The main experimental variables in a CO₂ production experiment with different compost treatments are the type of compost treatment (independent variable) and the CO₂ production (dependent variable). Controls and additional factors, like water and soil temperature, should be managed to ensure experiment validity.
The experiment described concerns the cumulative production of CO₂ from soil with different compost treatments. The experimental variables in this type of experiment typically include, but may not be limited to, the type of compost treatment applied to each soil layer (independent variable), and the resulting amount of CO₂ produced (dependent variable). The control in such an experiment could be a soil layer with no compost applied, to compare the difference in CO₂ production due to the treatments.
Other factors that should be controlled to ensure the experiment's validity include the amount of water, soil temperature, and the presence of plants or insects. These factors are potential variables that Gary did not control, which could influence the outcome of the experiment.
The complete question contains image attached below-