Martin et al. 2004

Martin JG, Bolstad PV, Norman JM. 2004. A carbon dioxide flux generator for testing infrared gas analyzer-based soil respiration systems. Soil Science Society of America Journal 68: 514-518.

These authors constructed a system for calibrating soil CO2 flux using a closed-chamber Infrared Gas Analyzer (IRGA), in this case a Li-Cor 6400. Two detectors were used: one to monitor the CO2 concentration inside the reservoir, and one as the test machine placed on top of the artificial soil on top of the reservoir. The basic construction was quite simple, compared with the systems of Butnor and Johnsen (2004) and Widen and Lindroth (2003). Essentially, this system is just a cylinder topped by a level layer of uniform glass beads. CO2 is added to the reservoir beneath, apparently by the simple method of exhaling into the input valve, and diffuses through the reservoir (mixed by a small fan) and through the glass bead layer.

Much of the refinement of this system concerns the placement of mixing fans to 1) ensure the reservoir is well-mixed but not pressurized and 2) disrupt the boundary layer on top of the glass beads. Boundary layer effects are blamed for some of the measurement error reported here.

The test system underestimated low fluxes, and overestimated high fluxes. These authors suggest that wind-speed differences inside vs. outside the closed chamber, and associated boundary layer differences, are the main drivers of these measurement errors. They also strongly caution that variation in the set point inside the reservoir has a large effect on measured flux rates. Under field conditions, with soil composed of smaller particles, these errors are expected to be less important.