to use multiple pieces of experimental evidence from classroom experiments and other experiments to develop a scientific argument ( Appendix 2).

Conclusion

The plant growth and soil respiration experiments align with NGSS and the CER framework to engage students in an authentic science experience. Students will develop soil and plant disciplinary knowledge and gain experience with science practices. Together, students will craft a scientific argument using data generated both by their class and by professional scientists. This integrated approach connects students to ongoing, relevant research in soil and plant science.

This work was funded by the U.S. Department of Agriculture National Institute of Food and Agriculture Research Initiative–Coordinated Agricultural Projects Bioenergy Alliance Network of the Rockies grant no. 2013-68005-21298.

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Appendix 1.

Protocol for measuring soil and biochar pH. Example data from plant growth experiment are included.

Why Measure Soil & Biochar pH?

pH is a good metric of the chemical environment of the soil. It is often considered a “master variable” in determining soil microbial community composition and activity (Fierer & Jackson, 2006; Wu et al., 2011). As part of the pre-experiment observation, ask your students to measure and compare the pH of different soil types and biochar. You could also have students measure soil pH at the beginning and end of the plant growth experiment (example data shown below) or soil respiration experiments. Both plants and microorganisms acidify the soil environment through biological processes such as respiration Paul, 2014. We would expect to see a decrease in soil pH over the experimental period. However, the pH of the biochar itself will alter the pH of the biochar–soil mixture when added. Biochar is typically basic and often increases the pH of the soil, but results are variable (Lehmann & Joseph, 2015). The effect of biochar on soil pH is often pointed to as a mechanism for observed plant and soil responses to biochar addition. Use the pH results to help your students interpret the results of the plant growth and soil respiration experiments. Consider the question “How does biochar change soil pH, and how might this affect plant growth and soil respiration?”

Measuring pH

Using pH paper or a pH probe, measure the pH of 20 mL of deionized water. Record the pH of the water. Combine 10 g of soil or biochar that has been dried at room temperature with the 20 mL of deionized water. Stir or shake the water and soil or biochar solution. Let the solution settle for 10 minutes before measuring pH. If using a probe, place the probe in the solution without letting it touch the settled soil or biochar. Compare the pH of the water to the pH of the soil or biochar solution. Determine whether the soil or biochar increased, decreased, or did not affect the pH of the deionized water. Example pH results are given in Figure A1.

Figure A1.

Soil pH data collected by students during classroom plant growth experiments. Points represent means. Error bars represent one standard deviation from the mean. Numbers above the points indicate sample size. Here, biochar only meaningfully decreases soil pH in the sand.

Figure A1.

Soil pH data collected by students during classroom plant growth experiments. Points represent means. Error bars represent one standard deviation from the mean. Numbers above the points indicate sample size. Here, biochar only meaningfully decreases soil pH in the sand.

Appendix 2.

The following lab activity example follows the CER framework and is designed to guide students through the plant growth experiment. The lab can be modified for the soil respiration experiment. Example student answers are given in italics. These examples are not the only possible responses but rather goal responses that have been crafted on the basis of real middle school student responses. Modifications for the high school level could include asking students to first write a hypothesis, then a prediction, and discuss the variability in the data (standard deviations).

Scenario: You are a farmer. A salesperson knocks on your door selling a new “miracle” soil additive that she claims will both increase your crop yield and help the environment. After your discussion with the salesperson, you learn that the substance is biochar. Biochar is made from organic waste such as wood chips or agricultural byproducts that are burned in the presence of little or no oxygen.

The salesperson leaves you a small sample of the solid biochar so you can “see for yourself” the huge, amazing gains your crops will make with biochar. After hearing the saleperson's pitch, you decide to try this new “miracle substance” in a small plot before you use it on your entire farm. You decide to conduct an experiment to test the effectiveness of the addition of biochar to your mung bean crop.

Question to investigate: How does biochar affect plant growth in different soil types?

Prediction: Make a prediction about what will happen to the growth of your mung bean plants once biochar is added to the soil.

If biochar is added to the top soil (topsoil, potting soil, sand), then the mung beans will grow bigger than the mung beans in the soil without biochar.

