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.
Students can compare their findings with other experimental
results from working scientists (Figure 10). As is often the case,
0 200 400 600 800 1000 1200
garden − control (n = 5)
garden − biochar (n = 3)
compost − control (n = 4)
compost − biochar (n = 3)
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 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.