Figure 2. The arrangement used to eliminate the CO₂ gradient from a central cylinder of soil.

The blackened PVC pipes are set in the bottom of 10 cm by 2 m deep boreholes and act as chimneys ventilating the boreholes. CO₂ diffuses into the boreholes from the soil and is vented to the atmosphere. In this test arrangement, CO₂ concentrations in the central cylinder were monitored by 5 sampling arrays (the syringe needles were under the short, capped PVC pipes and blue plastic cups). In this situation, the gradient disappeared in less than a day. The person in the photo has given written informed consent, as outlined in the PLOS consent form, to publication of his photograph.