Challenges to Anticipate and Solve

  1. Free-riders: In a group setting, it is common for some students (especially weaker students and quiet or shy students) to step back and let the other group members do the work and contribute answers. One solution is to make that person the recorder or notetaker. Step in and say something like, “Make sure you have someone recording your data. Mike, why don’t you do that,” and hand him the clipboard.  That gives that person an active role to play and sometimes it even becomes a leadership position.

  2. Statistical Guidance: I have my students run a t-test on their results using EXCELTM (Microsoft, Inc.). Most of our students have been exposed to the t-test before but they usually still need some guidance so I make sure they do this procedure in lab while I am around. This step is optional and some faculty may not want to bother with it. I include a handout with step-by-step instructions for performing the t-test (click here for t-test instructions (DOC)).


Introducing the Experiment to Your Students

Typically I cover much of the introductory material (decomposition and the carbon cycle) in lecture before the lab activity, so students are somewhat familiar with it. In lab we begin indoors where I review the concepts briefly. I show them the incubation chambers and review how soda lime works. Finally I tell them that they will be working in groups to design their own experiment to determine how environmental factors affect soil respiration.

Then we head outside to a nearby forest on campus. I have found that students need some guidance in knowing what to look for. I point out differences in soil type, leaf litter types and amounts, sunlight, slope, and topography and ask them how the microclimate might differ in each case. I give them 15 – 20 minutes to walk around, observe, and as a group come up with an idea for an experiment. They write out their experimental design while outside and then type it up as homework before turning it in.

An option for an upper level class is to give them less information about the environmental factors (i.e., edit the Introduction section to omit these), let them generate their hypotheses “from scratch” and run their experiments. This option will likely require more time for discussion.


Data Collection and Analysis Methods Used in the Experiment

A forest setting works best because the herbaceous vegetation is sparse and the soil organic matter content is high. Any green plant in the chamber may remove CO2 through photosynthesis, so it is important to remove or avoid all plants (unless you want to measure photosynthesis). Even a small stand of trees is adequate. However, I have also used the technique successfully in grasslands, gardens and lawns. When herbs are present they can be removed by pinching or cutting them off at ground level. In a lawn or grassland you can treat small patches with herbicide several days ahead of time. Note that when a plant is killed, the rapid decomposition of its roots will create a spike in decomposition between 2 and 5 days later. So soil respiration must be measured immediately after cutting the plant or after 5 days have elapsed.

Weather is an important consideration. When soil temperatures are above 16ºC, a 24-hour incubation period is usually adequate. At cooler temperatures, a 48- to 96-hr incubation period may be required. Avoid rainy days or waterlogged soils. Excessive moisture as during a rain storm will turn the soda lime to mush. Flooded conditions also inhibit CO2 diffusion from soil.

You can expect a mass change in the soda lime of about 0.01 to 0.3 g so a 0.001 g balance should work fine. A 0.0001 g balance will give better precision but is not necessary in most cases.

It may be difficult for all the students to get out to the site after the 24 or 48 hour incubation period. If my lecture falls on that day, I will take them out during the lecture period to collect the soda lime. This step in the procedure does not take much time. Another option is to ask for just a few student volunteers to do the collection for everybody.

Because the chambers will be sitting out unattended for one or more nights, it is important to consider the problem of vandalism. It is best to place chambers away from trails, high-traffic areas, or places where people may congregate. If they must be set out in public view then it is best to post a small sign explaining that this is an experiment and “please do not disturb”. Don’t forget to contact your Physical Plant or Groundskeeper or they may pick up the chambers thinking that they are litter (I learned this the hard way)!


Questions for Further Thought

  1. Instructors may need to help students summarize their environmental measurements and to figure out exactly what they mean. For example, students may not immediately grasp the idea that a south-facing slope gets more direct sunlight than a north-facing slope and that this could effect soil temperature and moisture.

  2. It is important for students to recognize that plant roots can contribute to soil respiration as well as microorganisms.

  3. Students can be referred to the Introduction section of the lab exercise or to the internet to find answers to this question.

  4. The world’s soil organic carbon pool mass can be found in the Introduction section or students could be assigned to research several estimates.

  5. Students may need assistance properly formatting a table (they often want to include raw values instead of summary data like means, for example). Instructors can find a sample table and formatting requirements in the Lab Report Guidelines document [click here to download DOC] or they can develop their own.

  6. With some leading questions Instructors can get students to realize that soil temperature probably changes from day to night and that soil moisture may change over a 24 – 96 hour incubation period.

  7. Based on discussions held during the first lab session, students should have some ideas for these already. I will usually point them to some pages in their textbook about microclimate and microtopography or supply some references for them related to slope, aspect, soil bulk density, vegetation, etc.

  8. I have found students often have difficulty coming up with good key words for internet searches because they are unfamiliar with the “jargon” of a particular field, so I usually will supply them with some. They may also need training on how to find and use search engines. Library staff can be very helpful here.