Initial explanation for prediction:

I think this will happen because

biochar increases how much water and nutrients is held by the soil, which helps plants grow.

Independent and dependent variables:

Independent variable: time, soil type, and amount of biochar

Dependent variable: plant growth (plant height, plant weight)

Results:

Create a line graph that plots the average height of plants over time. The independent variable (x-axis) is the day of growth. The dependent variable (y-axis) is the plant height (mm). Include error bars that represent one standard deviation from the mean. [An example of a student graph is given in Figure A2.]

Figure A2.

Plant height data for potting soil treatment collected by students during plant growth experiment. Points represent means. Error bars represent one standard deviation from the mean. Numbers in legend are number of student replicates within each treatment.

Figure A2.

Plant height data for potting soil treatment collected by students during plant growth experiment. Points represent means. Error bars represent one standard deviation from the mean. Numbers in legend are number of student replicates within each treatment.

Discussion & Explanation:

Offer an explanation for the class results and explain why it is either consistent or inconsistent with your original prediction. What did you find out after completing your experiment? Would you recommend adding biochar to fields of mung plants? Use data and evidence from the class experiment and from the results of other experiments by other scientists to support your reasoning.

Claim: What is your claim about biochar? Under what conditions (e.g., in what types of soils) is biochar an effective soil additive to increase plant biomass?

Biochar is not a good way to increase plant growth in topsoil. The mung beans did not grow larger when they were in topsoil with biochar compared with topsoil without biochar.

Evidence: What evidence (data) do you have to support your claim about biochar? Provide evidence from your class data in the form of a graph or table.

The plants were shorter when grown with biochar compared to without biochar in all the different soil types. The amounts of shoots and roots was almost the same in soils with biochar and without biochar.

Reasoning: Provide a biochar recommendation (whether to use it or not) to your neighbor who also farms mung beans. The neighbor asks, “Why do you make this claim about biochar?” Explain the reasons why you made this suggestion to your neighbor. Why is biochar effective in the conditions you identified and less effective in other conditions?

Based on my experiment, I would not recommend adding biochar to your soil because it did not make the plants grow any bigger. In the soils we tested, biochar did not increase plant growth. I think that the soils we used are already fertile and support good plant growth, so adding biochar didn't help. We learned that biochar can help keep soils moist. If your soils are dry, then adding biochar might be useful for keeping water in the soil so the plant can get the water it needs. We also learned that biochar doesn't affect all plants the same way. If you are growing a different plant than mung beans, biochar might help plant growth more.

Reflection:

  • What variables that you did not test or control for might have influenced your results?

    We did not test the fertility and amount of nutrients in the soil types.

  • Describe two things you would do differently if you repeated the experiment.

    I would try to measure or control some things about the soil types, like the amount of nutrients in the different soil types.

  • What other experiments would you need to conduct before you make a decision about the use of biochar? How would they help you make a better-informed decision?

    I would need to do the experiment with different plants and soils. I would also need to try the same experiment on dry soils because other experiments show that biochar can keep soils moist in dry conditions.

  • Provide two new questions you have since completing the experiment and analyzing the class data.

    Why didn't the biochar increase plant growth in the sand? Sand isn't very fertile and I thought the biochar would help add nutrients that would make the plants grow better. Why do plants grow more roots, but not more plants, in sand than in potting soil? If the sand is less fertile and the plants need more roots, how are they able to still grow the same amount of leaves?

Appendix 3.

Example data table worksheet for plant growth experiment. Data tables will differ for the soil respiration experiment depending on the data collection device used.

Student names: __________________________________

Date of planting: _________________________________

Date plants broke soil surface: _____________________

Soil TypeTreatmentDay 7 Height (cm)Day 10 Height (cm)Day X Height (cm)Final Root Mass (g)Final Shoot Mass (g)Root : Shoot
Sand Control       
Sand Biochar       
Potting Control       
Potting Biochar       
Top Control       
Top Biochar       
Soil TypeTreatmentDay 7 Height (cm)Day 10 Height (cm)Day X Height (cm)Final Root Mass (g)Final Shoot Mass (g)Root : Shoot
Sand Control       
Sand Biochar       
Potting Control       
Potting Biochar       
Top Control       
Top Biochar