Challenges of Making Flux Measurements

Soil CO2 production is heavily influenced by environmental factors (soil temperature, soil moisture, organic content, etc.) and biological factors (above ground canopy size, growth activity, etc.). Soil CO2 efflux is a physical process driven primarily by the CO2 concentration diffusion gradient between the upper soil layers and the atmosphere near the soil surface. The fundamental challenge for making accurate soil CO2 flux measurements is that the deployment of chambers must cause minimal disturbance to environmental conditions that have an impact on CO2 production and transport inside the soil profile.

Chambers Designed to Minimize Environmental Perturbations

LI-8100 chambers are designed to minimize perturbations to the surrounding environmental conditions and measurement artifacts that can affect the natural soil CO2 production and diffusion processes.

• Both survey and long-term chambers close automatically, eliminating variations caused by manual chamber placement.

• New patent-pending pressure vent design minimizes pressure pulses at chamber closing, and allows chamber pressure to track the ambient pressure under calm and windy conditions.

• CO2 flux rate is calculated at the CO2 concentration of the surrounding ambient air. This minimizes effects resulting from the necessary increase in chamber CO2 concentration during a measurement.

• A bowl-shaped chamber provides good mixing without using fans, thus eliminating potential for chamber pressure perturbation.

• Air flow is generated by a rotary pump that provides a steady, consistent air flow with much lower pulsations than ordinary diaphragm pumps.

• Temperature artifacts are minimized by careful consideration of materials and coatings.

• The perforated baseplates of the Long-Term Chambers minimize perturbations to the soil environment around the chamber, including the prevention of a concentration gradient-induced impedance of soil CO2 flux.

 

Maintaining Pressure Equilibrium Inside and Outside the Chamber

Pressure equilibrium between air inside a soil CO2 flux chamber and the surrounding air outside the chamber must be maintained during the measurement if measured flux is to accurately represent the rate occurring naturally outside the chamber. A simple open vent tube connecting to the chamber often has been used to maintain the pressure equilibrium. This approach is effective only under calm conditions. Under windy conditions, negative pressure excursions occur as wind blows over the vent tube’s external open end because of the Venturi effect. This causes a mass flow of CO2 -rich air from the soil into the chamber, leading to a significant overestimation of soil CO2 flux. In fact, some researchers have recommended eliminating the vent tube after recognizing the potential problem from the Venturi effect.

LI-COR has developed a patent-pending vent design which has a tapered cross section (illustration). Conservation of mass requires the average air flow rate to drop as air enters the vent. According to Bernoulli’s equation, as the rate of air flow slows down, a major portion of the dynamic pressure is converted to static pressure, raising the static pressure with which the chamber equilibrates. The vent design is radially symmetric to eliminate sensitivity to wind direction. Data from field experiments on differential pressure measurements between air inside the chamber and the outside ambient air show that chambers equipped with this vent maintain the pressure of outside air under calm and windy conditions. LI-COR’s vent virtually eliminates the Venturi effect.

Xu, L., M. D. Furtaw, R. A. Madsen, R. L. Garcia, D. J. Anderson, and D. K. McDermitt (2006), On maintaining pressure equilibrium between a soil CO2 flux chamber and the ambient air, J. Geophys. Res., 111, D08S10, doi:10.1029/2005JD006435.

Chamber Drive Mechanisms

The different drive mechanisms controlling the opening and closing of the long-term chambers do not affect the measurements. Both chambers lower slowly onto the measurement collar to minimize pressure pulsations that change soil CO2 flux rates. The vertical drive of the 8100-101 chamber may work better between and within crop rows by positioning the chamber to open and close without affecting adjacent plants. The 8100-104 has six settings for the chamber’s open position. This gives researchers an advantage when placing the chamber under short plant canopies or other situations where these open positions may help to avoid terrain or other obstructions.

8100-101 Long-Term Chambers between and within soybean rows.

 

Baseplate Perforations

Structurally, the environmental impact of the 8100-101 and 8100-104 chambers is reduced with perforations in the baseplates to minimize disturbance of the environmental conditions surrounding the chambers. The perforations allow minimal perturbations to natural sunlight, precipitation, wind, etc., and they prevent a concentration gradient-induced impedance of CO2 flux from the soil, which can occur under a nonperforated, uniform plate.
View of the 8100-104 Long-Term Chamber baseplate.	View of the 8100-101 Long-Term Chamber baseplate.
Diffusion simulation (Fick’s Law), showing a representation of CO2 flux rates at the soil/air interface (red = low flux rate, blue = high flux rate). In the graphic at left, a solid baseplate shows low (suppressed) flux rates with a nearly uniform effect under the baseplate. Perforations in the baseplate (right) greatly reduce this effect, preventing a concentration gradient-induced impedance of soil CO2 flux.