  9. Same comments as for #8 above.

Assessment of Student Learning Outcomes

The use of a Prime Trait Assessment (PTA) has been extremely useful to me over the years because it speeds up my grading process, makes my grading more consistent and objective, and, by using it in several classes, allows me to compare lab reports among classes and over time for departmental assessment purposes. Because it is in spreadsheet form, it can be filled in electronically without having to resort to paper (the spreadsheet also automatically adds up the points!)

The PTA is a form of rubric that assesses student performance relative to certain “prime traits”. Because the PTA is so long and detailed there is a bit of learning curve for new users but after a few uses it becomes a time-saver. My suggestion is to read through the lab report and make your normal comments and corrections in the text and margins. Then go through the rubric and check all the errors that were found in the paper. Assign points for each category (as described below) and total them up for the score.

Here’s how the scoring works: Let’s say in the Introduction section of the lab report you find two incorrect statements and that the significance of the research was unclear and the hypotheses were missing. You would put two X’s in the “Incorrect or contradictory statements” box, an X in the “significance of the research was unclear” box, an X in the “three or more of the above errors” box (because of the previous three X’s), and an X in the “No statement of hypotheses” box. The score for the Introduction section would be 3 points out of a possible 10 because that was the lowest score of all the boxes that were checked. This is where the grading rationale of the PTA is somewhat unconventional. Instead of losing points for each infraction and accumulating those deductions, the student is graded based on his/her most egregious error. This tends to highlight those errors and shows students what aspects are most important (the prime traits) in a report.

Notes on scoring: Students at first are a little shell-shocked when they receive their numeric grades because the bell-curve is shifted to the left with this rubric. A grade of 50 – 60% is common for a first draft lab report and scores of 95% or more are very rare. I make sure to convert their numeric grade to a letter grade. They feel a little better when they see that a 61% is comparable to a B-.

By filling in the PTA electronically and using it in several classes including a sophomore class and senior class, I have been able to track the progress of individual students through the curriculum with respect to writing lab reports. I also have a long-term time series accumulated that shows trends over time.


Evaluation of the Lab Activity

I have used three types of formative evaluation in this exercise. The question I ask myself (and the students) is: how can I help students achieve the three learning objectives?

The first type of formative evaluation is already imbedded in the exercise and that is the Experimental Design assignment. As I grade that assignment I can evaluate and provide feedback to the students on two aspects of learning objectives 1 and 3: writing hypotheses and writing a methods section. During Session 2 we go over this graded assignment and that gives students a chance to correct mistakes and ask questions.

The second formative evaluation is a Quiz/Survey given at the beginning of Lab Session 2. It is intended to assess the degree to which they have achieved learning objective 2 and also to identify any problem areas. The quiz portion contains five objective questions to assess content knowledge. The survey portion contains two questions asking students about 1) anything that is not clear, 2) the hardest part of the activity so far. No grade was associated with the quiz in my courses but an instructor could use it as a graded assignment.

I instituted the Quiz/Survey the last time I conducted this exercise and found it helpful. The quiz portion showed the poorest performance on the soda lime technique question. The survey showed that students were not sure how the environmental variables were going to be incorporated. As a result I was able to go over those topics in lecture immediately following Lab Session 2. I also revised the Introduction by adding more detail about the soda lime technique, including Figure 3. Finally I spent time during Lab Session 2 helping students summarize and interpret their environmental measurement data.

The third formative evaluation is already embedded in the exercise: students turn in two drafts of their lab report. The first draft is graded and returned to them with comments and the PTA spreadsheet. They then have 1 week to revise the report and submit a final draft which is graded with the same PTA spreadsheet. This simple procedure transforms the lab report assignment from a “shot in the dark” at a grade to a learning experience.


Translating the Activity to Other Institutional Scales or Locations

The experiment can be adapted for indoors when the weather does not cooperate. Plant trays or plastic bins containing 2 to 4 cm of soil material set up in a lab or greenhouse make suitable substrates. Students can compare CO2 emission rates among contrasting soil types, amounts of organic matter, soil depths, soil temperatures (using a plant germination heating mat), or moisture levels. The indoor setting would also be more suitable for students with physical disabilities.

I have also used the technique successfully in grasslands, gardens and lawns. When herbs are present they can be removed by pinching or cutting them off at ground level. In a lawn or grassland you can treat small patches with herbicide several days ahead of time. Note that when a plant is killed the rapid decomposition of its roots will create a spike in decomposition between 2 and 5 days later. So soil respiration must be measured immediately after cutting the plant or after 5 days have elapsed.

An alternative that greatly speeds up the experiment but requires more expensive equipment is to use CO2 detectors to measure CO2 accumulation in the headspace of the chambers instead of soda lime. The incubation time is reduced from 24 hours to 5 minutes with this technique. For example, relatively inexpensive detectors that can be connected to portable computers or handhelds can be obtained from Vernier, Inc.(www.vernier.com). In this situation CO2 concentration in the chamber headspace is monitored for 5 minutes and the rate of CO2 emission is calculated.

Because of the inexpensive materials and simple techniques needed for this experiment, it is well-suited for junior high or high school classes.