Butnor and Johnsen 2004

Butnor JR, Johnsen KH. 2004. Calibrating soil respiration measures with a dynamic flux apparatus using artifical soil media of varying porosity. European Journal of Soil Science, 55: 639-647.

These authors describe a system for the absolute calibration of CO2-flux from soils for closed-chamber type measurement systems. The underlying principle of the large box they constructed is that there is no pressure difference between the footspace below the simulated soil and the atmosphere above. This means CO2 moves through the soil only by diffusion, and not by other processes.

The artificial soil was constructed from inorganic components of known and nearly-constant physical characteristics, and without water. This avoids the many confounding variables associated with real soils, such as microbial activity and changes in physical parameters with water content and water movement. However, there are still a large number of variables to account for in calculating and measuring CO2 flux through a porous material.

In general, the tested closed-chamber system, a Li-Cor 6400 Photosynthesis System, underestimated CO2 flux rates except when flux rates were very low. The authors speculate that this reversal of estimation situation was generated by the chamber’s airflow disturbing and capturing shallow pockets of soil air that would not otherwise be measured at very low flux rates.
Overall, this system appears to be an improvement over previous attempts to absolutely calibrate measures of soil CO2 flux. However, it is a large, expensive, and complicated system.